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rolling bearings are universally adopted to serve as revolute joints in almost all mechanisms or machines, because they offer a convenient solution to the problem of minimizing friction and, simultaneously, providing a large load-carrying capacity at any kinematic regime, including slow or alternate rotations. however, in offshore wind turbines not only they reach large dimensions but also they move within strong electromagnetic fields created by the turbine generators. for example, considering the last amplification stadium epicyclic gearbox, they may serve to sustain elements rotating around floating shafts (planetary) which also move around a fixed principal shaft (solar). this article illustrates an original experimental test bench that simulates sliding and rolling contacts through which a test current is flowing. unexpected and interesting results disclose how this particular field is challenging and how more investigations are still required to achieve an adequate and complete interpretation. the understanding of this phenomenon could give rise to modification to the composition and the microstructure of rollers and rings employed in offshore wind turbines. | an accelerated test stand to assess wear in offshore wind turbines rolling bearings |
it has long been the view that energetic charged particle radiation can darken ice. recent work on the inner saturnian satellites links levels of mev electron and proton bombardment with global brightness (hedman et al. 2020). at saturn, the subject lends itself to careful analysis as inner satellites like janus and mimas form macrosignatures while smaller moons like aegaeon and methone do not. a macrosignature is a circumplanetary decrease in the flux level along a moon's orbit and it suggests much lower weathering on the body by that particle species in the corresponding energy range. energetic proton precipitation rates onto jovian and saturnian moons might also be connected with ice state. laboratory results suggest that energetic proton bombardment can lead to a transition from crystalline to amorphous ice at temperatures typical of satellite surfaces in the outer solar system. research with infrared data on galileo found europa and ganymede to have amorphous ice in the top layer, possibly due to their location within the jovian radiation belts, while callisto does not (hansen and mccord 2004). ground-based work confirmed these ice distributions (ligier et al. 2016; 2019). in this talk, we will review some of the recent in situ and remote observations of ice on the inner satellites of jupiter and saturn, describe how the ice might be processed by different types of particles, and discuss current controversies. hedman, m. m. et al. (2020), photometric analyses of saturn's small moons: aegaeon, methone, and pallene are dark; helene and calypso are bright, astron. j., 159. hansen, g. b., and t. mccord (2004), amorphous and crystalline ice on the galilean satellites: a balance between thermal and radiologic processes, j. geophys. res., 109. ligier, n., et al. (2016), vlt/sinfoni observations of europa: new insights into the surface composition, astron. j., 151. ligier, n., et al. (2019), surface composition and properties of ganymede: update from ground-based observations with the near-infrared imaging spectrometer sinfoni/vlt/eso, icarus, 333. | weathering of ice on the jovian and saturnian satellites |
a surprisingly strong influx of organic-rich material into saturn's upper atmosphere from its rings was observed during the proximal obits of the grand finale of the cassini mission. measurements by the ion and neutral mass spectrometer (inms) gave insights into the composition of the material, but it remains to be resolved what fraction of the inferred heavy volatiles should be attributed as originating from the fragmentation of dust particles in the instrument versus natural ablation of grains in the atmosphere. in the present study, we utilize measured light ion and neutral densities to further constrain the abundances of heavy volatiles in saturn's ionosphere through a steady-state model focusing on helium ion chemistry. we first show that the principal loss mechanism of he+ in saturn's equatorial ionosphere is through reactions with species other than h2. based on the assumption of photochemical equilibrium at altitudes below 2,500 km, we then proceed by estimating the mixing ratio of heavier volatiles down to the closest approaches for cassini's proximal orbits 288 and 292. our derived mixing ratios for the inbound part of both orbits fall below those reported from direct measurements by the inms, with values of ∼2 × 10-4 at closest approaches and order-of-magnitude variations in either direction over the orbits. this aligns with previous suggestions that a large fraction of the neutrals measured by the inms stems from the fragmentation of infalling dust particles that do not significantly ablate in the considered part of saturn's atmosphere and are thus unavailable for reactions. | utilizing helium ion chemistry to derive mixing ratios of heavier neutral species in saturn's equatorial ionosphere |
planetary spin can inform our understanding of planet accretion histories, which determine final masses and atmospheric compositions, as well as the formation of moons and rings. at present, the physics behind how gas giant planets spin up is still very poorly understood. we know that when giant planets form, they accrete material and angular momentum via a circumplanetary disk, causing the planet to spin up. in order to prevent planet spins from reaching break-up velocity, some mechanism must regulate these spins. however, there is currently no well-formulated picture for how planet spins evolve. this is in part due to the fact that there are very few measurements of giant planet spin rates currently available. outside the solar system, to date there has only been one published spin measurement of a directly imaged planet, beta pic b. we use keck/nirspec to measure spin rates for a sample of bound and free-floating directly imaged planetary mass objects, providing a first look at the distribution of spin rates for these objects. | the world is spinning: constraining the origin of supermassive gas giant planets at wide separations using planetary spin |
recent measurements of jupiter's gravitational moments by the juno spacecraft and seismology of saturn's rings suggest that the primordial composition gradients in the deep interior of these planets have persisted since their formation. one possible explanation is the presence of a double-diffusive staircase below the planet's outer convection zone, which inhibits mixing across the deeper layers. however, hydrodynamic simulations have shown that these staircases are not long-lasting and can be disrupted by overshooting convection. in this short talk, i will show that planetary rotation could be another factor for the longevity of primordial composition gradients. using rotational mixing-length theory and 3d hydrodynamic simulations, i will demonstrate that rotation significantly reduces both the convective velocity and the mixing of primordial composition gradients. | rotation reduces convective mixing in jupiter and other cold gas giants |
the cassini-huygens mission ended in a science-rich blaze of glory on september 15, 2017. covering a period of roughly ten months, the grand finale and ring grazing orbits marked the final phases of cassini's 13-year mission, ending with the first time in-situ exploration of the region between the rings and planet. on its final orbit, cassini plunged into saturn's atmosphere at 126,000 km/hr (35 km/sec), vaporizing and satisfying planetary protection requirements, while sending back its final bits of unique science data for as long as possible. on november 29, 2016, a single titan flyby sent cassini into a series of 20 highly inclined, elliptical ring grazing orbits with peripases just outside saturn's main rings at distances within 10,000 km of saturn's f ring. these orbits provided high-resolution views of saturn's f and a rings, providing prime viewing conditions of fine scale ring structures such as propellers, and included the closest flybys of tiny ring moons. a final close flyby of titan on april 22, 2017 propelled cassini across saturn's main rings. in its 22 grand finale orbits, the spacecraft repeatedly dove between saturn's innermost rings and upper atmosphere to answer fundamental questions that were unattainable earlier in the mission. the grand finale explored a region of the saturn system that was unexplored by any previous outer planet spacecraft. saturn's gravitational and magnetic fields were measured to unprecedented accuracy, providing information from which constraints on the interior structure of the planet, mass distribution in the rings, and the structure of the internal magnetic field could be obtained. the grand finale orbits provided the highest resolution observations ever of both saturn's c and d rings and saturn's atmospheric weather layer. direct in-situ sampling of the ring particle composition, upper atmosphere and the innermost radiation belts was also achieved. this talk will present key science findings and new mysteries gleaned from the ring grazing and grand finale orbits. the research described in this paper was carried out in part at the jet propulsion laboratory, california institute of technology, under a contract with the national aeronautics and space administration. copyright 2018 california institute of technology. government sponsorship is acknowledged. | cassini's grand finale: discoveries and highlights one year later |
"purest anorthosite" (pan) are rocks of the lunar primary crust with over 98 vol% plagioclase. understanding the distribution and purity of pan on a local scale can help put constraints on the efficiency of plagioclase separation from the lunar magma ocean and help to illuminate the makeup of the moon's primary crust. here we use lunar reconnaissance orbiter camera (lroc) narrow angle camera (nac) photometry to investigate sites where pan has been identified spectrally. we determine hapke photometric parameters and minimize the effects of topography on local albedo variations using nac images and photometric processing with local incidence and emission angles derived from nac dtms. the areas investigated are two sites in the orientale basin inner rook ring (irr) and one site at the central peaks of jackson crater (jksn). recent impact craters and the steepest slopes on the massifs have the highest single scattering albedo values (w) at the irr sites, but areas interpreted to be mature pan regolith have values of w=0.52-0.64. variations in w not associated with maturity indicate differences in composition. pan on both shallow and steep slopes at jksn have values of w=0.75-0.84. jackson crater, however, is only a few hundred myr old, compared to orientale structures, which are 3.75 byr old, and the jksn central peaks are fresh and immature. to investigate mineralogy, spectra from the moon mineralogy mapper (m3) were examined. jksn has strong pyroxene absorptions on the central peak and clear plagioclase absorptions (1250 nm) on steep central-peak slopes. spectra from a pan massif of the irr range from featureless, in some locations, to variable depths of the 1250 nm absorption feature. several factors cause variations in absorption strength (e.g., grain size, differences in maturity, or mixtures of crystalline and amorphous plagioclase). we examine these data in detail to assess the purity of pan and composition of coexisting rock types at these two sites. in areas of mature regolith, we find an inverse correlation between w and clementine-derived wt.% feo (i.e., high w is associated with low wt.% feo), similar to that observed from lunar soils at the apollo landing sites. | investigating local variations of lunar rock types at sites of "purest anorthosite" detection |
ductile and brittle deformation reshapes our planet as a consequence of plate tectonics. while the relative age of these processes can be inferred with field and petrographic relationships, absolute timing of these multiphase processes is usually unclear. most attempts to directly date these events have failed because mineral closure temperatures are too low for classical thermochronology. this work aims to directly date timing and duration of brittle deformational processes recorded by calcite fibres from carbonate rocks. the study area is the al hajar mountains in oman and the method used is the u-pb small scale isochrone (ssi) method recently developed at guf. this area is known to be affected by multiple deformation events from the cretaceous and therefore is a perfect target to apply this methodology. the samples are cross-cutting calcite veins and fibres (orientated thin/thick sections) where in situ analyses were performed, using an excimer laser coupled to an element2 sf-icpms. this method is based on the fact that carbonates, that (re-)crystallised and equilibrated during an event, contain low but variable amounts of u and µ (238u/204pb). therefore, several analyses ( 10 to 50) will yield variable u/pb ratios and thus form an isochrone in the 207pb/206pb vs 238u/206pb space. the age is interpreted to be lower intercept with the concordia curve and the intercept with the y-axis to be the initial pb isotope composition. the geological background frame the deformational events from a late cretaceous ophiolite obduction, followed by extension, to a neogene shortening, where the timing of the uplift of the al hajar mountains is a matter of debate. the results of this work show that top-south, layer-parallel shearing due to ophiolite obduction occurred in the coniacian to early campanian. afterwards, top-ne early paleocene layer-parallel veins developed during post-obduction exhumation. eocene to early miocene horizontal shortening formed strike-slip structures that cross-cut and reactivate older faults, mainly causing the current high topography of the al hajar mountains. the youngest structures are crack-seal fibres in hairline veins dated as late miocene to middle pleistocene [1]. the results show that the ssi technique successfully constrains complicated successions of brittle deformational structures. [1] hansman, r. j. et al. (geology, 2018). absolute age of multiple generations of brittle structures by u-pb dating of calcite. | timing and duration of brittle deformational events. laser u-pb ssi ages on calcite (al hajar mountains, oman) |
the dawn spacecraft has been orbiting dwarf planet ceres since march 2015. images from the dawn's framing camera (fc) have been used to create shape models of ceres using stereophotogrammetry [1] and stereophotoclinometry [2,3] techniques. these shape models have since been used for orthorectifcation of the fc images and computing photometric corrections [4], conducting local geomorphologic studies [e.g. 5] as well as constraining the internal structure of ceres [6,7].the goal of this paper is to characterize the roughness of ceres topography at a range of scales. we follow the methodology from [8] to compute scale-dependent roughness. first, we compute differential slopes, which are differences of slopes between two (long and short) baselines. the roughness is then computed as the interquartile width (using 25th and 75th percentiles) of the differential slope distribution within a certain area (0.5^circ bins in our case). we computed roughness between the following short and long baselines: 246 meters and 574 meters, 574 meters and 902 meters; 902 meters and 1558 meters. these baselines are multiples of 82 meters (or 0.01^circ).unlike for the moon or mercury, the roughness maps for ceres do not reveal any large-scale dichotomy, such as, for example, lunar highlands versus lunar maria. the roughness is mostly controlled by regional scale geology. we observe that young craters such as dantu, occator, haulani, ikapati and azacca are surrounded by ejecta that is than the surrounding terrains, which suggests a high degree of ejecta fluidization. curiously, this behavior is different from that of lunar crater ejecta. lunar craters are often surrounded by rings of smoother proximal ejecta, while distal ejecta becomes rougher with increasing distance from the crater [9]. the difference in the ejecta roughness pattern is likely explained by the different target composition. indeed, it is likely that ceres' crustal material, which is volatile-rich, could reach low viscosity or even partial melting upon impact heating. also, contrary to large lunar basins, kerwan-the largest and oldest confirmed basin-has no distinct roughness signature. on the other hand, the next two largest younger large basins urvara and yalode do have roughness signatures. the younger and smaller of the two-urvara-has a relatively smooth interior, whereas the bigger and older-yalode-appears rougher than the surrounding terrains.in summary, the produced maps reveal that roughness of ceres' topography is mostly controlled by regional geologic processes. the roughness maps can be used to facilitate the mapping of crater ejecta as well as to determine relative surface ages.references:[1] preusker et al., 2016, lpsc 47, 1954; [2] park et al., 2016, nature 537 (7621), 515; [3] park et al., 2018, in prep.; [4] schröder et al., 2017, icarus 288, 201-225; [5] buczkowski et al., 2016, science 353, 6303, aaf4332; [6] ermakov et al., 2017, jgr:planets 122,11, 2267-2293; [7] fu et al., 2017, epsl 476, 153-164; [8] kreslavsky & head, 2003, grl 30,15,1815; [9] kreslavsky et al., 2013, icarus 226, 52-66. | roughness of ceres from high-resolution shape models |
the cosmic facility was developed at nasa ames to study interstellar, circumstellar and planetary analogs in the laboratory [1]. cosmic stands for “cosmic simulation chamber” and is dedicated to the study of neutral and ionized molecules and nanoparticles under the low temperature and high vacuum conditions that are required to simulate space environments. cosmic integrates a variety of instruments that allow forming, processing and monitoring simulated space conditions in the laboratory. it is composed of a pulsed discharge nozzle (pdn) expansion that generates a plasma in a free supersonic jet expansion coupled to high-sensitivity, complementary in situ diagnostics tools, used for the detection and characterization of the species present in the expansion: a cavity ring down spectroscopy (crds) and fluorescence spectroscopy systems for photonic detection and a reflectron time-of-flight mass spectrometer (retof-ms) for mass detection [2].recent advances achieved in laboratory astrophysics using cosmic will be presented, in particular the advances that have been achieved in the domain of the diffuse interstellar bands (dibs) [3] and in monitoring, in the laboratory, the formation of dust grains and aerosols from their gas-phase molecular precursors in environments as varied as circumstellar outflows [4] and planetary atmospheres [5, 6]. plans for future laboratory experiments on cosmic molecules and grains in the growing field of laboratory astrophysics (nir-mir crds, laser induced fluorescence spectra of cosmic molecule analogs and the laser induced incandescence spectra of cosmic grain analogs will also be addressed as well as the implications of the on-going studies for astronomy.references: [1] salama f., in organic matter in space, iau s251, kwok & sandford eds.cup, 4, 357 (2008).[2] ricketts c., contreras c., walker, r., salama f., int. j. mass spec, 300, 26 (2011)[3] salama f., galazutdinov g., krelowski j., biennier l., beletsky y., in-ok song, the astrophys. j., 728, 154 (2011)[4] cesar contreras & farid salama, the astrophys. j. suppl. ser., 208, 6 (2013)[5] sciamma-o'brien e., ricketts c., and salama f. icarus, 243, 325 (2014)[6] sciamma-o'brien e., upton k. and salama f. icarus, in press (2017) | advances in interstellar and planetary laboratory astrophysics with ames’ cosmic facility |
today, we believe that the onset of life requires free energy, water, and complex, probably carbon-based chemistry. in the interstellar medium, complex organic molecules seem to mostly form in reactions happening on the icy surface of dust grains, such that they are released into the gas phase when the dust is heated. the resulting “snow lines”, marking regions where ices start to sublimate, play an important role for planet growth and bulk composition in protoplanetary disks. however, they can already be observed in the envelopes of the much younger, low-mass class 0 protostars that are still in their early phase of heavy accretion. the information on the sublimation regions of different kinds of ices can be used to understand the chemistry of the envelope, its temperature and density structure, and may even hint at the history of the accretion process. accordingly, it is a crucial piece of information in order to get the full picture of how organic chemistry evolves already at the earliest stages of the formation of sun-like stars. as part of the calypso large program (http://irfu.cea.fr/projets/calypso/), we have obtained observations of c18o, n2h+ and ch3oh towards the class 0 protostar ngc 1333-iras4b with the iram plateau de bure interferometer at sub-arcsecond resolution. of these we use the methanol observations as a proxy for the water snow line, assuming methanol is trapped in water ice. the observed anti-correlation of c18o and n2h+, with n2h+ forming a ring around the centrally peaked c18o emission, reveals for the first time the co snow line in this protostellar envelope, with a radius of ~300 au. the methanol emission is much more compact than that of c18o, and traces the water snow line with a radius of ~40 au. we have modeled the emission using a chemical model coupled with a radiative transfer module. we find that the co snow line appears further inwards than expected from the binding energy of pure co ices. this may hint at co being frozen out in h2o or co2 dominated ices. our observations can thereby yield clues on the widely unknown composition of interstellar ices, being the initial seeds of complex organic chemistry. | probing the water and co snow lines in the young protostar ngc 1333-iras4b |
planets form and obtain their compositions in gas-rich disks around young stars. the molecular gas and associated chemistry are directly tied to the resulting exoplanet architectures, including what types of planets form, their atmospheres, and potential habitability. while molecular line observations often provide the best probes of disk characteristics relevant to planet formation, such as surface density, ionization, temperature, and c/n/o ratios, most line observations have been limited to coarse angular resolutions. thus, the detailed structure of the gas component in disks remains largely unexplored, especially toward the inner, planet-forming regions (<100 au). here, i present alma observations of over 50 molecular lines at 10 au scales in five protoplanetary disks, as part of the molecules with alma at planet-forming scales (maps) large program. these observations represent the most comprehensive disk chemistry survey conducted at these scales to date. i show that chemical substructures in the form of rings and gaps are ubiquitous and extremely varied in their radial locations, widths, and contrasts. this suggests that planets form in diverse chemical environments across disks and at different radii within the same disk. the favorable inclinations of the maps disks also provide a direct view of their vertical gas distributions. in each disk, i map this vertical structure, from midplane to disk atmosphere, by extracting emitting heights of several co isotopologues and use these to derive 2d gas temperatures, which are critical inputs for disk models. as part of this work, i helped develop an automated emitting surface finder, which i then apply to a large sample of disks with alma archival data. i show that disks exhibit a wide range of co gas heights and since the vertical distribution of gas influences the chemical reservoirs available to nascent planets, this implies further diversity in local planet-forming environments. overall, these results demonstrate the utility of high-resolution alma observations of molecular lines in disks, and going forward, we aim to expand this approach to larger and more representative samples to obtain statistical constraints on the chemistry of planet formation. | zooming in on the chemistry of planet formation: the alma view of molecular gas in protoplanetary disks |
in september 2017, the cassini spacecraft entered saturn's atmosphere, providing new insights into the composition and structure of the planet's upper atmosphere. prior to atmospheric entry, cassini executed a series of 22 highly inclined orbits through the previously unexplored region between saturn and its rings, yielding the first ever direct sampling of saturn's atmosphere. data returned from the ion and neutral mass spectrometer (inms) aboard cassini have already revealed surprising results, including an unexpected external contribution of molecules into saturn's upper atmosphere and the presence of complex hydrocarbons in saturn's atmosphere. we present here the first results of the composition and thermal structure of saturn's upper atmosphere from inms measurements during cassini's final orbits and atmospheric entry, including the distribution of h2, he, and ch4 in saturn's equatorial thermosphere (yelle et al. 2018). measurements of the h2 density in this region indicate temperatures ranging from 340 to 370 k. ch4 is observed to not be in diffusive equilibrium above saturn's homopause, indicating the presence of an external source for this molecule. the downward external flux of this molecule, most likely coming from the ring system, is 1013 m-2s-1. the upper atmospheric density and the relative abundances of saturn's major constituents will be presented. the unexpected complexities of saturn's mass spectrum at higher masses will also be discussed. | the composition and thermal structure of saturn's upper atmosphere from cassini/inms measurements |
high precision exoplanet light curves offer the opportunity for detailed characterization far beyond estimating the radius and orbital period of exoplanets and are now able to estimate parameters such as reflectivity and temperature. such details provide information about the planet's composition and potential habitability. currently, only very hot exoplanets have had their temperatures directly estimated and exoplanet heat maps are just beginning to be developed. one method of characterizing the temperature distribution of an exoplanet assumes that the exoplanet may be treated as having only two distinct temperatures, one for the dayside and one for the nightside. in such a model, each side of the exoplanet emits thermal radiation like a blackbody at a constant temperature. i seek to employ a new method of characterizing the thermal emissions of exoplanets by considering n temperature zones, as opposed to the much coarser dayside/nightside model. the zones make up a series of rings centered along the line connecting the center of the exoplanet to the center of its host star. each ring will be treated as a blackbody of constant temperature and the zone with the greatest temperature will be that closest to the host star. the new model will be used to characterize a set of hot exoplanets and a comparison of the new model to the former dayside/nightside model will be made to determine the precision necessary to differentiate the two models using light curve data. | thermal variations of extremely close-in exoplanets |
observations from the texas echelon cross echelle spectrograph (texes) on nasa"s irtf and the visir instrument on the vlt are used to characterize the saturn"s seasonal changes. radiative transfer modelling (using nemesis [8]) provides the northern hemisphere temperature progression of the atmosphere over 10 years, both during and beyond the cassini mission. comparisons between imaging observations taken one saturn year apart (1989-2018) show the extent of the interannual variability of saturn"s northern hemisphere climate for the first time.1. introductionwith the culmination of cassini's unprecedented 13-year exploration of the saturn system in september 2017, and with no future missions currently scheduled to visit the ringed world, the requirement to build upon cassini's discoveries now falls upon earth-based observatories. mid-infrared observations have been used to characterise features such as the extreme temperatures within an enormous storm system in 2011 [1,6], the cyclic variations in temperatures and winds associated with the 'quasi-periodic oscillation' (qpo) in the equatorial stratosphere [2] and the onset of a seasonal warm polar vortex over the northern summer pole [3].saturn's axial tilt of 27° subjects its atmosphere to seasonal shifts in insolation [4], the effects of which are most significant at the gas giant's poles. the north pole emerged from northern spring equinox in 2009 (planetocentric solar longitude ls=0°), and northern summer solstice in may 2017 (ls=90°), providing earth-based observers with their best visibility of the north polar region since 1987, with its warm central cyclone and long-lived hexagonal wave [5,6].studying these interconnected phenomena within saturn's atmosphere (particularly those that evolve with time in a cyclic fashion) requires regular temporal sampling throughout saturn's long 29.5-year orbit. we present here a showcase of research from the wealth of archived observations from both texes and visir obtained over the past decade.1.1 temperature progressionmethane (ch4) is used to determine stratospheric temperatures due to its even distribution across the planet, as well as its well understood emissive behavior. figure 1 shows the visible changes in the stratospheric and tropospheric conditions as seen by visir over 3 years; these images are representative of a range of filters between 7-20 µm, which can be stacked and inverted to derive the 3d temperature distribution in the upper troposphere and stratosphere. using this technique, we probe changes in the atmospheric 3d temperature distribution across the planet disc in visir observations taken from april 2008 (ls=343°) to july 2018 (ls=102°); thereby discerning the spatial variability as well as temporal. visir observations concurrent with the cassini/cirs observations will be used to cross-check the time-series from cassini, which can be extended beyond the end-of-mission with the newer visir observations. these profiles will provide a new measure of long-term temperature variability in the context of an established model.1.2 interannual variabilityspectroscopic maps of the northern summer hemisphere from texes instrument on the irtf collected in september 2018 have provided a unique opportunity, as they were acquired exactly one saturn year apart from the 1989 observations of gezari et al, (1989) [7], which were the first ever 2d images of saturn in the mid-ir. examining the differences in brightness temperatures and composition will indicate the extent of any interannual variation for saturn"s northern hemisphere. this study will also provide unique insight into the timescale of the qpo which will be contrasted with a previously suggested biennial cycle [2]. the seasonal temperature progression measured in section 1.1 also enables us to place this interannual variability in a wider context and provides further opportunity for insightful comparison with the comparatively shorter-term temperature variability.figure 1: visir observations from p95-102 sensing the troposphere (right) and stratosphere (left). polar warming is evident in the stratosphere; but is considerably smaller than that seen during southern summer and in the historical record of the 1980s. the warm polar hexagon is seen at the north pole, the first such observation from the ground. work to remove residual striping is ongoing and has been successfully applied to the 2017 images. 2015-16 images have been published by fletcher et al., 2017 [2].acknowledgementsthis research is funded by a european research council consolidated grant under the european union"s horizon 2020 research and innovation program, grant agreement 723890. we would like to thank co-author mael es-sayeh for his significant contribution to this research.references[1] fletcher et al., 2012, icarus 221, p560-586[2] fletcher et al., 2017, nature astronomy, 1, p765-770[3] fletcher et al. 2015, icarus. 251, 131-153[4] fletcher et al., 2015, https://arxiv.org/abs/1510.05690[5] fletcher et al., 2008, science. 319, 79-81[6] fouchet et al., 2016, icarus, 277, p196-214[7] gezari et al., 1989, nature, 342, 777-780[8] irwin et al. 2008, jqsrt 109:1136-1150[9] orton et al., 2008, nature 453, p198 | saturn's seasonal atmosphere: cassini cirs contrasts to vlt and irtf observations |
a strongly coupled unmagnetized rotating dust ring interacting through a yukawa potential has been observed in a laboratory rf-driven dusty plasma. radial confinement is provided by the plasma potential well that forms above a 70 mm (o.d.) groove with a center pedestal. it is well known that in magnetized plasmas the exb drift force often provides the driving force for rotation, however with no external magnetic field, the literature has yet to establish an easily identifiable driving force to over-come the drag forces present in the system. the angular velocity and angular momentum of various rings has been analyzed and possible driving forces for rotation are examined. | unmagnetized rotating plasma dust ring |
it is now known that many spiral density waves in saturn's c ring are forced by resonant gravitational interactions with the planet's nonradial oscillations. observations of their periods have the power to reveal new information concerning subtle aspects of the planet's internal structure. the properties of a certain class of nonradial oscillations, consisting of the inertial and rotational normal modes (i-modes and r-modes), are particularly sensitive to small departures of the internal density stratification from adiabatic, and may be especially useful in revealing such departures and their implications for any compositional gradients residing inside saturn. we calculate the frequencies and eigenfunctions (shapes) of the i-modes and r-modes for a given suite of internal structure models and determine the lindblad resonance locations and periods of the density waves they excite in the c ring. we find that for reasonable models of saturn's internal structure, the i-mode and r-mode frequencies overlap the range of frequencies of the slowest density waves observed in the c ring, that is, those having pattern speeds similar to saturn's rotation rate. in addition, the presence of observable density waves forced by r-modes requires saturn to possess (at least) two statically stable (non-convective) regions, one in the deep interior and the other in the outer molecular hydrogen envelope. the inner zone is most likely stabilized by a significant compositional gradient, the outer zone by a compositional gradient, by baroclinic processes, or both. unresolved questions remain. currently, the model predicts that if energy is equipartitioned among the i-modes, then some slow density waves should be detectable at radii inside ~84500 km in the c ring. none have been observed inside this radius. in addition, the source of excitation of the normal modes remains to be identified. we are currently exploring whether they can draw their energy from baroclinic instability in the outer stable zone. these questions will be addressed at length at the meeting. this work is supported by the nasa ssw program. | nonradial oscillations of saturn: forcing of the slowest spiral density waves in the c ring by i-modes and r-modes and the implications for the internal structure |
tree rings from europe and north america show reduced growth from ~536 to 555 a.d. if the reduction were solely due to high aerosol optical depth produced by sulfate from explosive volcanic eruptions, ice cores should record intense sulfate deposition ~ every 3 years, the stratospheric residence time of sulfate from one large eruption. although there were large, s-emitting eruptions in early 536 a.d. (high northern latitude) and in 540 a.d. (low latitude) [1] the rest of the time shows little evidence for large amounts of s emission. however, sulfate is not the only aerosol that obscures the sun and increases optical depth. aerosols composed of caco3, kaolinite and silica are all whitish and reflect sunlight. in samples from the gisp2 ice core covering 532 to 542 a.d. we found evidence for the injection of low latitude siliceous marine microfossils into the stratosphere at five age horizons: 3 at 535.4 a.d., 22 at 536.1 a.d., 46 at 538.6 a.d., 9 at 539.1 a.d. and 11 at 541.8 a.d. at the 535.4 a.d. horizon, we imaged 1695 particles of caco3. most are caco3 cleavage rhombs ranging from 0.1 to 10 μm in diameter. the largest pieces of caco3 resemble recrystallized nannoplankton such as coccoliths. the low latitude fossils and caco3 rhombs have two potential sources: 1) low latitude submarine eruptions that did not inject significant s into the stratosphere but did inject sediment [2], or 2) impacts of extraterrestrial bodies into low latitude continental shelves. the latter idea might explain the impossible (from astronomical calculations) eclipses, planetary transits or lunar sightings recorded by the chinese on dec. 534, feb. 15, 538, july 25, 538, feb. 2, 539, jan. 26, 548, from may 23-june 21, 548 and feb. 17, 550 (r. juhl, written comm.). these accounts may instead record close passes (or hits) of extraterrestrial bodies. both ideas could explain the stratospheric injection of aerosols needed to produce the intense solar dimming from march 536 to june 537 a.d. these ideas can be further tested by searching for low latitude diatoms, calcareous nannoplankton and kaolinite within high latitude, basic ash layers dating to the early 6th century. [1] sigl et al, 2015. nature 523, 543-549. [2] van eaton et al., 2013. high-flying diatoms: widespread dispersal of microorganisms in an explosive volcanic eruption. geology, 41, 1187-1190. | what are the sources of 91 low latitude fossils found in the gisp2 ice core and dating from 535 to 542 a.d.? |
the molecular composition, haze abundance, and thermal structure of saturn's upper atmosphere is determined by the solar insolation. as with any planet, the amount of sunlight impinging on the atmosphere is determined by both the planet's distance from the sun in addition to seasonal effects cause by the planet's axial tilt. the rings of saturn shadows regions of the planet for long periods of time while illuminating the opposite hemisphere. this research attempts to quantify the extent to which ring shadow and illumination from the rings impact atmospheric properties. where possible, we use spacecraft measurements to place constraints. | photochemistry and heating in saturn's atmosphere: ring shadow and ring reflection |
the saturn system possesses a number of small unique moons, including the coorbitals janus and epimetheus; the ring moons pan and daphnis; and prometheus, pandora, and atlas, which orbit near the edge of the main ring system. during the last phases of the cassini mission, when the spacecraft executed close passes to the f-ring of saturn, five "best-ever" flybys of these moons occurred. pan, daphnis, atlas, pandora, and epimetheus were approached at distances ranging from 6000-40,000 km. the visual infrared mapping spectrometer (vims) captured data from the spectral range spanning 0.35-5.1 microns, as well as capturing solar phase angles not observed before. when combined with spectra from different regions of the moons obtained throughout the mission, the vims observations reveal substantial changes in the depth of water-ice absorption bands and color over the moons' surfaces. these measurements show the accretion of main-ring material onto the moons, with leading sides exhibiting stronger water-ice signatures in general. atlas and pandora have red visible spectra similar to the a-ring and unlike other icy moons, which are blue, further revealing accretion of main ring material onto the small inner moons. in general the visible spectra of the moons gets bluer with distance from saturn until the surface of the moons is dominated by contamination from the e-ring, which is composed of fresh ice. there is a weak correlation between color and albedo, with lower-albedo moons being redder, suggesting the existence of a dark reddish contaminant from the main ring system. the solar phase curves of the moons are similar to those of larger icy moons (unfortunately no opposition surge data was gathered). 2017 california institute of technology. government sponsorship acknowledged. | five fabulous flybys of the small inner moons of saturn by the cassini spacecraft |
the surface of the moon was shaped by large-scale impact basins and 50-60 of such basins are identified with well-preserved morphologies. the formation of impact basins is influenced by target properties and most obviously by the size, composition (mass), and velocity of the impactor. beside the surface expression (crater morphology) as the most direct outcome of an impact event, also changes in the gravity field, local petrography and thermal structure are products of impact. in particular, high-resolution topography data, gathered by the lola, constrain the crater morphology and yield new estimates about the ejecta thickness; gravity data from the grail provide further insight and additional constraints on the impact cratering formation mechanism. in this study, orientale basin is used as benchmark for further systematic numerical modeling studies of all impact basins on the moon. with respect to the ejecta thickness distribution, based on morphologic data sets, a formation model of orientale predicts an impactor of 100 km in diameter and a velocity of 12 km/s. alternatively, johnson et al. (2016) focuses on the subsurface structure, especially on the location of orientale's rings and suggests a smaller impactor of 74 km. these two separate modeling approaches differ in terms of the assumed kinetic energy of the impactors: zhu et al. (2015) assume twice the kinetic energy for the impactor than johnson et al. (2016). a lower kinetic energy results in a smaller transient crater size and, thus, a smaller amount of ejected material. in other words, less kinetic impact energy results in a thinner ejecta blanket in the vicinity of the orientale basin. in this study, we aim at the development of a new model that satisfies both observational constraints, including the ejecta distribution and subsurface structure based on the gravity data. in addition, we quantify the deposition of ejecta and the production and distribution of impact melt. | formation of impact basins on the moon - insights from numerical modeling, gravity and remote sensing data |
photosynthesis has dominated the biological carbon cycle for 100s of millions of years. photorespiration, the process by which previously fixed glycine is carboxylated to co2 within the mitochondrion, is fundamental to all photosynthetic organisms and serves to decrease the overall efficiency of photosynthesis. increasing atmospheric co2 has long been known to diminish rates of photorespiration, and thus increase the net amount of carbon available for the construction of plant tissues. monitoring of changes in the stable carbon isotopic composition of atmospheric co2 at the iconic mauna loa observatory suggested a global decrease in photorespiration by the terrestrial biosphere (keeling et al., 2017, pnas). this work implicated increased carbon isotope discrimination under rising co2 to the diminished photorespiration by the terrestrial biosphere. growth chamber experiments confirmed the modeled effect of diminished photorespiration on carbon isotope value in c3 land plants, but a direct measure of photorespiration within the terrestrial biosphere is lacking. here we present a new global dataset of carbon isotope values measured on annually dated growth rings of trees growing from > 100 sites across the planet. these data reveal a significant increase in carbon isotope discrimination in forests worldwide, consistent in magnitude and trend with the predicted effect of 100+ years of co2 increase on photorespiration rates. we identify regions of the planet with lower and higher rates of change, and find these deviations to be consistent with changes in stomatal conductance and water use efficiency associated with regional climate change. this work provides a quantitative approach to understanding carbon isotope trends across the last 100+ years of changing climate and co2, by incorporating the effect of photorespiration on carbon isotope discrimination. | using tree rings to quantify 100 years of photorespiration decline |
the cassini inms data set gathered during the grand finale phase of the mission provides information about the atmospheric composition that can be used to infer effects in the interior of saturn. the he/h2 ratio measured in the upper atmosphere can be used to infer information about the he/h2 in the well mixed interior subject to uncertainties in our knowledge of the tropospheric and stratospheric thermal and turbulent mixing profiles. preliminary values are consistent with a jupiter-like he/h2 ratio. furthermore, the unexpectedly large influx (5000 to 40,000 kg per second) of methane, water, ammonia, carbon dioxide, and organic nanograins can have a dramatic effect on the carbon budget in the troposphere and stratosphere over time scales of 100 million years. these observations suggest that large ring capture events in past epochs may have a dramatic effect on the composition of the atmospheric veneer. | implications from cassini ion neutral mass spectrometer (inms) grand finale measurements on atmospheric and interior processes at saturn |
titan's lakes and seas are composed of predominantly methane and ethane [1], however thermodynamic modeling suggests that various other molecules exist in the lakes at lower abundances. some of these molecules include: acetylene, ethylene, benzene, and acetonitrile e.g., [2-5]. as the lakes evaporate, they may leave behind "bathtub rings" that are bright in the cassini vims 5 µm band [6]. experimental work [5, 7-10] has improved our understanding of titan's evaporite composition and has opened up a new class of "hydrocarbon minerals" [11] to be studied. using a custom-built titan chamber [12], we condense various combinations of liquid (methane and/or ethane) and solid (e.g., acetylene) hydrocarbons onto a dish (90 k) within the chamber where the compounds are constantly exposed to a 1.5 bar n2 atmosphere. we use ftir spectroscopy (nicolet 6700, 1-2.5 µm), mass measurements, and cameras to characterize spectral absorptions, band shifts, solubility values, and the morphology of the sample. here, we present a culmination of results from evaporite experiments associated with the dissertation research for this project. ethylene experiments [9] show that the evaporite forms more quickly in pure methane, the evaporite was identified by spectral band shifts, and our evaporation rates match those in the literature. acetylene:benzene co-crystal experiments [10] show that the co-crystal forms within minutes at 135 k, is stable down to 90 k, and was identified by several new spectral bands and sample morphology changes. acetonitrile experiments show that when combined with methane and allowed to evaporate, the residual spectrum is quite different from pure acetonitrile. these results can be applied to co-condensation processes in titan's atmosphere, and the ongoing effort to better characterize the composition of titan's evaporites. knowledge of these spectral and optical changes will be useful for future missions such as dragonfly, which can closely examine molecular minerals like co-crystals on titan's surface. this work was funded by the nessf grant #80nssc17k0603. 1: stofan et al. 2007 nature, 445, 61 ; 2: cordier et al. 2016 icarus, 270, 41 ; 3: singh et al. 2017 gca, 208, 86 ; 4: clark et al. 2010 jgr planets, 115, e10005 ; 5: cable et al. 2020 acs earth & space chem., in press ; 6: barnes et al. 2009a icarus, 201, 217 ; 7: cable et al. 2018 acs earth & space chem., 2, 366 ; 8: cable et al. 2019 acs earth & space chem., 3, 2808 ; 9: czaplinski et al. 2019 acs earth & space chem., 3, 2353 ; 10: czaplinski et al. 2020 psj, under revision ; 11: maynard-casely et al. 2018 am. mineralogist, 3, 343 ; 12: wasiak et al. 2013 adv. space res., 7, 1213 | experimental study of evaporites on titan |
we present and discuss the characteristics and the capabilities of the laboratory facility, cosmic, that was developed at nasa ames to generate, process and analyze interstellar, circumstellar and planetary analogs in the laboratory [1]. cosmic stands for “cosmic simulation chamber” and is dedicated to the study of neutral and ionized molecules and nano particles under the low temperature and high vacuum conditions that are required to simulate space environments. cosmic integrates a variety of state-of-the-art instruments that allow forming, processing and monitoring simulated space conditions for planetary, circumstellar and interstellar materials in the laboratory. cosmic is composed of a pulsed discharge nozzle (pdn) expansion that generates a plasma in free supersonic jet expansion coupled to two high-sensitivity, complementary in situ diagnostics: a cavity ring down spectroscopy (crds) and laser induced fluorescence (lif) systems for photonic detection and a reflectron time-of-flight mass spectrometer (retof-ms) for mass detection [2].recent laboratory astrophysics results that were obtained using cosmic will be presented, in particular the progress that has been achieved in the domain of the diffuse interstellar bands (dibs) and in monitoring, in the laboratory, the formation of dust grains and aerosols from their gas-phase molecular precursors in environments as varied as stellar/circumstellar outflows [3] and planetary atmospheres [4]. plans for future, next generation, laboratory experiments on cosmic molecules and grains in the growing field of laboratory astrophysics will also be addressed as well as the implications of the current studies for astronomy.references:[1] salama f., in organic matter in space, iau symposium 251, kwok & sandford eds.cambridge university press, vol. 4, s251, p. 357 (2008) and references therein.[2] ricketts c., contreras c., walker, r., salama f., int. j. mass spec, 300, 26 (2011)[3] cesar contreras and farid salama, the astrophys. j. suppl. ser., 208, 6 (2013)[4] sciamma-o'brien e., ricketts c., and salama f. icarus, 243, 325 (2014) | laboratory astrophysics studies with the cosmic facility: interstellar and planetary applications. |
understanding how astrophysical processes like diffusion, evaporation, and condensation of volatile ices affects the ratios of isotopes can help us better understand observations of isotopic variation in astrophysical systems. for example, the sublimation of water-ice from astrophysical and planetary surfaces such as interstellar dust, comets and asteroids at low temperatures is expected theoretically to result in significant shifts in the isotopic composition of the residual water ice. using a uhv system with closed-cycle he cryostat and a cavity ring-down isotope spectrometer (crd), we developed a protocol for measuring changes in the ratios of d/h, 18o/16o, and 17o/16o that result from the sublimation of water ice at 155 k. in this presentation, we discuss the challenges and lessons learned to achieving experimental conditions that mimic astrophysical environments. results from our experiments, including instantaneous fractionation factors associated with hydrogen and oxygen isotopes will be presented. the role that experimental conditions such as sample size and surface temperature and composition play will also be discussed. to assess whether the experimental conditions present in the uhv chamber are adequate for simulating sublimation into the near perfect vacuum of space, we developed a detailed model of the isotopic fluxes associated with the water-ice surface, vacuum chamber walls, and pumping provided by a magnetically levitated turbo pump. this model incorporates a transition-state-theory (tst) based representation of surface potential energies and associated isotopic fractionation factors. using this model, we estimated the magnitude of water recondensation onto the cold-surface and calculated the associated effects on the isotopic composition of the residual water ice compared to the theoretical values expected for pure sublimation. | mulit-isotopic fractionation of water from sublimation at low temperatures |
we show existence of energetic quasi-sunward o+ ion jets (~10 kev or higher) reflected below the martian bow shock by analyzing ion velocity distribution functions (vdfs) measured by the superthermal and thermal ion composition (static) instrument on the mars atmosphere and volatile evolution mission (maven). in the solar wind near mars, maven often observes energetic quasi-sunward o+ ion jets. in the magnetosheath near the o+ ion jets event, a partial ring distribution is sometimes seen in the o+ ion vdfs. the partial ring distribution is not on a ring distribution corresponding to newly-born pickup ions but on another ring distribution corresponding to pickup ions with an initial velocity (i.e. not newly-born pickup ions). we interpret that the partial ring distribution is produced by the reflection of the precipitating o+ ions below the bow shock. after being injected into the magnetosheath and sometimes subsequently into the induced magnetosphere from the solar wind, the precipitating o+ ions are subject to the deflection by fields in these regions, and consequently, a part of precipitating o+ ions are reflected back to the solar wind with the incident energy. we find that the ion reflection preferably occurs near the subsolar region where the velocity of the shocked solar wind becomes slow (~100 km/s) and the magnetic field becomes strong (~20 nt), making the gyro radius of the precipitating o+ ions (> 10 kev) comparable to the spatial scale of the magnetosheath and the induced magnetosphere (~2000 km on the dayside). we discuss the importance of the o+ ion jets in terms of understanding the oxygen escape processes at mars. | sunward o+ ion jets reflected below the martian bow shock: maven observations |
the cassini-huygens mission ended in a science-rich blaze of glory on september 15, 2017. covering a period of roughly ten months, the grand finale and ring grazing orbits marked the final phases of cassini's 13-year mission, ending with the first time in-situ exploration of the region between the rings and planet. on its final orbit, cassini plunged into saturn's atmosphere at 126,000 km/hr (35 km/sec), vaporizing and satisfying planetary protection requirements, while sending back its final bits of unique science data. ring grazing orbits. on november 29, 2016, a single titan flyby sent cassini into a series of 20 highly inclined, elliptical ring grazing orbits with peripases just outside saturn's main rings at distances within 10,000 km of saturn's f ring. these orbits provided high-resolution views of saturn's f and a rings, providing prime viewing conditions of fine scale ring structures such as propellers, and included the closest flybys of tiny ring moons. grand finale orbits. a final close flyby of titan on april 22, 2017 propelled cassini across saturn's main rings. in its 22 grand finale orbits, the spacecraft repeatedly dove between saturn's innermost rings and upper atmosphere to answer fundamental questions that were unattainable earlier in the mission. the grand finale was like a brand-new mission, exploring a region of the saturn system that was unexplored by any previous outer planet spacecraft.saturn's gravitational and magnetic fields were measured to unprecedented accuracy, providing information from which constraints on the interior structure of the planet, mass distribution in the rings, and the structure of the internal magnetic field could be obtained. the grand finale orbits provided the highest resolution observations ever of both saturn's c and d rings and saturn's atmospheric weather layer. direct in-situ sampling of the ring particle composition, upper atmosphere and the innermost radiation belts was also achieved.cassini's final orbit. during the final plunge into saturn, cassini became the first atmospheric probe with all of fields and particle instruments gathering data for as long as possible.this talk will present key science findings and new mysteries gleaned from the ring grazing and grand finale orbits. the research described in this paper was carried out in part at the jet propulsion laboratory, california institute of technology, under a contract with the national aeronautics and space administration. copyright 2018 california institute of technology. government sponsorship is acknowledged. | new discoveries in cassini's grand finale year |
the morphology and composition of star and inner disk also have an impact on the disk structure at larger radii. in fact, the shadows, spirals, rings that have been recurrently imaged at >5 au can be traced back to stellar mass and metallicity as well as to the presence of a misaligned inner belt within a disk cavity. we show the trends found over a large sample of objects and discuss the possibility that the inner disk may also regulate the planet formation by affecting the outer disk properties. | the outer disk knows about the inner disk |
we study the plasma interactions of galilean satellites by means of multi-species global hybrid simulations. we consider multi-species background plasma composed of oxygen and sulphur ions and multi-component neutral atmospheres. the neutral atmospheres via several ionization processes then represents a source of dense population of cold pick-up ions. we apply different background plasma properties (density, temperature, magnetic field magnitude and orientation) in order to cover the variability in conditions experienced by the satellites when located in different regions of the jovian plasma torus. we examine global structure of the interactions, formation of alfvén wings, development of plasma temperature anisotropies and corresponding kinetic instabilities, and the fine phenomena caused by the multi-specie nature of the local plasma. the results are compared with in situ measurements made by the galileo spacecraft. | global multi-species hybrid modelling of plasma interactions at galilean moons |
for cassini's grand finale, the spacecraft completed 22 orbits that passed between saturn and the d ring from north to south. prior to these orbits, it was expected that the ion neutral mass spectrometer (inms) would observe only h2 and he from saturn's atmosphere. instead, mass surveys early in the grand finale detected signal at heavy masses (including 15 u - methane; 28 u - co, n2, and c2h4; and 44 u - co2 and c3h8) with increasing mixing ratios above altitudes of 3000 km above the 1 bar level. these data are consistent with an influx of material from saturn's d ring (the innermost ring) to the upper atmosphere. current calculations suggest the total mass flux may be on the order of 104 kg s-1. the magnitude of this inflow process has important implications for the lifetime and evolution of the rings, as well as for the chemistry of saturn's upper atmosphere. our presentation will include an overview of this dataset, with an emphasis on constraints on the composition of inflowing ring material. data from the final five orbits, which reached the lowest altitudes below 1800 km, include signal across the full inms 1 to 99 u mass range. the ring material is comprised of methane and other organics (nearly 50 % by mass), as well as water, ammonia, nitrogen and/or carbon monoxide, and carbon dioxide. model fits to the data suggest a predominantly aliphatic composition for the organics, with an enrichment in c relative to solar abundances. longitudinal and/or temporal variations in the abundance of ring material may support a connection to the d68 ringlet. work to detect possible latitudinal variation in composition will also be presented. current analyses suggest that the final plunge, which terminated at 11 n latitude and did not cross the equatorial ring plane, is consistent with depletion of refractory species relative to the final five orbits, which had closest approach at 6 s latitude and crossed the ring plane. to determine whether the differences in these datasets are due to real compositional differences or instrumental effects, we are using the simulator for chemical reactions in the antechamber of a mass spectrometer (scram), which models the evolution of the population within the inms antechamber, including effects from adsorption and desorption. we will discuss modeling results and their implications for the ring influx composition. | cassini ion neutral mass spectrometer measurements of d ring influx to saturn's atmosphere |
the floors of large impact structures are largely flat and contain one or more morphological rings. the formation of the innermost topographic ring, the so-called peak ring, and the causes of target rock weakening leading to observed flat crater floors are not well understood. constraining these mechanisms is the prime structural geological objective of expedition 364 "drilling the k-pg impact crater", using the chicxulub impact structure, mexico, as a terrestrial analogue for the formation of planetary impact basins. a total of 829 meters of core was recovered from borehole m0077a drilled into the peak ring of the chicxulub crater. from bottom to top, the core is crudely composed of: (1) pervasively shocked granitoid target rock hosting meter- to decameter-thick impact melt rock and suevite dike-like bodies, (2) a 130 m thick impact melt rock and suevite unit overlying the target rocks, and (3) a 112 m thick section of post-impact pelagic carbonate rocks. based on visual appraisal of the drill core, we determined prominent impact-induced deformation structures in target rock pertaining to rock fluidization during cratering. in addition to microscopic planar structures formed by shock metamorphism, the target rocks are replete with impact-induced, mesoscopic planar deformation structures. these structures include: (1) cataclastic deformation zones, (2) striated shear faults, (3) crenulated mineral folia-tions, and (4) ductile shear band structures. structural overprinting criteria point to a relative age for these structures. zones of cataclasite are consistently displaced or utilized by shear faults. cataclasite bands in target rock fragments included in suevite are cut by the latter and a striated target rock fragment was found in impact melt rock. suevite and impact melt were emplaced in zones of dilation, often localized by shear faults. collectively, these observations suggest that cataclastic deformation was followed by shear faulting, followed in turn by emplacement of sue-vite and melt into dilation zones. this succession of deformation mechanisms is corroborated by the observation that suevite and impact melt bodies are devoid of cataclasite and shear faults. these lithologies were still viscous when they were deformed by ductile band structures. thus, the shear band structures formed after the shear faults. based on the structural overprinting rela-tionships, we attempt to relate the mesoscopic planar structures to cratering stages known from impact mechanics. | target-rock fluidization during peak-ring formation of the chicxulub crater inferred from expedition 364 drill core |
the innermost narrow ringlet in saturn's ring system is located around 67,630 km from saturn's center and is designated d68. observations of this ringlet early in the cassini mission showed that it was composed primarily of small particles and was roughly the same brightness all the way around the planet. however, images of this ring taken after 2015 revealed a series of clumps in this ringlet that were up to four times brighter than the background ring. over the course of the 18 months that they could be observed, these localized enhancements of material drifted and spread very slowly, indicating that all the visible material consisted of particles spanning a sub-kilometer range of semi-major axes. this material was probably released from the surfaces of multiple source bodies at least several meters wide orbiting within or close to this ringlet. these clumps may therefore provide new insights into how this narrow dusty ringlet is maintained, and might also be a source of the material seen by the in-situ instruments during cassini's grand finale. | what has been happening to saturn's innermost ringlet? |
seismology has been the premier tool of study for understanding the interior structure of the earth. the sun, and even other stars. in this thesis we develop the framework for the first ever seismic inversion of a rapidly rotating gas giant planet. we extensively test this framework to ensure that the inversions are robust and operate within a linear regime. this framework is then applied to saturn to solve for its interior density and sound speed profiles to better constrain its interior structure. this is done by incorporating observations of its mode frequencies derived from linblad and vertical resonances in saturn's c-ring. we find that although the accuracy of the inversions is mitigated by the limited number of observed modes, we find that saturn's core density must be at least 8.97 +/- 0.01 g cm-1 below r/rs = 0.34 and its sound speed must be greater than 54.09 +/- 0.01 km s-1 below r/rs = 0.23. these new constraints can aid the development of accurate equations of state and thus help determine the composition in saturn's core. in addition. we investigate mode excitation and whether the k -mechanism can excite modes on jupiter. while we find that the k-mechanism does not play a. role in jovian mode excitation, we discover a different opacity driven mechanism. the radiative suppression mechanism, that can excite modes in hot giant planets orbiting extremely close to their host stars if they receive a stellar flux greater than 109 erg cm-2 s-1. finally, we investigate whether moist convection is responsible for exciting jovian modes. mode driving can occur if, on average, one cloud column with a 1-km radius exists per 6423 km2 or if 43 storms with 200 columns, each with a radius of 25 km, erupt per day. while this seems unlikely given current observations, moist convection does have enough thermal energy to drive jovian oscillations, should it be available to them. | seismic inferences of gas giant planets: excitation & interiors |
in the final phase of the cassini mission in 2016-2017, the spacecraft will first move to orbits with periapse outside the f-ring and then to orbits grazing the upper atmosphere of saturn. these trajectories will provide the unique opportunity to sample the structures and composition of saturn's ionosphere but also those of the main rings. from an assessment of the neutral gas and plasma environment of the rings, we investigate the formation of molecular ions and nano-grains in the areas to be probed by cassini during its f-ring and proximal orbits | ion composition of the thermal plasma in the f-ring region of saturn |
we review the different key findings made by the cassini-huygens spacecraft that allowed to shed light on the formation conditions of saturn, its rings and surrounding satellite system. among them, cassini's measurements of the enceladus' plumes isotopic composition show that this satellite likely agglomerated from building blocks formed beyond 10 au in the protosolar nebula. the strong noble gas impoverishment measured in titan's atmosphere by the huygens probe also suggest that this satellite could have formed from solids initially produced in the protosolar nebula and that would have been partly devolatilized during their migration and accretion within saturn's circumplanetary disk. we also discuss the future missions that could help answer the remaining open questions regarding the origin of saturn's system. | origin of the saturnian system after the cassini-huygens mission |
dawn xm2 high resolution imaging of 92-km diameter occator crater on ceres at better than 10-m pixel scales provides an opportunity to test hypotheses based on earlier lamo mapping at 35 m scales. among these are the origins of the extensive lobate floor deposits (post-impact volcanism or impact "melt"), of bright carbonate-rich deposits (ballistic emplacement or lateral "flow") and tof the central pit/dome complex (laccolithic "pingo-like" inflation or extrusion). extensive stereo imaging is key for determining vertical relationships. the extensive floor deposits resemble impact melt on the moon on macro-scales but have features unique to ceres, mostly likely related to the distinct "wet" composition and rheology of the materials. flow features abound. lobate flows are perched on and over terrace blocks and across the crater floor. resistant-layer capped scarps, pit clusters, cryptic shallow ring structures, and sinuous rilles are common across plains that can be smooth, knobby or ridged. numerous small pits may make crater counting age determinations difficult. large mounds of enigmatic origin may be impact debris poking above the floor deposit, "pingo-like" uplifts, or both. cryptic shallow circular features of different sizes suggest localized withdrawal of or convection within or secondary impacts into a solidifying deposit. extensive post-impact deformation in the form of fractures and the bright deposits are also well mapped. the central dome is relatively simple morphologically. flow front or textural change at dome edge are not as yet recognized, suggesting laccolithic or "pingo" inflation might be more likely. very small pits formed on the crest of the dome indicate late venting. exposures in deep fresh post-impact fractures suggest that the bright carbonate deposits elsewhere are a few 10's of m thick at most. the carbonate deposits are complex, with conflicting indication of ballistic deposition and overflow of some ridges. xm2 data over occator are spectacular and while interpretations are still immature, initial analyses indicate that surface features on the floor deposits may be consistent with physical and geochemical evolution of a cooling and solidifying impact "melt/mud" sheet composed of hydrated silicates, carbonates and salt blended with water, followed by post-impact deformation. | occator crater at 35 km altitude: dawn xm2 mapping of a pristine impact basin on a hydrous dwarf planet |
charles beichman (ipac/nasa exoplanet science institute), natalie batalha (nasa ames research center), elodie choquet (jet propulsion laboratory), camilla danielski (observatoire de meudon), andras gaspar (university of arizona), tom greene (nasa ames research center), sasha hinkley (university of exeter), klaus hodapp (university of hawaii), david lafrenière (université de montreal), pierre-olivier lagage (cea, saclay), jarron leisenring (university of arizona), laurent pueyo (space telescope institute), everett schlawin (university of arizona), josh schlieder (nasa goddard research center), gene serabyn (jet propulsion laboratory) and marie ygouf (ipac/nasa exoplanet science institute)the large diameter, low thermal background and broad suite of instruments will make the james webb space telescope (jwst) the premier observatory for the characterization of exoplanets and, in some cases, for their discovery. transit spectroscopy covering the 0.6 to 28 um range at resolutions varying from a few to few thousand will revolutionize the study of the atmospheric composition and physical properties of planets ranging from hot jupiters to temperate super earths like those discovered orbiting the nearby m dwarf trappist-1. coronagraphic and spectroscopic observations will target young jupiter-mass planets to characterize their properties as well as to search for new planets with masses as low as that of saturn or even uranus. coronagraphic observations will also study the debris disks associated with planets, studying their morphology and composition as well as looking for planets which might be responsible for structures such as rings, gaps and spiral arms. in addition to highlighting the cycle 1 programs from the guaranteed time observers and early release science investigations, we will discuss some of the dramatic discoveries jwst might make as it achieves its full potential in the years after launch. | finding and characterizing exoplanets with the james webb space telescope (jwst) |
the cosmic facility was developed at nasa ames to study interstellar, circumstellar and planetary analogs in the laboratory [1, 2]. cosmic stands for “cosmic simulation chamber” and is dedicated to the study of molecules, ions and nanoparticles under the low temperature and high vacuum conditions that are required to simulate space environments. cosmic integrates a variety of instruments that allow generating; processing and monitoring simulated space conditions in the laboratory. it is composed of a pulsed discharge nozzle expansion that generates a plasma in a free supersonic jet expansion coupled to high-sensitivity, complementary in situ diagnostic tools, used for the detection and characterization of the species present in the expansion: a cavity ring down spectroscopy (crds) and fluorescence spectroscopy systems for photonic detection, and a reflectron time-of-flight mass spectrometer (retof-ms) for mass detection [3, 4].recent advances achieved in laboratory astrophysics using cosmic will be presented, in particular in the domain of the diffuse interstellar bands (dibs) [5, 6] and the monitoring, in the laboratory, of the formation of dust grains and aerosols from their gas-phase molecular precursors in environments as varied as circumstellar outflows [7] and planetary atmospheres [8, 9, 10]. plans for future laboratory experiments on cosmic molecules and grains in the growing field of laboratory astrophysics (nir-mir crds, laser induced fluorescence spectra of cosmic molecule analogs and the laser induced incandescence spectra of cosmic grain analogs) will also be addressed as well as the implications for astronomy.references: [1] salama f., proceed. iau s251, kwok & sandford eds. cup, 4, 357 (2008).[2] salama f., et al., proceed. iau s332, y. aikawa, m. cunningham, t. millar, eds., cup (2018)[3] biennier l., et al., j. chem. phys., 118, 7863 (2003)[4] ricketts c. et al. ijms, 300, 26 (2011)[5] salama f., et al., apj., 728, 154 (2011)[6] edibles consortium, a&a 606, a76 (2017)[7] contreras, c., salama, f., apj. suppl. ser., 208, 6 (2013)[8] sciamma-o'brien e., ricketts c., salama f. icarus, 243, 325 (2014)[9] sciamma-o'brien e., upton k.t., salama f. icarus, icarus, 289, 214 (2017)[10] raymond a.w., et al.. apj., 853, 107 (2018)the authors acknowledge nasa smd/apra and ssw programs. | nasa ames’ cosmic laboratory astrophysics facility: recent results and progress |
using alma we detect for the first time co gas in a debris disc around a solar-type star. we find the gas is of secondary origin and we probe the co/rock composition of exocomets in the disc, finding it matches with solar system comets. the dust continuum is resolved in an axisymmetric ring that that is consistent with grain size segregation due to radiation pressure and with a scale height<0.14. | alma: exocometary gas in the hd 181327 debris ring |
"snow lines", marking regions where abundant volatiles freeze out onto the surface of dust grains, play an important role for planet growth and bulk composition in protoplanetary disks. however, they can already be observed in the envelopes of the much younger, low-mass class 0 protostars that are still in their early phase of heavy accretion. the information on the sublimation regions of different kinds of ices can be used to understand the chemistry of the envelope, its temperature and density structure, and may even hint at the history of the accretion process. as part of the calypso large program, we have obtained observations of c18o, n2h+ and ch3oh towards the nearest low-luminosity class 0 protostars with the iram plateau de bure interferometer at sub-arcsecond resolution. we observe an anti-correlation of c18o and n2h+ in four of these sources, with n2h+ forming a ring (perturbed by the outflow) around the centrally peaked c18o emission. this reveals the co snow line in these protostellar envelopes with unprecedented resolution. in addition, we observe compact methanol emission towards three of the sources. we have modeled the emission using a chemical model coupled with a radiative transfer module, using the temperature and density profiles self-consistently determined by kristensen et al. ([4]). we find that for all four sources the co snow line appears further inwards than expected from the binding energy of pure co ices. this may hint at co being frozen out on h2o surfaces or in mixed ices. our observations can thereby yield clues on the widely unknown composition of interstellar ices, being the initial seeds of complex organic chemistry. | probing the methanol and co snow lines in young protostars |
the cassini radar's altimetry mode has been successfully used for probing the depth and composition of titan's hydrocarbons seas. in may 2013, during the spacecraft's 91stflyby of titan (t91), the instrument demonstrates its capabilities as a radar sounder, presenting a unique opportunity to constrain direct measurements of the depth and composition of titan's second largest sea, ligeia mare. later, observations of kraken mare and punga mare were planned and executed in august 2014 (t104) and january 2015 (t108), respectively. while most of the seafloor was not detected at kraken, suggesting the sea was either too deep or too absorptive in these areas to observe a return from the seafloor, shallow areas near moray sinus did return subsurface detections. at punga mare, a clear detection of the subsurface was observed with a maximum depth of 120 m along the interrogated track of the sea. we will present an analysis of all three altimetric observations of titan's mare, as well a re-analysis of altimetry data acquired over southern ontario lacus. depths measurements and liquid composition are obtained using a novel technique which makes use of radar simulations and monte carlo based inversions. finally, we will show that the estimates obtained from the direct measurements described above can be used along with the radar's active (i.e. synthetic aperture radar) and passive (radiometry) modes to generate bathymetry maps of areas not observed by altimetry. | an overview of the bathymetry and composition of titan's hydrocarbon seas from the cassini radar altimeter |
we develop a 2-d io torus physical chemistry model that combines latitudinal and radial variations assuming azimuthal symmetry. we model the distribution along the magnetic field and solve the radial fokker-planck equation to model the radial transport of the plasma out past the orbit of europa. our model includes ionization, recombination, coulomb collisions, charge exchange reactions, and energy loss due to radiation. we plan to couple our physical chemistry model to a neutral cloud model and include molecular chemistry. the extended neutral cloud model will use gravity, ionization, and charge exchange reactions to shape the cloud given an input of particle distributions from a local model of the interaction to form the extended neutral cloud. given the location and pointing of a spectral imager and our model of the density and temperature as a function of radius and latitude of the io plasma torus we can integrate the emissivity along the line of sight to simulate a uv or visible spectrum using the chianti atomic database. we compare model output with remote and in situ observations of the io plasma torus to determine spatial and temporal variability of composition as well as the flow of mass and energy through the jovian magnetosphere. | a 2d io plasma torus physical chemistry model and comparisons with remote and in situ observations |
chiron is an active centaur object of radius ~110 km orbiting between saturn and uranus. its size and orbital elements are similar to those of the largest centaur known to date, chariklo (radius~120 km). in 2013, a stellar occultation revealed the surprising presence of two narrow and dense rings around the latter body (braga-ribas f. et al., nature, 2 april 2014), showing that rings are not an exclusivity of the giant planets and may be a more common feature than previously thought. a stellar occultation by chiron observed in 2011 actually revealed the presence of sharp features that could be caused by a shell of material or cometary jets around chiron (ruprecht et al. 2015), a conclusion supported by the fact that chiron (contrarily to chariklo) does exhibit a cometary-like activity. conversely, analyzing results from three stellar occultations (in 1993, 1994, 2011), ortiz et al. (2015) show that the detections of secondary events could be explained by the presence of a dense and narrow rings orbiting at about 325 km from chiron's center. our goals here are to (1) search for jets and faint material around chiron (at more than 2,000 km from the central body), (2) search for faint satellites, (3) constrain the presence of close-in ring structures, and (4) get multi-wavelength photometry to constrain the material composition. this would help us to assess how unique chariklo's rings are, and to see whether the material surrounding both objects has something to do with a cometary activity. | search for material around chiron |
the gemini planet imager (gpi) is a near-infrared imaging instrument used with the gemini south telescope in chile to provide direct imaging and integral field spectroscopy of exoplanetary systems. one of the central goals of the gpi mission is to gain an improved understanding of debris disks, which are remnants of the planet formation process. in this investigation, we present gpi polarimetric observations of the debris disk around the star hd 32297. previous imaging of the system revealed a nearly edge-on disk that extends into a fan-shaped nebulosity on scales of hundreds of au. the exquisite quality of total intensity gpi observations, which focus on the inner 150 au of the disk system, allows us to precisely establish the disk geometry, including evidence for eccentricity in the parent body ring as seen in other such disks. furthermore, taking advantage of the polarimetric capabilities of gpi, we measure the linear polarization fraction induced by dust scattering. we combine these observations with radiative transfer modeling based on the mcfost code and using a markov chain monte carlo (mcmc) method for a thorough and efficient exploration of the parameter space. by simultaneously modeling the gpi scattered light total intensity image, the corresponding polarization fraction map, and the spectral energy distribution of the system, we place constraints on the composition and density structure of the disk. this case study will contribute to an improved understanding of debris disks for the purpose of characterizing planetary system formation and evolution. | the gemini planet imager view of the hd 32297 debris disk system |
the obliquity, or angular separation between spin and orbit poles, of asteroid (16) psyche is currently 95 degrees. we are interested in knowing how much that angular separation varies, on time scales of 104 to 106 years. to answer that question, we have done several related analyses. on short time scales, the orbital element variations of psyche are dominated by perturbations from jupiter. jupiter's dominance has two basic causes: first is the large mass and relatively close position of jupiter, and second is a 19:8 mean motion resonance. jupiter completes 8 orbits in 94.9009 years, while psyche takes 94.9107 years to complete 19 orbits. as a result of this, all of the orbital elements of psyche exhibit significant periodic variations, with a 94.9 year period dominating. there are also significant variations at the synodic period, which is 8.628 years, or 1/11 of the resonant period. over a 1000 year time span, centered on the present, the eccentricity varies from 0.133 to 0.140, and the inclination varies from 2.961 to 3.229 degrees. on longer time scales, the orbital elements of psyche vary considerably more than that, due to secular perturbations from the planets. the secular variations are modeled as the response of interacting mass rings, rather than point masses. again, jupiter is the main perturbing influence on psyche. the eccentricity and inclination both oscillate, with dominant periods of 18.667 kyr. the range of values seen over a million year time span, is 0.057 to 0.147 for eccentricity, and 0.384 to 4.777 degrees for inclination. using a recent shape model, and assumption of uniform density, to constrain relevant moments of inertia, we estimate the spin pole precession rate parameter to be 8.53 arcsec/year. the current spin pole is at ecliptic {lon, lat} = { 32, -7} deg, whereas the orbit pole is at {lon, lat} = {60.47, 86.91} deg. the current obliquity is thus 94.3 degree. using nominal values of the input parameters, the recovered spin pole trajectory is such that, over a million year time span, centered on the present, the minimum and maximum values of obliquity are 92.36 and 98.56 deg. the obliquity oscillates with dominant periods of 18.45 and 48.40 kyr. | secular orbit and spin variations of asteroid (16) psyche |
v582 mon (kh 15d) is a binary t tauri star, composed of a k1 and k7 star embedded in a circumbinary ring that is inclined to the binary orbit. the age of the stars is 1-3 myr and their total mass is about 1.3 solar masses. physics of the precession indicates that the center of the ring is at about 3.8 au from the center of mass of the binary and the half-width of the ring is of order 2 au. we are just entering the important phase of evolution of this system in which star b is completely unobscured during at least part of each orbital cycle. a wealth of photometric and spectroscopic data on the system is reviewed and analyzed, primarily addressing two questions: 1) what is the size and composition of the grains composing the ring? and 2) is there evidence for a self-luminous planet having already formed in the disk beyond the ring? | an update on the v582 mon (kh 15d) binary t tauri system |
we present and discuss the characteristics and the capabilities of the laboratory facility, cosmic, that was developed at nasa ames to generate, process and analyze interstellar, circumstellar and planetary analogs in the laboratory [1]. cosmic stands for “cosmic simulation chamber” and is dedicated to the study of neutral and ionized molecules and nano particles under the low temperature and high vacuum conditions that are required to simulate space environments. cosmic integrates a variety of state-of-the-art instruments that allow forming, processing and monitoring simulated space conditions for planetary, circumstellar and interstellar materials in the laboratory. cosmic is composed of a pulsed discharge nozzle (pdn) expansion that generates a plasma in free supersonic jet expansion coupled to two high-sensitivity, complementary in situ diagnostics: a cavity ring down spectroscopy (crds) and laser induced fluorescence (lif) systems for photonic detection and a reflectron time-of-flight mass spectrometer (retof-ms) for mass detection [2].recent laboratory results that were obtained using cosmic will be presented, in particular the progress that has been achieved in the domain of the diffuse interstellar bands (dibs) [3] and in monitoring, in the laboratory, the formation of dust grains and aerosols from their gas-phase molecular precursors in environments as varied as stellar/circumstellar outflows [4] and planetary atmospheres [5]. plans for future, next generation, laboratory experiments on cosmic molecules and grains in the growing field of laboratory astrophysics will also be addressed as well as the implications of the current studies for astronomy.references: [1] salama f., in organic matter in space, iau symposium 251, kwok & sandford eds.cambridge university press, vol. 4, s251, p. 357 (2008) and references therein.[2] ricketts c., contreras c., walker, r., salama f., int. j. mass spec, 300, 26 (2011)[3] salama f., galazutdinov g., krelowski j., biennier l., beletsky y., in-ok song, the astrophys. j., 728, 154 (2011)[4] cesar contreras and farid salama, the astrophys. j. suppl. ser., 208, 6 (2013)[5] sciamma-o'brien e., ricketts c., and salama f. icarus, 243, 325 (2014)acknowledgements: the authors acknowledge the support of nasa smd. | recent progresses in laboratory astrophysics with ames’ cosmic facility |
cassini observations have found that the plasma and magnetic field conditions upstream of titan are far more complex than they were thought to be after the voyager encounter. rymer et al., (2009) used the cassini plasma spectrometer (caps) electron observations to classify the plasma conditions along titan's orbit into 5 types (plasma sheet, lobe, mixed, magnetosheath and misc.). nemeth et al., (2011) found that the caps ion observations could also be separated into the same plasma regions as defined by rymer et al. additionally the t-96 encounter found titan in the solar wind adding a sixth classification. understanding the effects of the variable upstream plasma conditions on titan's plasma interaction and the evolution of titan's ionosphere/atmosphere is one of the main objectives of the cassini mission. to compliment the mission we perform hybrid simulations of titan's plasma interaction to examine how the properties of the incident plasma (composition, density, temperature etc…) affect titan's ionosphere. we examine how much ionospheric plasma is lost from titan as well as the amount of mass and energy deposited into titan's atmosphere. | the effects of upstream plasma properties on titan's ionosphere |
"rbspice in the classroom: changing magnetic fields and electrical currents" is a hands-on exercise for middle school and high school science classrooms. students build a ballistic galvanometer using inexpensive common items that can be purchased at any craft store, and make qualitative observations of changing magnetic fields and the electrical currents they create. the goal of this work is to provide teachers new materials to use in their classrooms as tools for teaching students about electricity and magnetism. the experiment relates our earth as a planet to the role the magnetosphere plays in protecting us from space weather. the experiments show the ways in which van allen probes play an important part in exploring those relationships using such instruments as the radiation belt storm probes ion composition experiment (rbspice). the exercise is a vehicle for discussing electromagnetic induction, the behavior of the earth's magnetosphere coupled with storm-time conditions that produce the earth's ring current, and the mission objectives of the van allen probes rbspice instrument. | rbspice in the classroom: building a ballistic galvanometer using common household products |
tobias chant owen (toby) was a graduate student of g. p. kuiper, receiving his ph.d. in the dept. of astronomy, university of arizona, in 1965. his thesis was broadly titled "studies of planetary spectra in the photographic infrared", and primarily presented a study of the composition and other properties of jupiter, as well as the abundance and surface pressure of co2 on mars. the surface pressure on mars was a topic of debate at that time, with a wide range of diverse observational results from several investigators. the jupiter work in particular consisted of the analysis of kuiper's unpublished spectra that were made with photographic plates pushed to the longest wavelength possible, about 1120 nm, with ammonia-hypersensitized kodak z emulsions. toby used the long-pathlength absorption cells at the lunar and planetary lab to study the spectra of ch4 and nh3 at pressures and temperatures relevant to jupiter (and saturn), as well as to search for spectral signatures of potential minor components of their atmospheres. toby also obtained new spectra of io, ganymede, and saturn and its rings, extended to the long-wavelength limit of photographic emulsions. no new molecular absorptions were found, although owen basically confirmed kuiper's earlier result that saturn's rings are covered (or composed of) with h2o ice or frost. as he pursued a broad range of problems of planetary atmospheres, toby used existing and newly acquired spectra of the planets in the photographic and near-infrared wavelength regions, together with data he obtained in the laboratory with long-pathlength absorption cells, to resolve some outstanding issues of unidentified spectral features and to clarify issues of the compositions, temperatures, and atmospheric pressures of several bodies. this work laid the foundation for his later decades of studies of planetary atmospheres and comets with spacecraft as an active participant in many us and european missions. he was very influential in shaping the science goals of several missions, and in the interpretation of the results. | the early planetary research of tobias c. owen |
evaluating the potential habitability of the icy ocean worlds of the outer planets requires an understanding of three main components, the chemistry and composition of the material making up the body, the sources of chemical disequilibrium available for potential metabolic processes, and the geology that drives the interaction between the surface and the deeper interior of the body. for europa, we have focused on the latter area, creating a global geologic map at the scale of 1:15m. the geologic units can be divide into four categories: (1) crater materials and subunits of continuous crater ejecta and discontinuous crater ejecta—including the primary impact crater and its local deposits and farther ranging ejecta material; (2) various morphological types of chaos materials identified as high albedo, mottled, low albedo, and knobby chaos—disrupted terrains; (3) general band forming material and high albedo bands—linear to curvilinear zones with a distinct, abrupt albedo change from the surrounding region; and (4) ridged plains—distributed across all latitudes and characterized by subparallel to cross-cutting ridges and troughs. in addition, a number of structural features are identified including: depression margins, troughs, multi-ring structures, cycloids, band linea, ridges, undifferentiated linea, and microchaos. the distribution of microchaos is not uniform across the surface, showing a general association with band forming materials. concentrations might correspond to areas of crustal weakness or thinning. upon completion of the global geologic map, we identified seven distinct regional scale images/mosaics (100's of m/pixel) covering 10% of the surface that allow greater characterization of each global unit. preliminary assessment of the conamara chaos and part of the manannan regions show that the ridged plains contain a greater abundance of band units that can be used as distinct stratigraphic markers. in addition, many of the outcrops of microchaos are surrounded by a halo of smooth, dark material that appears to embay the surrounding terrain, providing greater insight into potential cryomagmatic processes. in this presentation we will discuss the link between the global and regional-scale geologic units, their distribution across europa, and implications for future europa exploration. | gaining insight into surface geologic processes on europa through geologic mapping at global and regional scales |
planets form out of disks around young stars that are composed of dust and gas. the dust in these disks grows and forms planetesimals that go on to become rocky planets or cores which accrete disk gas, creating giant planets. as planets form, they create a footprint in the shape of ring-like gaps in the protoplanetary disk by accumulating and clearing out the material around themselves. theoretical models of disk evolution are central to understanding planet formation, but we have not yet fully exploited observations of protoplanetary disks to test these prevailing models. we will bridge this gap between theoretical models of disk evolution and observable properties of protoplanetary disks by using data modeling to extract the most detailed observational constraints to date. we will measure dust grain growth and settling, two critical steps in planetesimal formation, disk masses, which set the reservoir for planet formation, and disk clearing, which may be linked to planet-disk interactions. we will use our existing model of irradiated, accretion disks to perform the largest, detailed modeling effort to date of spectral energy distributions (seds) of protoplanetary disks around young, low-mass pre-main sequence stars, the precursors of our own sun. our sample includes 430 disks around stars ranging in age from ~1 to 10 myr. seds will be compiled from archival nasa datasets (2mass, spitzer, wise, iras, and herschel) and supplemented by archival millimeter data. there are no surveys planned in the foreseeable future that will significantly improve the sed coverage. therefore, these archival data are ripe for harvesting the embedded science using our disk modeling tools. by the end of this study, we will have observational constraints on disk dust grain growth, settling, masses, and clearing which will lead us to a far more complete and quantitative view of disk evolution in the first few million years after a star is formed. this will bring us a few steps closer to understanding the origin of our own solar system and help set the stage for future studies with alma, jwst, and the next generation of space-based and 30-m class ground-based telescopes. | multiwavelength characterization of planet formation environments |
the ice giant planets, uranus and neptune, are priority targets for future exploration because they challenge our understanding of how planets form and evolve, unique physical properties are on display in their rings, satellites, and magnetospheres, and because ice giants are common around other stars. nasa and esa recently undertook a mission study whose complete report is available at https://www.lpi.usra.edu/icegiants/mission_study/.this study identifies the two highest-priority science objectives as * characterize the planet's interior. * determine the planet's bulk composition, including noble gases and isotopic ratios.ten additional objectives are * improve knowledge of the dynamo. * determine the atmospheric heat balance. * measure the tropospheric 3-d flow. * characterize the structures and temporal changes in the rings. * obtain an inventory of moons. * determine the surface composition of rings and moons. * map the shape and geology of satellites. * determine the density and internal structure of satellites. * characterize triton's atmosphere. * investigate solar wind-magnetosphere-ionosphere interactions.the study finds that uranus and neptune are equally compelling science targets. each planet has things to teach us which the other cannot, so ultimately both systems must be explored.the study investigated many mission architectures and finds that achieving these objectives would require a well instrumented orbiter making multiple flybys of each major satellite and dropping an atmospheric probe into the planet. the study does not recommend a specific payload, but discusses a range of possible instruments and suggests an orbiter payload of 150 kg is needed. the information in this abstract is predecisional and is provided for planning and discussion purposes only. | a study on exploring uranus and neptune: science objectives and mission requirements |
the lower mantle makes up more than a half of our planet's volume. mineralogical and petrological experiments on realistic bulk compositions under high pressure-temperature (p-t) conditions are essential for understanding deep mantle processes. such high p-t experiments are commonly conducted in a laser-heated diamond anvil cell, producing a multiphase assemblage consisting of 100 nm to submicron crystallite grains. the structures of these lower mantle phases often cannot be preserved upon pressure quenching; thus, in situ characterization is needed. the x-ray diffraction (xrd) pattern of such a multiphase assemblage usually displays a mixture of diffraction spots and rings as a result of the coarse grain size relative to the small x-ray beam size (3-5 micron) available at the synchrotron facilities. severe peak overlapping from multiple phases renders the powder xrd method inadequate for indexing new phases and minor phases. consequently, structure determination of new phases in a high p-t multiphase assemblage has been extremely difficult using conventional xrd techniques. our recent development of multigrain xrd in high-pressure research has enabled the indexation of hundreds of individual crystallite grains simultaneously through the determination of crystallographic orientations for these individual grains. once indexation is achieved, each grain can be treated as a single crystal. the combined crystallographic information from individual grains can be used to determine the crystal structures of new phases and minor phases simultaneously in a multiphase system. combining in-situ synchrotron x-ray diffraction and ex-situ transmission electron microscope techniques, we are able to explore phase changes and element partitioning in multi-component systems that represent the realistic deep lower mantle under the high p-t conditions. we will explain key challenges in studying multiphase systems and demonstrates the unique capabilities of high-pressure multigrain xrd through successful examples of its applications. | development of high-pressure multigrain x-ray diffraction forexploring the earth's interior |
the historic encounters of the new horizons spacecraft with the pluto system in 2015 and arrokoth in 2019 have yielded a wealth of new information about the physical and chemical makeup of these remarkably diverse members of the kuiper belt. i review the properties of both systems and examine their formation within the framework of the author's modern laplacian theory of solar system origin (prentice 1978 moon \& planets $\bf19$ 341). pluto's largest moon charon (c) is about half the physical size of pluto (p). the other moons are much smaller. their surfaces consist mostly of pure crystalline water ice (cook et al. 2018). this fact is consistent with these bodies once having watery mantles. the normalized angular momentum density of the p-c binary is 0.389 $\pm$ 0.004, combining the data of brozovic et al. (2015) and stern et al. (2015). this matches the value 0.3896 expected for a rotational fission origin (darwin 1910, lyttleton 1953). on this basis, prentice (2015, 46$^{th}$ lpsc id. 2664) successfully predicted that pluto's moons were once liquid worlds and that their surfaces to 'be very flat and smooth and consist solely of water ice'. here i show that the observed mean densities of p \& c are consistent with the fission of a single body (proto-pluto), if the bulk chemical composition of that body matches that of the condensate that forms in the outermost gas ring of the protosun (psc) shed at initial mean orbital distance of 35.6 au. this orbital radius translates to a present-day value of 43.2 au, allowing for secular expansion due to the psc's mass loss during gravitational contraction in order to achieve solar size. the compositional mass fractions are dry rock \& graphite [0.5418], h$_{2}$o ice [0.1858], co$_{2}$ ice [0.2211] and ch$_{4}$ ice [0.0513] (prentice 2016 dps \#48 id. 224.13). arrokoth (a) accreted as a single rapidly-spinning body within the same gas ring as proto-pluto. the accretion of particles in gas rings is a swift and efficient process due to the mean-orbit focussing property of the rings (prentice 2018 dps \#50 id. 113.03). heating due to the radioactive decay of $^{26}$al soon vaporized all ch$_{4}$ ice within proto-a. this rose to the surface to form a solid outer shell. next, co$_{2}$ ice sublimes but the rising vapour is hindered from escape because of the ch$_{4}$ shell. pressure build-up not only may have rendered the surface of proto-a 'globally smooth and crater free' (prentice 2019 aas 233$^{rd}$ meeting id. 467.01), as new horizons has now found arrokoth almost to be, but it could also have cleaved proto-a in two in its equatorial plane. the two parts then drifted apart under the action of centrifugal force on a cushion of gas until halted by solid drag, so forming the present-day, slowly-spinning bi-lobate object (doi: 10.1002/essoar.10501732.1). | a unified model for the formation of bi-lobate arrokoth and the pluto-charon binary |
analyses of data from cassini's ion and neutral mass spectrometer (inms) taken during several close flybys of enceladus suggest the presence of a potentially habitable ocean underneath the ice shell [1,2]. proper identification of the molecular species sampled from enceladus' plumes by inms is of utmost importance for characterizing the ocean's chemical composition. data from cassini's cosmic dust analyzer (cda) and visible and infrared mapping spectrometer (vims) have provided clues for possible plume chemistry, but further analysis of the inms data is necessary [3,4]. here we present a novel automated algorithm for comparing inms spectra and analogue laboratory spectra to a vast library of sample spectra provided by the national institute of standards and technology (nist). the algorithm implements a monte carlo simulation that computes the angular similarity between the spectrum of interest and a random sample of synthetic spectra generated at arbitrary mixing ratios of molecular species. the synthetic spectra with the highest similarity scores are then averaged to produce a convergent estimate of the mixing ratio of the spectrum of interest. here we will discuss the application of this technique to inms and laboratory data and the implication of our preliminary results for the ocean chemistry and habitability of enceladus. 1. waite, j., et al., 2009. liquid water on enceladus from observations of ammonia and 40ar in the plume. nature 460, 487-498. 2. waite, j., et al. 2017. cassini finds molecular hydrogen in the enceladus plume: evidence for hydrothermal processes. science 356, 155-159. 3. postberg, f., et al., 2008. the e ring in the vicinity of enceladus ii: signatures of enceladus in the elemental composition of e-ring particles. icarus 193(2), 438-454. 4. brown, r., et al., 2006. composition and physical properties of enceladus' surface. science 311, 1425-1428. | quantitative chemical analysis of enceladus' plume composition. |
the deep geological and geophysical studies of the continents and oceans have revealed a number of well-defined new regularities in the structure of the crust and upper mantle that do not find a clear explanation in the modern geodynamic concepts. - the earth is divided into two hemispheres with different structure of the lithosphere: the pacific hemisphere with the predominance of the thin crust ("oceanic hemisphere") and the opposite indo-atlantic hemisphere, where all the continents are situated ("continental hemisphere"). - the system of the mid-ocean ridges with approximately equal distances between them, 90°, is symmetrical relatively to the south pole. - the deepwater drilling showed that the crust in the oceans is different in age and composition, it was identified from the remnants of an ancient (archaean) crust and large areas of subcontinental crust. there are also some fragments of the ancient continental lithosphere. - the continents are characterized by the large thickness of the lithosphere (more than 200 km), composed of the lower density depleted matter. - experimental data on petro-physical properties of the lithosphere matter at high pressure and temperature show a large role of deep energy-intensive fluids in the formation of the lithosphere, its composition and physical properties. the laboratory data show an important role of the deep fluids in the formation of the sialic crust and depleted mantle rocks. a new model of the global tectonics explaining these data is proposed. the irregular advection of the deep fluids was the main factor of the different crustal type formation: the continental crust was formed in the regions with intensive deep fluids flows, whereas in the areas of weak flows (mainly, in the pacific hemisphere) the subcontinental and oceanic crusts were developed. the oceanic crust of another origin was formed in the mid-oceanic ridge arias. this crust is characterized by the magnetic field with the linear anomalies. the deep fluids provided also the depletion of the mantle matter and formation of the thick lithosphere with the lower density. growth of the lower density lithosphere led to its emergence in respect to the oceanic lithosphere and to the formation of the continents. two hemispheres with different lithosphere structure were formed due to the elliptical form of the planet orbit, causing periodic changes of the planet accelerations. the planetary formation of the hemispheres explains the correct form of the tectonically active pacific ring. the structural symmetry of the global rift system relative to the planet poles may be explained by the expansion of the planet that resulted in the spreading of the continental lithosphere and formation of the mid-oceanic ridges. the regular form of the pacific ring and the mid-oceanic ridges means that during the geological history there were no large lithosphere plates shifting, there was only spreading that was a result of the earth expansion. thus the main energy sources of the global tectonics were the deep fluids advection and the planet expansion. | unsolved problems of the global tectonics and a possibility of their solution |
we describe the discovery of a bright, young kuiper belt-like debris disk around hd 115600, a ∼ 1.4--1.5 m_{⊙}, ∼ 15 myr old member of the sco-cen ob association. our h-band coronagraphy/integral field spectroscopy from the gemini planet imager shows the ring has a (luminosity scaled) semi major axis of (∼ 22 au) ∼ 48 au, similar to the current kuiper belt. the disk appears to have neutral scattering dust, is eccentric (e ∼ 0.1--0.2), and could be sculpted by analogues to the outer solar system planets. spectroscopy of the disk ansae reveal a slightly blue to gray disk color, consistent with major kuiper belt chemical constituents, where water-ice is a very plausible dominant constituent. besides being the first object discovered with the next generation of extreme adaptive optics systems (i.e. scexao, gpi, sphere), hd 115600's debris ring and planetary system provides a key reference point for the early evolution of the solar system, the structure and composition of the kuiper belt, and the interaction between debris disks and planets. | extreme exoplanet direct imaging: new results with gpi and scexao and the path to imaging another earth |
the new horizons flyby of mu69 has yielded a wealth of new data on the physical structure and surface chemistry of this cold classical kbo (cckbo). one surprising discovery is the flattened shape of each lobe. another is the low spin rate. this means that the lobes must have lost orbital angular momentum, prior to merging (stern et al 2019, science 364 649). importantly, i note here that the mu69 ang. mom. density equals that of a single globe of period ~5.8 hrs. this matches the mean of single-lobed kbos (benecchi & sheppard 2013). i now assume that initially mu69 was a rough spheroid of mean diameter 21 km. i suggest that the cckbos accreted from a torus of co-orbiting condensate grains that settled gravitationally on to the mean circular orbit of the 1st gas ring shed by the contracting protosolar cloud (prentice 2018, dps #50 id.113-03; 2019, aas #233 id.467.01). the mean orbital radius, temperature and mean-orbit pressure of this gas ring are 35.6 au, 26.3 k and 1.27 x 10-9 bar. the bulk chemical composition of the condensate is nearly-dry rock [mass fraction: 0.5269], graphite [0.0163], h2o ice [0.1845], co2 ice [0.2210] and ch4 ice [0.0513]. its mass density is 1.73 g/cm3. a thermal evolution model for proto-mu69 is constructed assuming that all heat is derived from the decay of 26al. an initial temperature of 35 k is chosen. by 24,000 yr, ~50% of the inner ch4 ice mass has both melted and vaporized. it is proposed that although ch4 vapour rises to the surface, much of it recondenses before escaping into space, so forming an outer solid shell of thickness ~0.6 km. at time ~60,000 yr, central sublimation of co2 ice begins. within a further 4400 yr, all co2 ice out to the 0.5 radius point has sublimed. this gas is prevented from escaping because of the outer ch4 shell. i conjecture that gas pressure build-up cleaved mu69 into two in its equatorial plane. centrifugal force then caused these 'halves' to drift apart until stopped by the drag of the frozen shell. | can the sublimation of dry ice explain the flattened, bilobate structure and spin rate of 2014 mu69 |
the origin and evolution of saturn's rings is critical to understanding the saturnian system as a whole. here, we discuss the physical and chemical composition of the rings, as a foundation for evolutionary models described in subsequent chapters. we review the physical characteristics of the main rings, and summarize current constraints on their chemical composition. radial trends are observed in temperature and to a limited extent in particle size distribution, with the c ring exhibiting higher temperatures and a larger population of small particles. the c ring also shows evidence for the greatest abundance of silicate material, perhaps indicative of formation from a rocky body. the c ring and cassini division have lower optical depths than the a and b rings, which contributes to the higher abundance of the exogenous neutral absorber in these regions. overall, the main ring composition is strongly dominated by water ice, with minor silicate, uv absorber, and neutral absorber components. sampling of the innermost d ring during cassini's grand finale provides a new set of in situ constraints on the ring composition, and we explore ongoing work to understand the linkages between the main rings and the d ring. the d ring material is organic- and silicate-rich and water-poor relative to the main rings, with a large population of small grains. this composition may be explained in part by volatile losses in the d ring, and current constraints suggest some degree of fractionation rather than sampling of the bulk d ring material. | the composition of saturn's rings |
extreme adaptive optics instruments have revealed exquisite details on debris discs, allowing to extract the optical properties of the dust particles such as the phase function, the degree of polarisation and the spectral reflectance. these are three powerful diagnostic tools to understand the physical properties of the dust : the size, shape and composition of the dust particles. this can inform us on the population of parent bodies, also called planetesimals, which generate those particles through collisions. it is however very rare to be able to combine all those three observables for the same system, as this requires different high-contrast imaging techniques to suppress the starlight and reveal the faint scattered light emission from the dust. due to its brightness, the ring detected around the a-type star \hr{} is a notable exception, with both unpolarised and polarised images covering near-infrared wavelengths. here, we show how measurements of dust particles in the laboratory can reproduce the observed near-infrared photo-polarimetric properties of the \hr{} disc. experimental characterisation of dust allows to bypass the current limitations of dust models to reproduce simultaneously the phase function, the degree of polarisation and the spectral reflectance. | insight from laboratory measurements on dust in debris discs |
the kepler-51 system consists of three known "super-puff" exoplanets - ultra-low density objects that challenge current theories of planet formation and evolution, with masses measured exclusively from transit timing variations (ttvs). the transmission spectra of kepler-51b and d were observed with jwst to investigate their atmospheric composition and to search for high altitude hazes and/or planetary rings that may be responsible for their large observed radii. however, the jwst mid-transit time of kepler-51d was discrepant from ttv predictions by 2 hours, compared to the prediction uncertainty of 2.7 minutes. in order to match the new observations, a fourth planet must be included in the ttv models, with the uncertainty in its mass and period greatly increasing the uncertainty on the masses of the three known planets and their future transit times. as a result, not only is it more difficult to interpret the jwst spectra, but we now run the risk of missing future transits of the kepler-51 planets altogether. we therefore propose to observe the next transits of kepler-51c and d in late oct/early nov 2023 with hst. observing these two planets would provide the tightest constraints possible on the mass and period of the fourth planet and reduce the uncertainties on the masses and future transit times of the other three planets. these transits cannot be observed from the ground due to the faintness of the host star and the availability of large telescopes, nor can we wait for subsequent transits due to the increasing uncertainty of the transit time with time. with hst being the ideal space telescope for this observation and the timing of these transits, we request dd time. | the discovery of a fourth low-mass planet in an unusual super-puff system |
we use a three-dimensional hybrid plasma model to study the global aspects of jovian plasma interaction with ganymede. ganymede, the largest moon of jupiter, is a unique body for several reasons: (1) it is the only known moon that has its own intrinsic magnetic field, (2) its dipole magnetic moment is large enough to form an embedded magnetosphere within the magnetosphere of jupiter, and (3) it has a bound neutral atmosphere and an ionosphere, mainly composed of molecular and atomic oxygen, that interact with the co-rotating plasma of jupiter. since jupiter's magnetic dipole moment tilts nearly 10o from its rotation axis, ganymede passes two distinct plasma environments on its orbit around jupiter (which is slightly inclined to the jovian equator): (1) the plasma sheet, where the plasma density is nearly 5 cm-3 and plasma beta is slightly larger than one, and (2) outside the plasma sheet where the plasma density is lower than that in the sheet and plasma beta is smaller than one. the sonic and alfvénic mach numbers, however, are both smaller than one in both of these regions. thus, the formation of a bow shock upstream of ganymede is not expected. the plasma interaction with ganymede has been studied before using mhd simulations and the formation of a magnetopause, magnetotail, and alfvén wings were examined. we use our three-dimensional hybrid model to compare the global effects of the two plasma regimes on the interaction with ganymede. we compare our simulation results with galileo flyby observations, and explain differences between our mode with mhd simulation results. we also provide a global map of plasma precipitation into the surface of ganymede which has direct impact on ganymede's atmosphere/exosphere formation. | jovian plasma interaction with ganymede's magnetosphere |
uranus and saturn share similarities in terms of their atmospheric composition, which is primarily made up of hydrogen and helium, as well as their ring systems. uranus has 13 known rings, which are divided into narrow main rings, dusty rings, and outer rings. unlike saturn's broad ring system, uranus' inner narrow main rings are relatively narrow, and likely consist of dark, radiation-processed organics that range from centimeters to meters in size. we assume that uranus may have a mechanism similar to saturn where tiny particles fall on-to the planet due to its gravity and the dragging force of the upper atmosphere. the uncharged nano-dust particles in uranus' inner narrow rings will collide with neutral gas molecules in the exosphere and fall onto the planet. this work derives a monte carlo simulation of the orbital behavior of nano-dust particles in the inner narrow rings of uranus. the model shows that the braking of the dust grain motion takes place at altitudes between 6000 km and 8000 km, and the dust particles are gradually captured into corotation with the planetary atmosphere below 4000 km altitude. the larger the dust particles are, the lower the altitude at which they will be assimilated into co-rotation. the lifetime of 1-nm dust particles to 1000 km-altitudes is estimated to be about 32.5 ± 18.8 h, and that of 30 nm is about 2770.0 ± 213.9 h. | infalling of nano-dust because of air drag on uranus |
context. t tauri stars are low-mass young stars whose disks provide the setting for planet formation, which is one of the most fundamental processes in astronomy. yet the mechanisms of this are still poorly understood. su aurigae is a widely studied t tauri star and here we present original state-of-the-art interferometric observations with better uv and baseline coverage than previous studies.aims: we aim to investigate the characteristics of the circumstellar material around su aur, and constrain the disk geometry, composition and inner dust rim structure.methods: the mirc-x instrument at chara is a six-telescope optical beam combiner offering baselines up to 331 m. we undertook image reconstruction for model-independent analysis, and fitted geometric models such as gaussian and ring distributions. additionally, the fitting of radiative transfer models constrained the physical parameters of the disk.results: image reconstruction reveals a highly inclined disk with a slight asymmetry consistent with inclination effects obscuring the inner disk rim through absorption of incident star light on the near side and thermal re-emission/scattering of the far side. geometric models find that the underlying brightness distribution is best modelled as a gaussian with a full-width half-maximum of 1.53 ± 0.01 mas at an inclination of 56.9 ± 0.4° and a minor axis position angle of 55.9 ± 0.5°. radiative transfer modelling shows a flared disk with an inner radius at 0.16 au which implies a grain size of 0.14 μm assuming astronomical silicates and a scale height of 9.0 au at 100 au. in agreement with the literature, only the dusty disk wind successfully accounts for the near infrared excess by introducing dust above the mid-plane.conclusions: our results confirm and provide better constraints than previous inner disk studies of su aurigae. we confirm the presence of a dusty disk wind in the cicumstellar environment, the strength of which is enhanced by a late infall event which also causes very strong misalignments between the inner and outer disks. reduced data are available at the cds via anonymous ftp to cdsarc.cds.unistra.fr (ftp://130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/j/a+a/678/a6 | imaging the warped dusty disk wind environment of su aurigae with mirc-x |
alkaline-carbonatite complexes are a primary resource for rees and act as efficient sequesters of deep carbon, however their origin and structure remains widely debated and poorly understood. the arkansas alkaline province, an area of northeast trending igneous intrusions of lamproites, diamond-bearing kimberlites, and carbonatites, is a significant geological anomaly in a state otherwise composed entirely of sedimentary rock. the alkaline-carbonatite ring-dike complex located at magnet cove in the ouachita mountains of southwest arkansas has primarily been studied from a geochemical and petrological approach. this mid-cretaceous intrusion has also been mined for minerals such as niobium, rutile, brookite, pyrite, and perovskite. the roughly 12-km2 surface exposure of dense, iron-rich minerals produces gravity and magnetic anomaly highs approximately 25 km in diameter, suggestive of a sizable subsurface unit. though aeromagnetic and gravity surveys have been conducted regionally, few studies have taken a geophysical approach to better understanding the structure and extent of the mineralogically diverse complex at magnet cove. hendricks (1988) used a bouguer gravity anomaly contour map from the 1970's and a residual total field aeromagnetic map from 1981 to produce a 2d model of magnet cove as a laccolith intrusion, using a single density and magnetic susceptibility contrast. since hendricks' (1988) model, more recent and higher density ground-based gravity data is available from the pan-american center for earth and environmental studies (paces). our approach combines paces gravity data with aeromagnetic data to jointly forward model the subsurface of the magnet cove alkaline-carbonatite intrusion as a ring-dike structure in 2d and 3d. these models are constrained by mapped surface exposures and density and magnetic susceptibility measurements taken from characteristic rock types within the complex. preliminary results indicate that the observed gravity and magnetic profiles of the anomaly can be matched using a forward model of a ring dike, and are best resolved using a diapir-shaped intrusion. our modeling suggests that the intrusion could reach depths greater than 10 km, with an approximate volume of 1000 km3. | structural interpretation of an alkaline-carbonatite complex at magnet cove, arkansas using gravity and magnetic methods |
exo-planets are common, and they span a large range of compositions. the origins of the observed diversity of planetary compositions is largely unconstrained, but must be linked to the planet formation physics and chemistry. among planets that are earth-like, a second question is how often such planets form hospitable to life. a fraction of exo-planets are observed to be ‘physically habitable’, i.e. of the right temperature and bulk composition to sustain a water-based prebiotic chemistry, but this does not automatically imply that they are rich in the building blocks of life, in organic molecules of different sizes and kinds, i.e. that they are chemically habitable. in this talk i will argue that characterizing the chemistry of protoplanetary disks, the formation sites of planets, is key to address both the origins of planetary bulk compositions and the likelihood of finding organic matter on planets. the most direct path to constrain the chemistry in disks is to directly observe it. in the age of alma it is for the first time possible to image the chemistry of planet formation, to determine locations of disk snowlines, and to map the distributions of different organic molecules. recent alma highlights include constraints on co snowline locations, the discovery of spectacular chemical ring systems, and first detections of more complex organic molecules. observations can only provide chemical snapshots, however, and even alma is blind to the majority of the chemistry that shapes planet formation. to interpret observations and address the full chemical complexity in disks requires models, both toy models and astrochemical simulations. these models in turn must be informed by laboratory experiments, some of which will be shown in this talk. it is thus only when we combine observational, theoretical and experimental constraints that we can hope to characterize the chemistry of disks, and further, the chemical compositions of nascent planets. | the chemistry of planet formation |
the ζ ring is the innermost component of the uranian ring system. it is of scientific interest because its morphology changed significantly between the voyager 2 encounter in 1986 and subsequent earth-based observations around 2007. it is also of practical interest because some uranus mission concepts have the spacecraft pass through the inner flank of this ring. recent reexaminations of the voyager 2 images have revealed additional information about this ring that provide a more complete picture of the ring's radial brightness profile and phase function. these data reveal that this ring's brightness varies with phase angle in a manner similar to other tenuous rings, consistent with it being composed primarily of submillimeter-sized particles. the total cross section of particles within this ring can also be estimated from these data, but translating that number into the actual risk to a spacecraft flying through this region depends on a number of model-dependent parameters. fortunately, comparisons with saturn's g and d rings allows the ζ ring's particle number density to be compared with regions previously encountered by the voyager and cassini spacecraft. finally, these data indicate that the observed changes in the ζ ring's structure between 1986 and 2007 are primarily due to a substantial increase in the amount of dust at distances between 38,000 and 40,000 km from uranus' center. | examining uranus' ζ ring in voyager 2 wide-angle camera observations: quantifying the ring's structure in 1986 and its modifications prior to the year 2007 |
the gas-and dust rich disks that surround all young solar type stars are characterised by holes, gaps, rings and warps, that are believed to be signposts of on-going planet formation. exoplanet surveys show a remarkable diversity in exoplanet properties, and in exoplanetary system architectures. what are the processes that govern this rich diversity, and can we link the properties of planet forming disks to those of exoplanetary systems? how important is the environment for disk dispersal and planet formation? the successful launch and deployment of the james webb space telescope marks the opening of a new window on the birthplaces of planetary systems. the four scientific instruments on board of webb will provide spectra and images of unprecedented quality of the gas and dust in planet forming disks. the wavelength range covered by webb allows studies of the warm surface layers of the inner disk, corresponding to the terrestrial planet forming region in disks surrounding solar type stars. a very rich range of inner disk molecular species will be within reach, probing key elements such as c, n, o, and s. in addition, due to the high sensitivity in combination with medium spectral resolution, also minor molecular species will become accessible. these new data will allow addressing the chemical composition of the inner disk gas, and will allow a comparison to the outer disk gas chemistry, and to the composition of exoplanet atmospheres. the webb data also allow studies of the chemical composition of small dust particles in the inner disk surface layers. space missions such as iso, spitzer and herschel have shown the power of far-ir spectroscopy for studies of planet forming disks. new mission concepts can address a number of key questions, such as an inventory of the main water reservoir in disks, and a direct measure of the disk gas mass. | planet forming disks: the promise of jwst, and science cases for a next generation ir facility |
a stellar occultation of gaia dr3 2646598228351156352 by the centaur (2060) chiron was observed from the south african astronomical observatory on 2018 november 28 ut. here we present a positive detection of material surrounding chiron from the 74-inch telescope for this event. additionally, a global atmosphere is ruled out at the tens of microbars level for several possible atmospheric compositions. there are multiple 3σ drops in the 74-inch light curve: three during immersion and two during emersion. occulting material is located between 242 and 270 km from the center of the nucleus in the sky plane. assuming the ring-plane orientation proposed for chiron from the 2011 occultation, the flux drops are located at 352, 344, and 316 km (immersion) and 357 and 364 km (emersion) from the center, with normal optical depths of 0.26, 0.36, and 0.22 (immersion) and 0.26 and 0.18 (emersion) and equivalent widths between 0.7 and 1.3 km. this detection is similar to the previously proposed two-ring system and is located within the error bars of that ring-pole plane; however, the normal optical depths are less than half of the previous values, and three features are detected on immersion. these results suggest that the properties of the surrounding material have evolved between the 2011, 2018, and 2022 observations. | material around the centaur (2060) chiron from the 2018 november 28 ut stellar occultation |
io's intense volcanic activity results in one of the most colorful surfaces in the solar system. ultraviolet and visible-wavelength observations of io are critical to uncovering the chemistry behind its volcanic hues. here, we present global, spatially resolved ultraviolet-visible spectra of io from the space telescope imaging spectrograph on the hubble space telescope, which bridge the gap between previous highly resolved imagery and disk-integrated spectroscopy, to provide an unprecedented combination of spatial and spectral detail. we use this comprehensive data set to investigate spectral endmembers, map observed spectral features associated with so2 frost and other sulfur species, and explore possible compositions in the context of io surface processes. in agreement with past observations, our results are consistent with extensive equatorial so2 frost deposits that are stable over multidecade timescales, widespread sulfur-rich plains surrounding the so2 deposits, and the enrichment of pele's pyroclastic ring and the high-latitude regions in metastable short-chain sulfur allotropes. | spectroscopic mapping of io's surface with hst/stis: so2 frost, sulfur allotropes, and large-scale compositional patterns |
we report the first near-infrared detection of uranus's tiny moon mab, the presumed source of the blue and diffuse μ ring, using the nirc2 instrument at keck observatory. the detection was permitted by an updated shift-and-stack procedure allowing us to integrate on mab as it moved across the detector in 23 separate exposures taken over ∼2 h, as well as the very low (0.02∘) phase angle at the time of observation. at this phase angle, mab has an integrated i/f of 24 ± 3 km2 at 1.6 μm and ≲37 km2 at 2.1 μm. comparing these values with mab's visible reflectance as derived by hst reveals that mab is spectrally blue; its (0.5 μm)/(1.6 μm) color is more consistent with miranda's value than puck's value. mab is therefore more likely a ∼6-km radius body with a miranda-like surface than a 12-km radius body with a puck-like surface, in agreement with prior work based on infrared upper limits, but we caution that a puck-like color is only ruled out at the 2 σ level. we also report the first infrared photometry of perdita, finding an integrated i/f of 31 ± 3 km2 at 1.6 μm. | keck near-infrared detections of mab and perdita |
we present the fuv phase curves of saturn's rings as a function of radius relative to the center of the planet, for both the lit and unlit side of the rings. this updated analysis expands on our previous work (elliott and esposito 2021, dps) by examining the phase curves of the rings at a finer granularity. each of the examined ring regions, a, b and c, are broken down into smaller annuli to illustrate the variability of the phase curves as a function of radius. particularly in the b ring, which has several distinct regions with varying optical depth and particle size distributions (colwell et al. 2018), characterization of the radial dependence of the phase curves can be important in the compositional analysis of the rings. next, we use the phase dependence of the fuv reflectance at each ring annulus to derive the macroscopic roughness of the rings, which is important to understand because it represents the degree of surface shadowing that occurs which can reduce the effective brightness of rings. this darkening of the rings due to shadowing can be misinterpreted as a pollutant material mixed in with the icy regolith and therefore is important to quantify. by constraining the macroscopic roughness, or surface shadowing, we reduce it to a fixed parameter in our least-squares best-fit hapke model for the fractional pollution of the saturn's rings. obtaining the current state of the fractional pollution of the rings, we then run our markov-chain based model for meteoritic bombardment of the rings (elliott and esposito 2011) backward in time to estimate the age of saturn's rings. | cassini uvis fuv phase curve maps and the age of saturn's rings as a function of radius |
in the last five years alma revolutionised our comprehension of planet formation. the first breakthrough was delivered by the impacting image of the rings in the disk of hl tau, suggesting that planet formation occurs early, in disks of less than 1 myr. alma is revolutionising also our comprehension of the disk chemistry, which is crucial to answer another key question about planet formation: what chemical composition planets inherit from their natal environment? answering this question is the goal of the alma chemical survey of disk-outflow sources in taurus (alma-dot). alma-dot targets six class i, early class ii disks (0.1-1 myr) in a number of molecular tracers: co, cn, s-bearing, cs, h2cs, so2, deuterated water, hdo, and simple organics, h2co and ch3oh. the survey allowed us to obtain a comprehensive view of the radial distribution of molecules in young disks, to reveal the disk vertical structure with the first image of the "molecular" and "freeze-out" layer, and to derive the molecules gas-phase abundance ratios. these are compared with the abundance ratios in class 0 and ii sources, and in comets, to reconstruct the chemical evolution from protostars to planets. moreover, we report the first detection of methanol in a class i disk, a key organic molecule for the build-up of chemical complexity. the results obtained by alma-dot are the first step towards the characterisation of the disk chemical evolution and of the molecular heritage delivered to the assembling planets. | alma-dot: the alma chemical survey of disk-outflow sources in taurus |
planetary organic hazes can play a significant role in influencing a planet's radiative balance and climate, with their impact determined by the optical properties of the haze. the optical properties, in turn, are partly influenced by particle composition. our previous work, reed et al., demonstrated that trace amounts of hydrogen sulfide (h2s) in haze chemistry can substantially affect the haze's composition by generating organosulfur compounds and increasing the amount of organic haze produced. however, no study has measured the optical properties of an h2s-influenced organic haze. here we present results from laboratory experiments measuring the real (scattering, n) and imaginary (absorbing, k) refractive indices of haze analogs produced from photochemistry of gas mixtures composed of 0.1% ch4 and variable h2s (0-10 ppmv) in n2. the optical properties of the aerosol produced were measured in real-time using coupled photoacoustic and cavity ring-down spectroscopy with 405 and 532 nm wavelengths of light. our findings show that the total extinction of light (scattering plus absorption) by the aerosol increases as a function of h2s mixing ratio. we provide our best-fit equations for predicting n and k at 405 and 532 nm as a function of the sulfur to carbon molar ratio (s:c) of the precursor gas mixture. further, we demonstrate how these changes in optical properties could alter the transmittance of 405 and 532 nm light through a haze layer. these results have potential implications for modeling the climate, habitability, and spectra for exoplanets exhibiting organic haze. | the influence of hydrogen sulfide on the optical properties of planetary organic hazes: implications for exoplanet climate modeling |
earth's space environment system is comprised of a myriad of plasma populations. these populations encompass characteristic energies ranging over 6 orders of magnitude; from sub-ev plasma in the ionosphere, to 10-100 kev plasma in the ring current, and mev and higher particles in the radiation belts. further complicating this picture is the fact that the origin of near-earth plasma, be it the ionosphere or solar wind, remains a subject of significant debate. simulating these disparate plasma populations in a single global simulation requires a fusion of fluid and kinetic models. in this presentation we present an improved coupling of the polar wind outflow model (pwom), the comprehensive ionosphere magnetosphere interaction (cimi) model, and batsrus model of the magnetosphere. these models are coupled together using the space weather modeling framework (swmf), and represent a comprehensive model of the earth's space environment. the improved coupling allows us to investigate the origin of near-earth plasma by separately tracking solar wind plasma (h+) from ionospheric plasma (highlatitude h+ and o+, as well as plasmaspheric h+). focusing on storm-time simulations, we will examine hemispheric asymmetries in the outflow, the relative role of wave-particle interactions and solar euv in driving ionospheric outflow, and the impact on the ring current and radiation belts. we will moreover highlight the kinetic features embedded in the model and discuss current limitations future directions. | global modeling of the drivers and impacts of storm-time plasma composition |
scientists have long believed that there may be billions to trillions of rogue planets drifting through our galaxy, unattached to any host star. a recent study has now identified one such candidate potentially the first terrestrial-mass world weve spotted on the run.severing attachmentsartists impression of a free-floating, earth-like planet. [christine pulliam (cfa)]weve discovered more than 4,000 exoplanets in the last three decades, spanning a dramatic range of masses, sizes, temperatures, compositions, orbital properties, and more. the vast majority of them, however, share one feature: they all orbit a star.while this may seem like normal behavior after all, were rather attached to our own star, here on earth planetary formation models predict that there should be a large population of free-floating planets in our galaxy. according to the models, these typically sub-earth-mass planets get kicked out from their parent systems through interactions with other bodies (usually bullying gas giants).how can we observationally confirm this picture? without the beacon of a host stars light, free-floating planets are challenging to detect but theyre discoverable via a method called gravitational microlensing.gravitational microlensing is a powerful tool for detecting exoplanets. this illustration shows the bending of light from a background source by a planetary system in the foreground. [nasa exoplanet exploration]the lens is the thingwhen light from a background source passes by a massive body on its way to us, the intervening object acts as a gravitational lens, bending the light.in the case of microlensing, the intervening lens object is small a stellar- or planetary-mass object so the lensing doesnt produce a resolvable ring of light like in strong lensing. instead, we see a brief brightening of the background source as the lens passes in front of it. from the shape of the light curve, we can then infer lens and source properties.roughly 100 planets have been discovered in microlensing events so far but in most of these cases, the lensing mass is actually a combination of a planet and its host star. only a handful of objects have been found so far that might be free-floating planets, and theyve all been of relatively large mass.that is, until now.short blip, small planetthe 23-yr long ogle light curve of the microlensing event ogle-2016-blg-1928 (top) reveals a single brightening. a closeup of the magnified part of the light curve (bottom) shows the structure of the event and its best-fit model. [mrz et al. 2020]a recent study led by przemek mrz (california institute of technology) presents a new discovery gleaned from data from two gravitational lensing telescopes: the shortest-timescale microlensing event seen yet, ogle-2016-blg-1928.the event was located in high-cadence survey fields, so though the brightening timescale was just 41.5 minutes, the optical gravitational lensing experiment (ogle) and the korea microlensing telescope network (kmtn) managed to capture a joint total of 15 magnified data points. by modeling the light curve, the authors establish that ogle-2016-blg-1928 is either a free-floating planet, or its host is located at least 8 au away from it.assuming that the planet is located in the galactic disk (which the authors deem likely based on their data), its estimated to weigh ~0.3 earth mass, or roughly 3 times the mass of mars.so how do our prospects look for finding more of these free-floating low-mass planets and verifying the expectation that theyre plentiful? certainly, this ogle detection proves its possible and with the power of upcoming observatories like the nancy grace roman space telescope, odds are good that well be able to spot more of these drifting terrestrial worlds.citationa terrestrial-mass rogue planet candidate detected in the shortest-timescale microlensing event, przemek mrz et al 2020 apjl 903 l11. doi:10.3847/2041-8213/abbfad | a terrestrial-mass planet on the run? |
the role of the pre-solar chemistry in the chemical composition of solar system bodies is far to be understood. did they inherit at least part of their composition from the earliest stages of star formation? during each step of the process leading to the formation of a sun-like star and its planetary system, the molecular complexity increases from simple species up to interstellar complex organic molecules (icoms, o-, n-, s-bearing species whit at least 6 atoms). in turn, icoms can be considered as bricks that can be used to assemble pre-biotic molecules. in this context, the study of the molecular complexity of class 0 objects, where protostars are still deeply embedded in their parental core and still accreting their mass, is mandatory. as a matter of fact, protostellar radiation heats the surrounding medium creating the so-called hot-corinos, i.e. the regions of about 100 au with temperatures of at least 100 k, hot enough to (i) evaporate dust mantles, and (ii) trigger a warm gas phase-chemistry. as a consequence, the icoms abundances bloom, dramatically enriching the gas phase. the chemistry of the protostellar environments represents the first stage, and it needs to be compared with those of more evolved phase including relics of our pristine solar system, such as comets.the investigation of star-forming regions has enormously benefited from the recent advent of the iram-noema and alma (sub-) mm-interferometers, which allowed the observers to reach solar system scales. it is of paramount importance to combine high-sensitivity spectral surveys to collect large numbers of lines for each icom (for reliable identifications and to analyse excitation conditions) as well as to image their spatial distribution to investigate their association with different ingredients of the sun-like star formation recipe (e.g. warm envelopes and cavities opened from hot jets, accretion disks, disk winds). the overall goal is to analyse the protostellar disk, i.e. the region where a solar system will form billions of years later. the imaging of these regions with molecules simpler than icoms, such as co or cs is indeed paradoxically hampered by their high abundances and consequently high line opacities which do not allow the observers to disentangle all the emitting components at these small scales.in this respect, we will report recent results of a prototypical object such as iras4a, showing how complementing images obtained in the cm-spectral regime are to mimimise icoms emission hampering due to dust opacity. finally, we will focus on the protostellar disk hh212, observed with a spatial resolution down to about 10 au. a large number of icoms, such as ch3oh, ch3cho, hcooch3, ch3ch2oh, nh2cho, have been imaged: their emission is clearly associated with the upper and lower rings in the outer part of the disk imaged using continuum emission. indeed, the geometry of these features has led this source to be called a "space hamburger" in the popular press. we will discuss the possible explanations: (i) the disk has a vertically extended gaseous atmosphere (the "methanol buns") and no gaseous methanol on the disk midplane (the "dusty meat"); (ii) the optically-thick disk midplane obscures icoms" emission. the answer is essential to understand the gas composition in the equatorial plane, where planets will form. | complex organics in protostellar disks: the first stage of a long chemical journey to planetary systems |
duo to the large magnetic reynolds number, the magnetic helicity originating from the solar interior can be carried away through the photosphere into the corona. however, the relationship between the accumulated magnetic helicity flux through the photosphere and the magnetic helicity in the corona is still unclear. by selecting 36 newly emerging active regions in the 23rd solar cycle, we apply optical flow methods to derive the accumulated magnetic helicity through the photosphere (hm p ) by using the sequential longitudinal magnetograms, we use nonlinear force-free field extrapolation to obtain the 3d coronal magnetic field, and we adopt finite volume methods to calculate the instantaneous relative magnetic helicity in the corona (hm c ) by using vector magnetograms. it is found that the local correlation tracking (lct)-based hm pis larger than hm cin 1″, and that the differential affine velocity estimator-based hm pis more consistent with hm cthan the lct-based hm p . hm pis more consistent with hm cin evaluation from 2″ than from 1″. moreover, ${h}_{m}^{c}-{h}_{m}^{p}$ systematically shows consistency with the hemispheric helicity rule (over 55%), no matter which resolution and method are used. these estimations suggest that the consistency of hm cand hm pis partly dependent on the resolution of the magnetograms and the calculation methods. | comparison of two methods for calculating magnetic helicity in the solar corona |
polar faculae are the footpoints of magnetic-field lines near the sun's poles that are seen as bright regions along the edges of granules. the time variation in the number of polar faculae has been shown to correlate with the strength of the polar magnetic field and to be a predictor of the subsequent solar cycle. due to the small size and transient nature of these features, combined with different techniques and observational factors, previous counts of polar faculae differ in magnitude. further, there were no scalable techniques to measure the statistical properties of the faculae, such as the variation of the facular lifetime with time or solar activity. using data from the helioseismic and magnetic imager (hmi) onboard the solar dynamics observatory (sdo), we present two new methods for tracking faculae and measuring their properties. in the first, we calculate the pixel-by-pixel standard deviation of the hmi continuum intensity images over one day, visualizing the faculae as streaks. the lifetime of the facula is found by dividing the angular length of the streaks by the latitude-dependent rotation rate. we apply this method to the more visible pole each day for a week every six months, from september 2010 to march 2021. combining all of the measured facular lifetimes provides a statistical distribution with a mean of 6.0 hours, a fwhm of 5.4 hours, and a skew towards longer lifetimes, with some faculae lasting up to 1 day. in the second method, we overlay images of the progressive standard deviation with the hmi magnetogram to show the close relationship between the facular candidates and the magnetic field. the results of this method allow us to distinguish between motion due to the sun's rotation and "proper motion" due to faculae moving across the sun's surface, confirming that faculae participate in convective motions at the poles. counts of polar faculae using both methods agree with previous counts in their variation with the solar cycle and the polar magnetic field. these methods can be extended to automate the identification and measurement of other properties of polar faculae, which would allow for daily measurements of all faculae since sdo began operation in 2010. | two new methods for counting and tracking the evolution of polar faculae |
we study the relation between stellar dynamo-wave propagation and the structure of the stellar magnetic field. modeling dynamo waves by the well-known parker migratory dynamo, we vary the intensity of dynamo drivers in order to obtain activity-wave propagation toward the equator (as in the solar-activity cycle) or towards the poles. we match the magnetic field in the dynamo-active shell with that in the surrounding stellar material, using a simple dissipative magneto-hydrodynamic system for the transition region. introducing a weak asymmetry between the stellar hemispheres, we study phase shifts of the dipole, quadrupole, and octupole magnetic components at various distances from the star to demonstrate that a several-percent asymmetry in dynamo drivers is sufficient to obtain a realistic relation between solar dipole and quadrupole moments. we study the behavior of the stellar current sheets and show that for the poleward-propagating activity it is substantially different from solar ones. in particular, we demonstrate conditions in which the conical current sheets propagate opposite to the solar directions. | migrating dynamo waves and consequences for stellar current sheets |
we are conducting a comprehensive investigation of v815 her using photometric and spectroscopic data to understand the origin of the activity and what influences it in the short and long term. using tess photometry we performed light curve modeling in order to derive astrophysical and orbital parameters for the eclipsing binary subsystem v815 her b. using archival photometric data covering a century we carried out a time frequency analysis. spectral synthesis was applied to determine the basic astrophysical parameters of the rapidly rotating primary using high-resolution stella spectra recorded in 2018. photometric analysis revealed multiple cycles on timescales between ~6.5 and ~26 years. from tess photometry we obtained orbital solution for the v815 her b subsystem. the stella spectra covering the 200 day-long observing season enabled to create 19 time-series doppler images, which revealed a constantly changing spotted surface. from the consecutive image pairs we measured a weak solar-type surface differential rotation of the spotted star. we found evidence that the v815 her b component previously apostrophized as a third body is actually an eclipsing close binary subsystem of two m dwarfs with a period of 0.5 d, i.e., v815 her is a 2+2 hierarchical quadruple system. the system is apparently young, only a few times ten million years old, consistent with the spotted primary v815 her aa being a zero-age main sequence star. spot activity on the primary was found to be vivid. fast starspot decay suggests that convective-turbulent erosion plays a more significant role in such a rapidly rotating star. the weak differential rotation of v815 her aa is presumably confined by tidal forces of the close companion v815 her ab. the slowly increasing photometric cycle of 6.5 years on average is interpreted as a spot cycle of v815 her aa, which is probably modulated by the eccentric wide orbit. | a star under multiple influences. magnetic activity in v815 her, a compact 2+2 hierarchical system |
the ubiquitousness of solar inter-network horizontal magnetic field has been revealed by the space-borne observations with high spatial resolution and polarization sensitivity. however, no consensus has been achieved on the origin of the horizontal field among solar physicists. for a better understanding, in this study we analyze the cyclic variation of inter-network horizontal field by using the spectro-polarimeter observations provided by solar optical telescope on board hinode, covering the interval from 2008 april to 2015 february. the method of wavelength integration is adopted to achieve a high signal-to-noise ratio. it is found that from 2008 to 2015 the inter-network horizontal field does not vary when solar activity increases, and the average flux density of inter-network horizontal field is 87$\pm$1 g, in addition, the imbalance between horizontal and vertical field also keeps invariant within the scope of deviation, i.e., 8.7$\pm$0.5, from the solar minimum to maximum of solar cycle 24. this result confirms that the inter-network horizontal field is independent of sunspot cycle. the revelation favors the idea that a local dynamo is creating and maintaining the solar inter-network horizontal field. | does the variation of solar inter-network horizontal field follow sunspot cycle? |
the sunspot number record covers over three centuries.these numbers measure the activity of the sun. this activity follows the solar cycle of about eleven years. in the dynamo-theory, the interaction between differential rotation and convection produces the solar magnetic field. on the surface of sun, this field concentrates to the sunspots. the dynamo-theory predicts that the period, the amplitude and the phase of the solar cycle are stochastic. here we show that the solar cycle is deterministic, and connected to the orbital motions of the earth and jupiter. this planetary-influence theory allows us to model the whole sunspot record, as well as the near past and the near future of sunspot numbers. we may never be able to predict the exact times of exceptionally strong solar flares, like the catastrophic carrington event in september 1859, but we can estimate when such events are more probable. our results also indicate that during the next decades the sun will no longer help us to cope with the climate change. the inability to find predictability in some phenomenon does not prove that this phenomenon itself is stochastic. | sunspot cycles are connected to the earth and jupiter |
exoplanet hunting surveys, including kepler and k2, have begun a new era for stellar magnetic activity studies, particularly for white light flares. continuous light curves with months to years baselines, and unmatched photometric precision, has led to discoveries including "superflares" on solar-type stars, the evolution of flare activity with stellar age, and the most statistically complete census of flares to date for thousands of stars. the tess mission is now capturing new light curves for most of the known benchmark flare stars, extending this space-based study of stellar activity across the sky. here we revisit many of the best-studied flare stars from the kepler mission with tess. we find the flare recovery capability for tess is well matched in comparison to the kepler archive. further, the 10-year baseline of observations between kepler and tess allow a new search for stellar activity cycles via changes in flare rate, which we expect to vary by an order of magnitude based on the solar example. as tess continues to gather high fidelity light curves across 85% of the sky, we anticipate flare rate variations to be an important new method for detecting stellar activity cycles, and further informing our understanding of stellar dynamos. | combining kepler and tess: 10 years of stellar flare studies from space |
we present the results of our observations of the maser radio emission source g188.946+0.886 in hydroxyl (oh) molecular lines with the radio telescope of the nançay observatory (france) and in the h2o line at λ = 1.35 cm with the rt-22 radio telescope at the pushchino observatory (russia). an emission feature in the 1720-mhz satellite line of the oh ground state has been detected for the first time. the radial velocity of the feature, v lsr = 3.6 km s-1, has a "blue" shift relative to the range of emission velocities in the main 1665- and 1667-mhz oh lines, which is 8-11 km s-1. this suggests a probable connection of the observed feature in the 1720-mhz line with the "blue" wing of the bipolar outflow observed in this region in the co line. we have estimated the magnetic field strength for three features (0.90 and 0.8 mg for 1665 mhz and 0.25 mg for 1720 mhz) from the zeeman splitting in the 1665- and 1720-mhz lines. no emission and (or) absorption has been detected in the other 1612-mhz satellite oh line. three cycles of h2o maser activity have been revealed. the variability is quasi-periodic in pattern. there is a general tendency for the maser activity to decrease. some clusters of h2o maser spots can form organized structures, for example, chains and other forms. | investigation of oh and h2o masers in the star-forming region g 188.946+0.886 |
ground level enhancements (gles) are extreme solar energetic particle (sep) events that are of particular importance in space weather. in solar cycle 24, two gles were recorded on 2012 may 17 (gle 71) and 2017 september 10 (gle 72), respectively, using a range of advanced modern instruments. here we conduct a comparative analysis of the two events by focusing on the effects of large-scale magnetic field configuration near active regions on particle acceleration and release. although the active regions are both located near the western limb, temporal variations of sep intensities and energy spectra measured in situ display different behaviors at early stages. by combining a potential field model, we find the coronal mass ejection (cme) in gle 71 originated below the streamer belt, while in gle 72 it originated near the edge of the streamer belt. we reconstruct the cme shock fronts with an ellipsoid model based on nearly simultaneous coronagraph images from multiple viewpoints and further derive the 3d shock geometry at the gle onset. the highest-energy particles are primarily accelerated in the shock-streamer interaction regions, i.e., likely at the nose of the shock in gle 71 and the eastern flank in gle 72, due to quasi-perpendicular shock geometry and confinement of closed fields. subsequently, they are released to the field lines connecting to near-earth spacecraft when the shocks move through the streamer cusp region. this suggests that magnetic structures in the corona, especially shock-streamer interactions, may have played an important role in the acceleration and release of the highest-energy particles in the two events. | effects of coronal magnetic field configuration on particle acceleration and release during the ground level enhancement events in solar cycle 24 |
the 2021 extreme precision radial velocity (eprv) working group's report, and the more recent 2022 sag21 findings, emphasizes that stellar activity is a primary hurdle to confirming and characterizing earth analogs (earth mass planets orbiting in the habitable zones of sun-like stars). as such, scientific interest has returned to stellar measurements, with a goal to characterize stellar activity and remove its signature(s) from rv data. the only star for which we can currently study such activity (and its impact on rvs) in great detail is the sun, which represents only a small subset of local spectral types. for more distant stars, we can infer the presence of star spots and flares from time series photometry and sometimes produce crude images of the photospheres using methods such as interferometry and doppler-tomography. yet this is a far cry from understanding stellar activity in detail across a wide range of stellar types. while numerical tools (e.g. gaussian processes) can be very helpful, their results can be misleading without a broad foundation of observed stellar variability to compare against. we have proposed to nasa a smallsat explorer mission called starspot (the stellar activity recorder and spectro-photometric observatory) which could provide a missing link between solar and stellar activity studies by collecting high-precision, spectrally and temporally resolved data for a representative set of f-g-k-m dwarf stars. these spectral time series would cover sequential rotation cycles and revisit targets yearly to sample long term magnetic cycles. the goal is to advance our understanding of stellar activity via characterization of specific active region phenomena, which in return could help to break the current 1m/s stellar activity rv barrier and aid in better modeling of the transit light source effect. in the starspot concept, high stability spectroscopy with a passband from nuv to nir is enabled by a low-noise space platform. a telescope based on free-form optical surfaces provides a large aperture and a large field of view, allowing for simultaneous observations of comparison stars with similar properties. this, and a constant solar reference, enables us to remove remaining systematics, which are already highly suppressed by design. | starspot: a space-based spectrophotometric platform for stellar activity characterization |
in this study, new photometric observations of shallow contact binary cn andromedae (cn and) were performed and multi-color (bv r) ccd light curves (lcs) were obtained. simultaneous analysis of new lcs and published radial velocity (rv) data reveals that the system is an early contact binary in which both components have recently filled their inner roche lobes. asymmetric lcs were modeled by a dark spot on the primary component and a hot spot on the secondary component that probably resulted from magnetic activity and mass transfer, respectively. modeling of lcs and rv data allows us to estimate the following absolute parameters: m 1 = 1.40 ± 0.02 m ⊙, m 2 = 0.55 ± 0.01 m ⊙, r 1 = 1.45 ± 0.02 r ⊙ and r 2 = 0.94 ± 0.02 r ⊙. a decreasing orbital period with a rate of dp/dt = -1.5 × 10-7 d yr-1 can be seen as evidence that the system is evolving into a contact binary with higher contact degree. cyclic oscillation of the o - c data was interpreted by the applegate mechanism and light-time effect due to an unseen component around the close binary system. the hypothetical third component is probably a fully convective red dwarf star with a minimal mass of 0.1 m ⊙. cn and is at the early phase of the contact stage of its evolution and is an interesting example for studying the formation and evolution of close binaries. | cn andromedae: a shallow contact binary with a possible tertiary component |
the sun is a g2v star with an effective temperature of 5780 k. as the nearest star to earth and the biggest object in the solar system, it serves as a reference for fundamental astronomical parameters such as stellar mass, luminosity, and elemental abundances. it also serves as a plasma physics laboratory. a great deal of researchers' understanding of the sun comes from its electromagnetic radiation, which is close to that of a blackbody whose emission peaks at a wavelength of around 5,000 å and extends into the near uv and infrared. the bulk of this radiation escapes from the solar surface, from a layer that is a mere 100 km thick. this surface from where the photons escape into the heliosphere and beyond, together with the roughly 400-500 km thick atmospheric layer immediately above it (where the temperature falls off monotonically with distance from the sun), is termed the solar photosphere. observations of the solar photosphere have led to some important discoveries in modern-day astronomy and astrophysics. at low spatial resolution, the photosphere is nearly featureless. however, naked-eye solar observations, the oldest of which can plausibly be dated back to 800 bc, have shown there to be occasional blemishes or spots. systematic observations made with telescopes from the early 1600s onward have provided further information on the evolution of these sunspots whose typical spatial extent is 10,000 km at the solar surface. continued observations of these sunspots later revealed that they increase and decrease in number with a period of about 11 years and that they actually are a manifestation of the sun's magnetic field (representing the first observation of an extraterrestrial magnetic field). this established the presence of magnetic cycles on the sun responsible for the observed cyclic behavior of solar activity. such magnetic activity is now known to exist in other stars as well. superimposed on the solar blackbody spectrum are numerous spectral lines from different atomic species that arise due to the absorption of photons at certain wavelengths by those atoms, in the cooler photospheric plasma overlying the solar surface. these spectral lines provide diagnostics of the properties and dynamics of the underlying plasma (e.g., the granulation due to convection and the solar p-mode oscillations) and of the solar magnetic field. since the early 20th century, researchers have used these spectral lines and the accompanying polarimetric signals to decode the physics of the solar photosphere and its magnetic structures, including sunspots. modern observations with high spatial (0.15 arcsec, corresponding to 100 km on the solar surface) and spectral (10 må) resolutions reveal a tapestry of the magnetized plasma with structures down to tens of kilometers at the photosphere (three orders of magnitude smaller than sunspots). such observations, combined with advanced numerical models, provide further clues to the very important role of the magnetic field in solar and stellar structures and the variability in their brightness. being the lowest directly observable layer of the sun, the photosphere is also a window into the solar interior by means of helioseismology, which makes use of the p-mode oscillations. furthermore, being the lowest layer of the solar atmosphere, the photosphere provides key insights into another long-standing mystery, that above the temperature-minimum (~500 km above the surface at ~4000 k), the plasma in the extended corona (invisible to the naked eye except during a total solar eclipse) is heated to temperatures up to 1,000 times higher than at the visible surface. the physics of the solar photosphere is thus central to the understanding of many solar and stellar phenomena. | solar photosphere |
the existence of magnetic activity on the eclipsing binary dv psc has been known for almost two decades. however, until recently, no evidence of periodic behaviour relevant to this activity had been found. in this study, long-term photometric observations of dv psc are used to analyze the system's magnetic activity, seek a possible magnetic cycle and determine orbital and physical parameters. the combination of photometric and spectroscopic observations results in a unified model that describes the system over time in terms of variable spot activity. new times of minimum light are determined and an accurate astronomical ephemeris and updated o-c diagram are constructed for a total span of 19 years (1997-2017). the intense magnetic activity, as indicated by strong asymmetries in the light curves (o' connell effect), and the periodic variation of the o-c diagram are combined to explain the system's behaviour. the existence of a third body, orbiting the eclipsing binary in an eccentric orbit, as well as a magnetic cycle are the most likely scenario. | temporal evolution of the magnetically active eclipsing binary dv psc |
surface acoustic waves (saws) propagate along material surfaces or at solid-air, solid-liquid, solid-solid, and solid-vacuum interfaces. saws are confined to surfaces within the depth of one wavelength, and the saw velocity depends on the elastic properties of the material. hence saws can be used to determine thermomechanical properties like young's modulus and thermal conductivity of 2d materials. inspired by a recent electron microscopy experiment, we use molecular dynamics (md) simulations to investigate the effect of saws on the thermomechanical behavior of mos2 mono- and bilayers. the md results for young's modulus and thermal conductivity of 2d mos2 are in excellent agreement with experiments and density functional theory calculations. we find nanopores have a dramatic effect on the thermal conductivity, which drops by an order of magnitude in a nanoporous mos2 monolayer. we also examine the effect of saws on moiré patterns between 2d van der waals materials. this work was supported by the national science foundation, future manufacturing program, award 2036359. the simulations were performed at the centre for advanced research and computing of the university of southern california. | probing thermomechanical properties of two-dimensional van der waals architectures using surface acoustic waves |
miura-ori pattern is seen in nature be it the unfolding of a leaf or opening and closing of insect wings. similar patterns in the form of wrinkles and ridges are also observed in biaxially compressed rigid thin films supported on soft substrates. we have investigated the formation of wrinkles and ridges in graphene placed on a polyethylene substrate using quantum molecular dynamics (qxmd) simulation. qxmd is a massively parallel quantum molecular dynamics software with various ``extensions'' such as non-adiabatic dynamics for the study of light-induced electronic excitations. in our qxmd simulations, we have seen the formation of wrinkles and ridges in biaxially compressed graphene sheets and calculated the electronic density of states and stress and strain distributions in wrinkle and ridge states. we will also present results of large-scale classical molecular dynamics simulations of wrinkle and ridge states of graphene on the polyethylene substrate. this work was supported by the national science foundation, future manufacturing program, award 2036359. the simulations were performed at the centre for advanced research and computing of the university of southern california. | dynamics of wrinkle-ridge transition in graphene supported on a polymer: quantum molecular dynamics simulations |
photodissociation studies in molecular beams that employ position-sensitive particle detection to map product recoil velocities emerged thirty years ago and continue to evolve with new laser and detector technologies. these powerful methods allow application of tunable laser detection of single product quantum states, simultaneous measurement of velocity and angular momentum polarization, measurement of joint product state distributions for the detected and undetected products, coincident detection of multiple product channels, and application to radicals and ions as well as closed-shell molecules. these studies have permitted deep investigation of photochemical dynamics for a broad range of systems, revealed new reaction mechanisms, and addressed problems of practical importance in atmospheric, combustion, and interstellar chemistry. this review presents an historical overview, a detailed technical account of the range of methods employed, and selected experimental highlights illustrating the capabilities of the method. | invited review article: photofragment imaging |
context. one of the main challenges for polarimetric observations of active galactic nuclei (agn) is to properly estimate the amount of parasitic light that contaminates the polarization signal. removing this unpolarized flux is a complex task that has only been achieved in a couple of objects.aims: in this fifth paper of the series, we present a new version of the monte carlo code stokes that accounts for dilution by interstellar polarization and host starlight in radiative transfer modeling.methods: we upgraded our code by including spectral energy distribution (sed) templates for a set of representative host galaxies. the unpolarized light emitted by those hosts alters the observer polarization while being coherently radiatively coupled to the agn structure. we also included in our analysis tool a routine that may add, depending on the user's objectives, an interstellar component.results: using a generic agn model, we illustrate how interstellar polarization and starlight dilution impact the observed polarimetric signal of agn. we applied our code to ngc 1068, an archetypal edge-on agn and demonstrate that stokes can reproduce its sed, the expected wavelength-dependent polarimetric signatures, and the observed high-angular resolution polarimetric maps. using the flexibility of the code, we derived several intrinsic parameters such as the system inclination and the torus opening angle.conclusions: the new version of our publicly available code now allows observers to better prepare their observations, interpret their data and simulate the three-dimensional geometry and physics of agn in order to probe unresolved structures. additionally, the radiative interaction between the host and the agn can be used to probe the co-evolution of the system. | modeling optical and uv polarization of agns. v. dilution by interstellar polarization and the host galaxy |
the origin of terrestrial bioorganic homochirality is one of the most important and unresolved problems in the study of chemical evolution prior to the origin of terrestrial life. one hypothesis advocated in the context of astrobiology is that polarized quantum radiation in space, such as circularly polarized photons or spin-polarized leptons, induced asymmetric chemical and physical conditions in the primitive interstellar media (the cosmic scenario). another advocated hypothesis in the context of symmetry breaking in the universe is that the bioorganic asymmetry is intrinsically derived from the chiral asymmetric properties of elementary particles, that is, parity violation in the weak interaction (the intrinsic scenario). in this paper, the features of these two scenarios are discussed and approaches to validate them are reviewed. | origin of terrestrial bioorganic homochirality and symmetry breaking in the universe |
linear polarization has been measured in several gamma-ray burst (grb) afterglows. after a few days, polarization arises from the forward shock emission that depends on the post-shock magnetic field. the latter can originate both from compression of existing fields, here the interstellar medium (ism) magnetic field, and from shock-generated instabilities. for short grbs, previous modelling of the polarization arising from the forward shock considered a random field fully or partially confined to the shock plane. however, the ism magnetic field likely consists of both random and ordered components. here we study the impact of a more realistic magnetic field having both ordered and random components. we present our semi-analytical model and compute polarization curves arising for different magnetic field configurations. we find that the presence of an ordered component, even significantly weaker than the random one, has distinct signatures that could be detectable. in the presence of an ordered component not in the observer plane, we show that (i) for an observer inside the jet, the polarization angle θp either remains constant during all the afterglow phase or exhibits variations smaller than the 90° swing expected from a random component solely; (ii) for an off-axis observer, the polarization angle evolves from $\theta _\mathrm{ p}^{\max }$, before the jet break to its opposite after the jet break. we also find that the upper limit polarization for grb 170817 requires a random field not fully confined to the shock plane and is compatible with an ordered component as large as half the random one. | impact of the ism magnetic field on grb afterglow polarization |
magnetic fields are crucial in numerous astrophysical processes within the interstellar medium (ism). however, the detailed determination of magnetic field geometry is notoriously challenging. based on the modern magnetohydrodynamic (mhd) turbulence theory, we introduce a novel statistical technique, the 'y-parameter', to decipher the magnetic field inclination in the ism and identify dominant turbulence modes. the y-parameter, calculated as the ratio of anisotropies of different stokes parameter combinations, displays contrasting trends with the mean-field inclination angle in alfvénic and compressible turbulence modes. a y-parameter value around 1.5 ± 0.5 provide a statistical boundary to determine the dominant mhd turbulence modes. we have discovered specific correlations between the y-parameter value and the inclination angle that unveil the dominant turbulence mode. this methodology, when applied to future radio polarization surveys such as lofar and ska, promises to significantly enhance our knowledge of 3d magnetic field in the ism and improve our understanding of interstellar turbulence. | diagnosis of 3d magnetic field and mode composition in mhd turbulence with y-parameter |
aims: the optical emission of black hole transients increases by several magnitudes during the x-ray outbursts. whether the extra light arises from the x-ray heated outer disc, from the inner hot accretion flow, or from the jet is currently debated. optical polarisation measurements are able to distinguish the relative contributions of these components.methods: we present the results of bvr polarisation measurements of the black hole x-ray binary maxi j1820+070 during the period of march-april 2018.results: we detect small, ∼0.7%, but statistically significant polarisation, part of which is of interstellar origin. depending on the interstellar polarisation estimate, the intrinsic polarisation degree of the source is between ∼0.3% and 0.7%, and the polarisation position angle is between ∼10 ° -30°. we show that the polarisation increases after mjd 58222 (2018 april 14). the change is of the order of 0.1% and is most pronounced in the r band. the change of the source stokes parameters occurs simultaneously with the drop of the observed v-band flux and a slow softening of the x-ray spectrum. the stokes vectors of intrinsic polarisation before and after the drop are parallel, at least in the v and r filters.conclusions: we suggest that the increased polarisation is due to the decreasing contribution of the non-polarized component, which we associate with the the hot flow or jet emission. the low polarisation can result from the tangled geometry of the magnetic field or from the faraday rotation in the dense, ionised, and magnetised medium close to the black hole. the polarized optical emission is likely produced by the irradiated disc or by scattering of its radiation in the optically thin outflow. | evolving optical polarisation of the black hole x-ray binary maxi j1820+070 |
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