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the rosetta ion and electron sensor (ies) has been measuring solar wind ions intermittently since exiting from hibernation in may 2014. on 19 august, when rosetta was ~80 km from the comet 67p/churyumov-gerasimenko, which was ~3.5 au from the sun, ies began to see ions at its lowest energy range, ~4-10 ev. we identify these as ions created from neutral species emitted by the comet nucleus, photoionized by solar uv radiation in the neighborhood of the rosetta spacecraft (s/c), and attracted by the small negative potential of the s/c resulting from the population of thermal electrons. later, ies began to see higher-energy ions that we identify as having been picked up and accelerated by the solar wind. ies continues to measure changes in the solar wind and the development of the pickup ion structure. | the rosetta ion and electron sensor (ies) measurement of the development of pickup ions from comet 67p/churyumov-gerasimenko |
we summarize unique aperture data on the degree of linear polarization observed for distant comets c/2010 s1, c/2010 r1, c/2011 kp36, c/2012 j1, c/2013 v4, and c/2014 a4 with heliocentric distances exceeding 3 au. observations have been carried out at the 6-m telescope of the special astrophysical observatory of the russian academy of sciences (nizhnij arkhyz, russia) during the period from 2011 to 2016. the measured negative polarization proves to be significantly larger in absolute value than what is typically observed for comets close to the sun. we compare the new observational data with the results of numerical modeling performed with the t-matrix and superposition t-matrix methods. in our computer simulations, we assume the cometary coma to be an optically thin cloud containing particles in the form of spheroids, fractal aggregates composed of spherical monomers, and mixtures of spheroids and aggregate particles. we obtain a good semi-quantitative agreement between all polarimetric data for the observed distant comets and the results of numerical modeling for the following models of the cometary dust: (i) a mixture of submicrometer water-ice oblate spheroids with aggregates composed of submicrometer silicate monomers; and (ii) a mixture of submicrometer water-ice oblate spheroids and aggregates consisting of both silicate and organic monomers. the microphysical parameters of these models are presented and discussed. | retrieval of microphysical characteristics of particles in atmospheres of distant comets from ground-based polarimetry |
context. cyanogen (nccn) is the simplest member of the dicyanopolyynes group, and has been proposed as a major source of the cn radical observed in cometary atmospheres. although not detected through its rotational spectrum in the cold interstellar medium, this very stable species is supposed to be very abundant.aims: the chemistry of cyanogen in the cold interstellar medium can be investigated through its metastable isomer, cncn (isocyanogen). its formation may provide a clue on the widely abundant cn radical observed in cometary atmospheres.methods: we performed an unbiased spectral survey of the l1544 proto-typical prestellar core, using the iram-30 m and have analysed, for this paper, the nitrogen chemistry that leads to the formation of isocyanogen. we report on the first detection of cncn, nccnh+, c3n, ch3cn, c2h3cn, and h2cn in l1544. we built a detailed chemical network for nccn/cncn/hc2n2+ involving all the nitrogen bearing species detected (cn, hcn, hnc, c3n, cncn, ch3cn, ch2cn, hccnc, hc3n, hnc3, h2cn, c2h3cn, hcnh+, hc3nh+) and the upper limits on c4n, c2n. the main cyanogen production pathways considered in the network are the cn + hnc and n + c3n reactions.results: the comparison between the observations of the nitrogen bearing species and the predictions from the chemical modelling shows a very good agreement, taking into account the new chemical network. the expected cyanogen abundance is greater than the isocyanogen abundance by a factor of 100. although cyanogen cannot be detected through its rotational spectrum, the chemical modelling predicts that it should be abundant in the gas phase and hence might be traced through the detection of isocyanogen. it is however expected to have a very low abundance on the grain surfaces compared to hcn. | isocyanogen formation in the cold interstellar medium |
the structure of the upper layer of a comet is a product of its surface activity. the rosetta lander imaging system (rolis) on board philae acquired close-range images of the agilkia site during its descent onto comet 67p/churyumov-gerasimenko. these images reveal a photometrically uniform surface covered by regolith composed of debris and blocks ranging in size from centimeters to 5 meters. at the highest resolution of 1 centimeter per pixel, the surface appears granular, with no apparent deposits of unresolved sand-sized particles. the thickness of the regolith varies across the imaged field from 0 to 1 to 2 meters. the presence of aeolian-like features resembling wind tails hints at regolith mobilization and erosion processes. modeling suggests that abrasion driven by airfall-induced particle “splashing” is responsible for the observed formations. | the structure of the regolith on 67p/churyumov-gerasimenko from rolis descent imaging |
exposure to bisphenol a (bpa) has been related to male reproductive disorders. since this endocrine disruptor also displays genotoxic and epigenotoxic effects, it likely alters the spermatogenesis, a process in which both hormones and chromatin remodeling play crucial roles. the hypothesis of this work is that bpa impairs early embryo development by modifying the spermatic genetic and epigenetic information. zebrafish males were exposed to 100 and 2000 μg/l bpa during early spermatogenesis and during the whole process. genotoxic and epigenotoxic effects on spermatozoa (comet assay and immunocytochemistry) as well as progeny development (mortality, dna repairing activity, apoptosis and epigenetic profile) were evaluated. exposure to 100 µg/l bpa during mitosis slightly increased sperm chromatin fragmentation, enhancing dna repairing activity in embryos. the rest of treatments promoted high levels of sperm dna damage, triggering apoptosis in early embryo and severely impairing survival. regarding epigenetics, histone acetylation (h3k9ac and h3k27ac) was similarly enhanced in spermatozoa and embryos from males exposed to all the treatments. therefore, bpa male exposure jeopardizes embryonic survival and development due to the transmission of a paternal damaged genome and of a hyper-acetylated histone profile, both alterations depending on the dose of the toxicant and the temporal window of exposure. | genetic and epigenetic alterations induced by bisphenol a exposure during different periods of spermatogenesis: from spermatozoa to the progeny |
the current giant planet region is a transitional zone where transneptunian objects (tnos) cross in their way to becoming jupiter family comets. their dynamical behavior is conditioned by the intrinsic dynamical features of tnos and also by the encounters with the giant planets. we address the giant planet crossing (gpc) population (those objects with 5.2 au <q <30 au ) studying their number and their evolution from their sources, considering the current configuration of the solar system. this subject is reviewed from previous investigations and also addressed by new numerical simulations of the dynamical evolution of scattered disk objects (sdos). we obtain a model of the intrinsic orbital element distribution of gpcs. the scattered disk represents the main source of prograde gpcs and centaurs, while the contribution from plutinos lies between one and two orders of magnitude below that from the sd. we obtain the number and size distribution of gpcs from our model, computing 9600 gpcs from the sd with d >100 km and ∼108 with d >1 km in the current population. the contribution from other sources is considered negligible. the mean lifetime in the centaur zone is 7.2 myr, while the mean lifetime of sdos in the gpc zone is of 68 myr. the latter is dependent on the initial inclination, being the ones with high inclinations the ones that survive the longest in the gpc zone. there is also a correlation of lifetime with perihelion distance, where greater perihelion leads to longer lifetime. the dynamical evolution of observed gpcs is different for prograde and retrograde objects. retrograde gpcs have lower median lifetime than prograde ones, thus experiencing a comparatively faster evolution. however, it is probable that this faster evolution is due to the fact that the majority of retrograde gpcs have low perihelion values and then, lower lifetimes. | centaur and giant planet crossing populations: origin and distribution |
the compositions of planet-forming disks are set by a combination of material inherited from the interstellar medium and material reprocessed during disk formation and evolution. indeed, comets and primitive meteorites exhibit interstellar-like isotopic ratios and/or volatile compositions, supporting that some pristine material was incorporated intact into icy planetesimals in the solar nebula. to date, the survival of volatile interstellar material in the disk stage has not been modeled using realistic disk physics. here, we present a modeling framework to track the destruction of interstellar ices on dust grains undergoing transport processes within a disk, with a particular focus on explaining the incorporation of pristine material into icy planetesimals. we find that it is difficult to explain inheritance through the local assembly of comets, as ice destruction is rapid for small (<10 μm) grains in the inner few tens of au. instead, a plausible pathway to inheritance is to form pebbles at larger disk radii, which then drift inward to the comet-forming zone with their ices mostly preserved. small grains beyond ~100 au can experience ice photodissociation at the tens of percent level; however, little of the ice is actually lost from the grain, likely making this a robust site for in situ ice chemistry. our models also indicate that many complex organic species should survive passage through the disk intact. this raises the possibility that organics synthesized in the interstellar medium can be delivered to terrestrial planets by icy-body impact and thus potentially participate in origins of life chemistry. | ice inheritance in dynamical disk models |
in an effort to better understand the evolution of composite supernova remnants (snrs) and the eventual fate of relativistic particles injected by their pulsars, we present a multifaceted investigation of the interaction between a pulsar wind nebula (pwn) and its host snr g327.1-1.1. our 350 chandra x-ray observations of snr g327.1-1.1 reveal a highly complex morphology: a cometary structure resembling a bow shock, prong-like features extending into large arcs in the snr interior, and thermal emission from the snr shell. spectral analysis of the non-thermal emission offers clues about the origin of the pwn structures, while enhanced abundances in the pwn region provide evidence for a mixing of supernova ejecta with pwn material. the overall morphology and spectral properties of the snr suggest that the pwn has undergone an asymmetric interaction with the snr reverse shock (rs), whichcan occur as a result of a density gradient in the ambient medium and/or a moving pulsar that displaces the pwn from the center of the remnant. we present hydrodynamical simulations of g327.1-1.1 that show that its morphology and evolution can be described by a ∼17,000-year-old composite snr that expanded into a density gradient with an orientation perpendicular to the pulsar’s motion. we also show that the rs/pwn interaction scenario can reproduce the broadband spectrum of the pwn from radio to γ-ray wavelengths. the analysis and modeling presented in this work have important implications for our general understanding of the structure and evolution of composite snrs. | late-time evolution of composite supernova remnants: deep chandra observations and hydrodynamical modeling of a crushed pulsar wind nebula in snr g327.1-1.1 |
this paper shows how the dips and secular dimming in the kic8462852 light curve can originate in circumstellar material distributed around a single elliptical orbit (e.g. exocomets). the expected thermal emission and wavelength dependent dimming is derived for different orbital parameters and geometries, including dust that is optically thick to stellar radiation, and for a size distribution of dust with realistic optical properties. we first consider dust distributed evenly around the orbit, then show how to derive its uneven distribution from the optical light curve and to predict light curves at different wavelengths. the fractional luminosity of an even distribution is approximately the level of dimming times stellar radius divided by distance from the star at transit. non-detection of dust thermal emission for kic8462852 thus provides a lower limit on the transit distance to complement the 0.6 au upper limit imposed by 0.4 d dips. unless the dust distribution is optically thick, the putative 16 per cent century-long secular dimming must have disappeared before the wise 12 μm measurement in 2010, and subsequent 4.5 μm observations require transits at >0.05 au. however, self-absorption of thermal emission removes these constraints for opaque dust distributions. the passage of dust clumps through pericentre is predicted to cause infrared brightening lasting tens of days and dimming during transit, such that total flux received decreases at wavelengths <5 μm, but increases to potentially detectable levels at longer wavelengths. we suggest that lower dimming levels than seen for kic8462852 are more common in the galactic population and may be detected in future transit surveys. | modelling the kic8462852 light curves: compatibility of the dips and secular dimming with an exocomet interpretation |
we measure the absolute magnitude, h, distribution, dn(h) ∝ 10 αh , of the scattering trans-neptunian objects (tnos) as a proxy for their size-frequency distribution. we show that the h-distribution of the scattering tnos is not consistent with a single-slope distribution, but must transition around hg∼ 9 to either a knee with a shallow slope or to a divot, which is a differential drop followed by second exponential distribution. our analysis is based on a sample of 22 scattering tnos drawn from three different tno surveys—the canada-france ecliptic plane survey, alexandersen et al., and the outer solar system origins survey, all of which provide well-characterized detection thresholds—combined with a cosmogonic model for the formation of the scattering tno population. our measured absolute magnitude distribution result is independent of the choice of cosmogonic model. based on our analysis, we estimate that the number of scattering tnos is (2.4-8.3) × 105 for hr< 12. a divot h-distribution is seen in a variety of formation scenarios and may explain several puzzles in kuiper belt science. we find that a divot h-distribution simultaneously explains the observed scattering tno, neptune trojan, plutino, and centaur h-distributions while simultaneously predicting a large enough scattering tno population to act as the sole supply of the jupiter-family comets. | ossos. ii. a sharp transition in the absolute magnitude distribution of the kuiper belt’s scattering population |
inner solar system bodies are depleted in volatile elements relative to chondrite meteorites, yet the source(s) and mechanism(s) of volatile-element depletion and/or enrichment are poorly constrained. the timing, mechanisms and quantities of volatile elements present in the early inner solar system have vast implications for diverse processes, from planetary differentiation to the emergence of life. we report major, trace and volatile-element contents of a glass bead derived from the d'orbigny angrite, the hydrogen isotopic composition of this glass bead and that of coexisting olivine and silicophosphates, and the 207pb-206pb age of the silicophosphates, 4568 ± 20 ma. we use volatile saturation models to demonstrate that the angrite parent body must have been a major body in the early inner solar system. we further show via mixing calculations that all inner solar system bodies accreted volatile elements with carbonaceous chondrite h and n isotope signatures extremely early in solar system history. only a small portion (if any) of comets and gaseous nebular h species contributed to the volatile content of the inner solar system bodies. this article is part of the themed issue 'the origin, history and role of water in the evolution of the inner solar system'. | early accretion of water and volatile elements to the inner solar system: evidence from angrites |
we describe the development, launch into space, and initial results from a prototype wide field-of-view soft x-ray imager that employs lobster-eye optics and targets heliophysics, planetary, and astrophysics science. the sheath transport observer for the redistribution of mass is the first instrument using this type of optics launched into space and provides proof-of-concept for future flight instruments capable of imaging structures such as the terrestrial cusp, the entire dayside magnetosheath from outside the magnetosphere, comets, the moon, and the solar wind interaction with planetary bodies like venus and mars [kuntz et al., astrophys. j. (in press)]. | invited article: first flight in space of a wide-field-of-view soft x-ray imager using lobster-eye optics: instrument description and initial flight results |
intensive coal mining activities in the upper silesian coal basin (uscb) in southern poland are resulting in large amounts of methane (ch4) emissions. annual ch4 emissions reached 448 kt according to the european pollutant release and transfer register (e-prtr, 2017). as a ch4 emission hotspot in europe, it is of importance to investigate its emission sources and make accurate emission estimates. in this study, we use satellite-based total column-averaged dry-air mole fraction of ch4 (xch4) from the tropospheric monitoring instrument (tropomi) and tropospheric xch4 (txch4) from the infrared atmospheric sounding interferometer (iasi). in addition, the high-resolution model forecasts, xch4 and txch4, from the copernicus atmosphere monitoring service (cams) are used to estimate the ch4 emission rate averaged over 3 years (november 2017-december 2020) in the uscb region (49.3-50.8° n and 18-20° e). the wind-assigned anomaly method is first validated using the cams forecast data (xch4 and txch4), showing a good agreement with the cams global anthropogenic emission (cams-glob-ant) inventory. it indicates that the wind-assigned method works well. this wind-assigned method is further applied to the tropomi xch4 and tropomi + iasi txch4 by using the carbon dioxide and methane (comet) inventory derived for the year 2018. the calculated averaged total ch4 emissions over the uscb region is about 496 kt yr-1 (5.9×1026 molec. s-1) for tropomi xch4 and 437 kt yr-1 (5.2×1026 molec. s-1) for tropomi + iasi txch4. these values are very close to the ones given in the e-prtr inventory (448 kt yr-1) and the ones in the comet inventory (555 kt yr-1), and are thus in agreement with these inventories. the similar estimates of xch4 and txch4 also imply that for a strong source, the dynamically induced variations of the ch4 mixing ratio in the upper troposphere and lower stratosphere region are of secondary importance. uncertainties from different error sources (background removal and noise in the data, vertical wind shear, wind field segmentation, and angle of the emission cone) are approximately 14.8 % for tropomi xch4 and 11.4 % for tropomi + iasi txch4. these results suggest that our wind-assigned method is quite robust and might also serve as a simple method to estimate ch4 or co2 emissions for other regions. | quantifying ch4 emissions in hard coal mines from tropomi and iasi observations using the wind-assigned anomaly method |
comets are seen as depleted in nitrogen compared to the protosolar value, but a small number exhibit significantly higher than typical n2/co ratios: c/1908 r1 (morehouse), c/1940 r2 (cunningham), c/1947 s1 (bester), c/1956 r1 (arend-roland), c/1957 p1 (mrkos), c/1961 r1 (humason), c/1969 y1 (bennett), c/1973 e1 (kohoutek), c/1975 v1-a (west), c/1986 p1 (wilson), c/1987 p1 (bradfield), c/2001 q4 (neat), c/2002 vq94 (linear), c/2016 r2 (panstarrs), and periodic comets 1p/halley, 29p/schwassmann-wachmann 1, and 67p/churyumov-gerasimenko. this study examines the composition and dynamical histories of these n2-'rich' comets to unearth insights into their formation processes. using updated n2 fluorescence factors, we re-estimate the n2/co ratios of this sample and find that they are consistent with the expected values for comets based on estimations of the protosolar nebula. these also often display larger nucleus sizes and show rapid tail morphology variations due to their ionic nature. numerical simulations reveal no common dynamical history, suggesting that the n2/co ratio is independent of the number of inner solar system passages and that n2 is homogeneously distributed within these comets. these volatile-rich comets share an oort cloud origin which is consistent with their survival over the past 4.5 gyr. our study also suggests that there may be a bias using modern high-resolution spectrometers with narrow slits, which could potentially overlook the ion tail of comets. we advocate for the use of long-slit spectroscopy to potentially detect a wider range of n2-rich comets, thereby enriching our understanding of comet compositions and origins. | n2/co ratio in comets insensitive to orbital evolution |
we present a photometric and dynamical study of comet c/2010 u3 (boattini), which was seen active in prediscovery data as early as 2005 november at a new inbound record heliocentric distance of r h = 25.8 au. two outburst events around 2009 and 2017 were observed. the coma and tail of the comet consist of dust grains of ∼10 μm in radius, ejected protractedly at speeds ≲50 m s-1 near the subsolar point, and are subjected to the lorentz force, solar gravitation, and radiation pressure force all together. the prolonged activity indicates that sublimation of supervolatiles (e.g., co, co2) is at play, causing a net mass-loss rate ≳1 kg s-1. to sustain the mass loss, the nucleus radius has to be ≳0.1 km. the color of the cometary dust, similar to other long-period comets, is redder than the solar colors, but we also observed potential color variations when the comet was at 10 < r h < 15 au, concurrent with the onset of crystallization of amorphous water ice, if at all. using publicly available and our refined astrometric measurements, we estimated the precise trajectory of the comet and propagated it backward to its previous perihelion. we found that the comet visited the planetary region 1.96 ± 0.04 myr ago, with barycentric perihelion distance q = 8.364 ± 0.004 au. thus, c/2010 u3 (boattini) is almost certainly a dynamically old comet from the oort cloud, and the observed activity cannot be caused by retained heat from the previous apparition. | c/2010 u3 (boattini): a bizarre comet active at record heliocentric distance |
we have conducted the first comprehensive numerical investigation of the relative velocity distribution of dust particles in self-gravitating protoplanetary discs with a view to assessing the viability of planetesimal formation via direct collapse in such environments. the viability depends crucially on the large sizes that are preferentially collected in pressure maxima produced by transient spiral features (stokes numbers, st ∼ 1); growth to these size scales requires that collision velocities remain low enough that grain growth is not reversed by fragmentation. we show that, for a single-sized dust population, velocity driving by the disc's gravitational perturbations is only effective for st > 3, while coupling to the gas velocity dominates otherwise. we develop a criterion for understanding this result in terms of the stopping distance being of the order of the disc scaleheight. nevertheless, the relative velocities induced by differential radial drift in multi-sized dust populations are too high to allow the growth of silicate dust particles beyond st ∼ 10- 2 or 10-1 (10 cm to m sizes at 30 au), such stokes numbers being insufficient to allow concentration of solids in spiral features. however, for icy solids (which may survive collisions up to several 10 m s-1), growth to st ∼ 1 (10 m size) may be possible beyond 30 au from the star. such objects would be concentrated in spiral features and could potentially produce larger icy planetesimals/comets by gravitational collapse. these planetesimals would acquire moderate eccentricities and remain unmodified over the remaining lifetime of the disc. | collision velocity of dust grains in self-gravitating protoplanetary discs |
a comprehensive study of the organic chemistry and mineralogy of an ultracarbonaceous micrometeorite (ucamm d05ib80) collected from near the dome fuji station, antarctica, was carried out to understand the genetic relationship among organic materials, silicates, and water. the micrometeorite is composed of a dense aggregate of ∼5 μm-sized hollow ellipsoidal organic material containing submicrometer-sized phases such as glass with embedded metal and sulfides (gems) and mineral grains. there is a wide area of organic material (∼15 × 15 μm) in its interior. low-ca pyroxene is much more abundant than olivine and shows various mg/(mg + fe) ratios ranging from ∼1.0 to 0.78, which is common to previous works on ucamms. by contrast, gems grains in this ucamm have unusual chemical compositions. they are depleted in both mg and s, which suggests that these elements were leached out from the gems grains during very weak aqueous alteration, without the formation of phyllosilicates. the organic materials have two textures-smooth and globular with an irregular outline-and these are composed of imine, nitrile and/or aromatic nitrogen heterocycles, and amide. the ratio of nitrogen to carbon (n/c) in the smooth region of the organics is ∼0.15, which is five times higher than that of insoluble organic macromolecules in types 1 and 2 carbonaceous chondritic meteorites. in addition, the ucamm organic materials are soluble in epoxy and are thus hydrophilic; this polar nature indicates that they are very primitive. the surface of the material is coated with an inorganic layer, a few nanometers thick, that consists of c, o, si, s, and fe. sulfur is also contained in the interior, implying the presence of organosulfur moieties. there are no isotopic anomalies of d, 13c, or 15n in the organic material. interstellar photochemistry alone would not be sufficient to explain the n/c ratio of the ucamm organics; therefore, we suggest that a very small amount of fluid on a comet must have been necessary for the formation of the ucamm. the gems grains depleted in mg and s in the ucamm prove a very weak degree of aqueous alteration; weaker than that of carbonaceous chondrites. short-duration weak alteration probably caused by planetesimal shock locally melted cometary ice grains and released water that dissolved the organics; the fluid would likely have not mobilized because of the very low thermal conductivity of the porous icy body. this event allowed the formation of the large organic puddle of the ucamm, as well as organic matter sulfurization, formation of thin membrane-like layers of minerals, and deformation of organic nanoglobules. | formation of an ultracarbonaceous antarctic micrometeorite through minimal aqueous alteration in a small porous icy body |
despite a long tradition of dust instruments flown on-board space mission, the largest number of these can be considered unique as they used different detection techniques. giada (grain impact analyzer and dust accumulator), is one of the dust instruments on-board the rosetta spacecraft and is devoted to measure the dust dynamical parameters in the coma of comet 67p/churyumov-gerasimenko. it couples two different techniques to measure the mass and speed of individual dust particles. we report here the results of an extended calibration activity carried-out, during the hibernation phase of the rosetta mission, on the giada proto flight model (pfm) operative in a clean room in our laboratory. the main aims of an additional calibration campaign are: to verify the algorithms and procedures for data calibration developed before rosetta launch; to improve the comprehension of giada response after the increased knowledge on cometary dust, e.g. the composition of dust particles after stardust mission. these calibration improvements implied a final step, which consisted in defining transfer functions to correlate the new calibration curves obtained for the giada pfm to those to be used for giada onboard the rosetta spacecraft. the extended calibration activity allowed us to analyze giada data acquired in the 67p/c-g coma permitting to infer additional information on cometary dust particles, e.g. density and tensile strength. | giada - grain impact analyzer and dust accumulator - onboard rosetta spacecraft: extended calibrations |
hr 8799 is a young f0-type star with four directly imaged giant planets and two debris belts, one located exterior and another one interior to the region occupied by the planetary orbits. not only having an architecture similar to that of our solar system but also revealing dissimilarities such as high masses of planets, a huge radial extent, and a high mass of the outer debris belt, hr 8799 is considered to be a benchmark to test formation and evolution models of planetary systems. here, we focus on the outer debris ring and its relation to the planets. we demonstrate that the models of the outer disc, proposed previously to reproduce herschel observations, are inconsistent with the alma data, and vice versa. in an attempt to find a physically motivated model that would agree with both observational sets, we perform collisional simulations. we show that a narrow planetesimal belt and a radiation pressure induced dust halo cannot account for the observed radial brightness profiles. a single, wide planetesimal disc does not reproduce the data either. instead, we propose a two-population model, comprising a kuiper-belt-like structure of a low-eccentricity planetesimal population (`the classical kuiper belt') and a high-eccentricity population of comets (`scattered disc'). we argue that such a structure of the exo-kuiper belt of hr 8799 could be explained with planet migration scenarios analogous to those proposed for the kuiper belt of the solar system. | the scattered disc of hr 8799 |
we present resolved images of the hr 4796a debris disk using the magellan adaptive optics system paired with clio-2 and visao. we detect the disk at 0.77 μm, 0.91 μm, 0.99 μm, 2.15 μm, 3.1 μm, 3.3 μm, and 3.8 μm. we find that the deprojected center of the ring is offset from the star by 4.76 ± 1.6 au and that the deprojected eccentricity is 0.06 ± 0.02, in general agreement with previous studies. we find that the average width of the ring is 14+3-2% (11.1+2.4-1.6 au), also comparable to previous measurements. combining our new scattered light data with archival hubble space telescope images at ~0.5-2 μm, along with previously unpublished spitzer/mips thermal emission data and all other literature thermal data, we set out to constrain the chemical composition of the dust grains. after testing 19 individual root compositions and more than 8400 unique mixtures of these compositions, we find that good fits to the scattered light alone and thermal emission alone are discrepant, suggesting that caution should be exercised if fitting to only one or the other. when we fit to both data sets simultaneously, we find that silicates and organics are generally the most favored, while large abundances of water ice are usually not favored. these results suggest the hr 4796a dust grains are similar to interstellar dust and solar system comets, though improved modeling is necessary to place better constraints on the exact chemical composition of the dust. this paper includes data obtained at the 6.5 m magellan telescopes located at las campanas observatory, chile. | on the morphology and chemical composition of the hr 4796a debris disk |
millimeter and centimeter observations are discovering an increasing number of interstellar complex organic molecules (icoms) in a large variety of star-forming sites, from the earliest stages of star formation to protoplanetary disks and in comets. in this context it is pivotal to understand how the solid-phase interactions between icoms and grain surfaces influence the thermal desorption process and, therefore, the presence of molecular species in the gas phase. in the laboratory, it is possible to simulate the thermal desorption process, deriving important parameters such as the desorption temperatures and energies. we report new laboratory results on temperature-programmed desorption from olivine dust of astrophysical relevant ice mixtures of water, acetonitrile, and acetaldehyde. we found that in the presence of grains, only a fraction of acetaldehyde and acetonitrile desorb at about 100 k and 120 k, respectively, while 40% of the molecules are retained by fluffy grains of the order of 100 μm up to temperatures of 190-210 k. in contrast with the typical assumption that all molecules are desorbed in regions with temperatures higher than 100 k, this result implies that about 40% of the molecules can survive on the grains enabling the delivery of volatiles toward regions with temperatures as high as 200 k and shifting inwards the position of the snow lines in protoplanetary disks. these studies offer a necessary support to interpret observational data and may help our understanding of icom formation, providing an estimate of the fraction of molecules released at various temperatures. * released on. | thermal desorption of astrophysically relevant ice mixtures of acetaldehyde and acetonitrile from olivine dust |
enceladus harbors an ocean beneath its ice crust that erupts spectacular plumes from fissures at the south pole. the plume composition was measured by the cassini spacecraft, and provides evidence for the ocean's gas content, salinity, ph, and potential for life. understanding the ocean's composition is complicated by physicochemical processes that alter the plume composition during eruption, such as water vapor condensation in the icy fissures and gas exsolution from the ocean surface. we developed a model that includes key fractionation processes, in particular fractionation during gas exsolution, which has not been previously considered. our model predicts a moderately alkaline (ph 7.95-9.05), gas-rich ocean (~10-5-10-3 molal) with high concentrations of ammonium ions (10-2-10-1 molal). our derived dissolved gas concentrations are higher than in recent studies because we account for gas exsolution, which depletes gases in the plume compared to the ocean, and because our model conserves mass flow rates between gas exsolution from the ocean and eruption from the tiger stripe fissures. we find carbon dioxide and hydrogen concentrations that could provide sufficient chemical energy for oceanic life in the form of hydrogenotrophic methanogens. carbon dioxide concentrations of 10-5-10-3 molal indicate a more earth-like ph than the ph ~8.5-13.5 in previous studies. the inferred bulk ammonium and total inorganic carbon concentrations are consistent with cometary levels. this corroborates evidence from cometary deuterium-hydrogen (d/h) ratios that enceladus formed from comet-like planetesimals. our results suggest a gas-rich ocean that inherited its high volatile concentrations from comet-like building blocks. | chemical fractionation modeling of plumes indicates a gas-rich, moderately alkaline enceladus ocean |
modeling of the electrical conductivity (ec) of icy moon oceans has previously assumed that chloride, sulfate, and other ions released from rock leaching are the main solutes and carriers of ec. here, we show that accreted volatiles, such as carbon dioxide and ammonia, can add a significant fraction of solutes in bodies whose volatile content was in part supplied from cometary materials. these volatiles can increase the ec of aqueous solutions above 1 s/m. our salinity and ec estimates can serve as a basis for planning future magnetometer investigations at icy moons and dwarf planets. in particular, oceans expected in some of the uranian satellites and neptune's satellite triton could have ec above 3 s/m as a result of accretion of both carbon dioxide and ammonia, even if rock leaching during water-rock separation was limited, and if chlorine and sulfur abundances may be at ci carbonaceous chondritic levels. | contribution of non-water ices to salinity and electrical conductivity in ocean worlds |
aims: solar orbiter launched in february 2020 with the goal of revealing the connections between the sun's interior, atmosphere, and the heliosphere. the solar orbiter heavy ion sensor (his) is a time-of-flight ion mass spectrometer dedicated to measuring heavy ions in the solar wind.methods: we present an overview of the first measurements of heavy ion composition from his, reviewing the methods used to transform the spectra obtained on board into scientific data products and examining two solar wind case studies as well as the statistical properties of the heavy ion composition observed by his. we also carried out a comparison with prior measurements of heavy ions at l1.results: the his data set provides the first mass- and charge-resolved heavy ion measurements in the inner heliosphere.conclusions: these high temporal resolution data have the potential to transform our understanding of the connections between the solar wind and its origin at the sun, as well as the interaction between the solar wind and the environment around planets, comets, and in the interstellar medium. | first results from the solar orbiter heavy ion sensor |
context. increasingly, oort cloud comets are being discovered at great distances from the sun and tracked over ever wider ranges of heliocentric distances as observational equipment improves.aims: our goal is to investigate in detail how the original semimajor axis for near-parabolic comets depends on the selected data arc and the assumed form of the nongravitational (ng) acceleration.methods: among currently known oort cloud comets with large perihelion distances (q > 3 au), we selected 32 objects observed over the widest ranges of heliocentric distances in orbital legs before and after perihelion. for each of them, we determined a series of orbits using at least three basic types of datasets selected from available positional data (pre- and post-perihelion data and the entire dataset), and a few forms of ng acceleration representing water ice or co sublimation.results: we find that the motion of comets is often measurably affected by ng forces at heliocentric distances beyond 5 au from the sun. the most spectacular example is c/2010 u3 (boattini), whose perihelion distance is 8.44 au. ng effects are detectable for 19 of the 32 comets within the positional data. for five comets, we find asymmetric effects of ng forces - in three cases significantly greater before perihelion than afterward (c/2017 m4, c/2000 sv75, and c/2015 o1), and in two others the opposite (c/1997 ba6 and c/2006 s3). we also find that the well-known systematic effect of finding more tightly bound original orbits when including the ng acceleration than in purely gravitational solutions may be related to the specific form of the standard g(r) function describing the sublimation of ices. | oort cloud comets discovered far from the sun |
we show that if comets (or any small icy planetesimals such as kuiper belt objects) are composed of pebble piles, their internal radiogenic as well as geochemical heating results in considerably different evolutionary outcomes compared to similar past studies. we utilize a 1d thermophysical evolution code, modified to include state-of-the-art empirical measurements of pebble thermal conductivity and compression, the latter obtained through a new laboratory experiment presented here for the first time. results indicate that due to the low pebble thermal conductivity, the peak temperatures attained during evolution are much higher than in any previous study given the same formation time. assuming meteoritic radiogenic abundances, we find that only extremely small, sub-kilometre comets have the potential to retain the primordial, uniform, and thermally unprocessed composition from which they formed. comets with radii in excess of about 20 km are typically swept by rapid and energetically powerful aqueous hydration reactions. across the full range of comet sizes and formation times, evolutions result in the processing and differentiation of various volatile species, and a radially heterogeneous nucleus structure. our computations however also indicate that the assumed fraction of radionuclides is a pivotal free parameter, because isotopic analyses of the only available cometary samples suggest that no 26al was ever present in comet 81p/wild 2. we show that if comets formed early in the protoplanetary disc (within 1-3 myr), the radionuclide abundances indeed must be much smaller than those typically assumed based on meteoritic samples. we discuss the importance of our findings for the formation, present-day attributes and future research of comets. | are there any pristine comets? constraints from pebble structure |
because ion-neutral reaction cross sections are energy dependent, the distance from a cometary nucleus within which ions remain collisionally coupled to the neutrals is dictated not only by the comet’s activity level but also by the electromagnetic fields in the coma. here we present a 1d model simulating the outward radial motion of water group ions with radial acceleration by an ambipolar electric field interrupted primarily by charge transfer processes with h2o. we also discuss the impact of plasma waves. for a given electric field profile, the model calculates key parameters, including the total ion density, ni , the h3o+/h2o+ number density and flux ratios, r dens and r flux, and the mean ion drift speed, < {u}i> , as a function of cometocentric distance. we focus primarily on a coma roughly resembling that of the esa rosetta mission target comet 67p/churyumov-gerasimenko near its perihelion in 2015 august. in the presence of a weak ambipolar electric field in the radial direction the model results suggest that the neutral coma is not sufficiently dense to keep the mean ion flow speed close to that of the neutrals by the spacecraft location (∼200 km from the nucleus). in addition, for electric field profiles giving niand < {u}i> within limits constrained by measurements, the r dens values are significantly higher than values typically observed. however, when including the ion motion in large-amplitude plasma waves in the model, results more compatible with observations are obtained. we suggest that the variable and often low h3o+/h2o+ number density ratios observed may reflect nonradial ion trajectories strongly influenced by electromagnetic forces and/or plasma instabilities, with energization of the ion population by plasma waves. | a 1d model of radial ion motion interrupted by ion-neutral interactions in a cometary coma |
we monitor the inner coma of comet 41p/tuttle-giacobini-kresák searching for variations of its colour. fast changes in colour of the comet 41p/tuttle-giacobini-kresák provide important clues for better understanding of the microphysical properties of its dust. using the 61- and 70-cm telescopes we measured the apparent magnitude of the comet with the v and r johnson-cousins filters from 2017 january 29 to april 25. the inner coma (∼2000 km) reveals fast and significant variations of colour. the most significant change was found between 2017 march 3 and 4, when it changed from blue with a colour slope s ≈ (-10.15 ± 3.43) per cent per 0.1 μm to red with s ≈ (16.48 ± 4.27) per cent per 0.1 μm. this finding appears in good accordance with what was previously reported by ivanova et al. for long-period comet c/2013 uq4 (catalina), suggesting that fast and significant variations of colour of dust could be a common feature of short- and long-period comets. we model observations of comet 41p/tuttle-giacobini-kresák using the agglomerated debris particles and conclude that its inner coma consists of a mixture of at least two types of particles made of mg-rich silicates and organics or mg-fe silicates. | rapid variations of dust colour in comet 41p/tuttle-giacobini-kresák |
context. hyperactive comets are a small group of comets whose activity is higher than expected. they seem to emit more water than would normally be expected given the size of their nucleus. comet 46p/wirtanen (hereafter, 46p) is among these objects of interest. investigating its activity and composition evolution could provide clues about its origins and its formation region in the solar nebulae.aims: given the exceptional close approach of comet 46p to the earth in 2018, we aim to study the evolution of its activity and composition as a function of heliocentric distances before and after perihelion.methods: we used both trappist telescopes to monitor the comet for almost a year with broad and narrow-band filters. we derived the production rates of five gaseous species (oh, nh, cn, c3, and c2) using a haser model as well as the a(θ)fρ dust proxy parameter. the comet was also observed with the two optical high-resolution spectrographs uves and espresso, mounted on the 8-m eso vlt, to measure the isotopic ratios of c and n, along with the oxygen forbidden-line ratios and the nh2 ortho-to-para ratios.results: over nearly a year, we followed the rise and decline of the production rates of different species as well as the dust activity of 46p on both pre- and post-perihelion. relative abundances with respect to cn and oh along the orbit of the comet show constant and symmetric abundance ratios and a typical coma composition. we determined the rotation period of the nucleus using high-cadence observations and long series of cn images on several nights. we obtained a value of (9.18±0.05) hours at perihelion. using the high-resolution spectra of 46p coma, we derived c and n isotopic ratios of 100±20 and 150±30 as well as a green-to-red forbidden oxygen [oi] line ratio of 0.23±0.02. we measured a nh2 ortho-to-para ratio of 3.31 ±0.03 and derived an ammonia ratio of 1.19±0.03, corresponding to a spin temperature of 27±1 k.conclusions: narrow-band observations show that comet 46p is a hyperactive comet for which 40% of its nucleus surface is active. it has a typical composition, similar to other normal comets; however, an asymmetric behavior with respect to perihelion has been seen in its activity, which is typical of seasonal effects. photometric measurements show no evidence for a change in the rotation period of the nucleus during this apparition. high-resolution spectra show that 46p has typical nh2 ortho-to-para, [oi] lines ratios, and c and n isotopic ratios. | activity and composition of the hyperactive comet 46p/wirtanen during its close approach in 2018 |
despite extensive studies on the formation of organic molecules in various extraterrestrial environments, it still remains under debate when, where, and how such molecules were abiotically formed. a key molecule to solve the problem, hexamethylenetetramine (hmt) has not been confirmed in extraterrestrial materials despite extensive laboratory experimental evidence that it can be produced in interstellar or cometary environments. here we report the first detection of hmt and functionalized hmt species in the carbonaceous chondrites murchison, murray, and tagish lake. while the part-per-billion level concentration of hmt in murchison and tagish lake is comparable to other related soluble organic molecules like amino acids, these compounds may have eluded detection in previous studies due to the loss of hmt during the extraction processes. hmt, which can yield important molecules for prebiotic chemistry such as formaldehyde and ammonia upon degradation, is a likely precursor of meteoritic organic compounds of astrochemical and astrophysical interest. | extraterrestrial hexamethylenetetramine in meteorites—a precursor of prebiotic chemistry in the inner solar system |
many biological systems display intriguing chiral patterns and dynamics. here, we present an active nematic theory accounting for individual spin to explore the collective handedness in chiral rod-shaped aggregations. we show that coordinated individual spin and motility can engender a vortex-array pattern with chirality and drive ordering of topological defects. during this chiral process, the stationary trefoil-like defects self-organize into a periodic, hexagon-dominated polygonal network, which segregates persistently rotating cometlike defects in pairs within each polygon, leading to a translation symmetry at the global scale while a broken reflection symmetry at the local scale. such defect ordering agrees exactly with the voronoi tiling of two-dimensional space and the emergence of the hexagonal symmetry is deciphered in analogy with topological charge neutralization. we calculate energy barriers to the topological transition of the defect ordering and explain the existing metastable states with nonhexagonal polygons. our findings shed light on the chiral morphodynamics in life processes and also suggest a potential route towards tuning self-organization in active materials. | pattern formation and defect ordering in active chiral nematics |
context. during the most recent perihelion passage in 2009 of comet 67p/churyumov-gerasimenko (67p), ground-based observations showed an anisotropic dust coma where jet-like features were detected at ~ 1.3 au from the sun. the current perihelion passage is exceptional as the rosetta spacecraft is monitoring the nucleus activity since march 2014, when a clear dust coma was already surrounding the nucleus at 4.3 au from the sun. subsequently, the osiris camera also witnessed an outburst in activity between april 27 and 30, and since mid-july, the dust coma at rh ~ 3.7-3.6 au preperihelion is clearly non-isotropic, pointing to the existence of dust jet-like features.aims: we aim to ascertain on the nucleus surface the origin of the dust jet-like features detected as early as in mid-july 2014. this will help to establish how the localized comet nucleus activity compares with that seen in previous apparitions and will also help following its evolution as the comet approaches its perihelion, at which phase most of the jets were detected from ground-based observations. determining these areas also allows locating them in regions on the nucleus with spectroscopic or geomorphological distinct characteristics.methods: three series of dust images of comet 67p obtained with the wide angle camera (wac) of the osiris instrument onboard the rosetta spacecraft were processed with different enhancement techniques. this was made to clearly show the existence of jet-like features in the dust coma, whose appearance toward the observer changed as a result of the rotation of the comet nucleus and of the changing observing geometry from the spacecraft. the position angles of these features in the coma together with information on the observing geometry, nucleus shape, and rotation, allowed us to determine the most likely locations on the nucleus surface where the jets originate from.results: geometrical tracing of jet sources indicates that the activity of the nucleus of 67p gave rise during july and august 2014 to large-scale jet-like features from the hapi, hathor, anuket, and aten regions, confirming that active regions may be present on the nucleus localized at 60° northern latitude as deduced from previous comet apparitions. there are also hints that large-scale jets observed from the ground are possibly composed, at their place of origin on the nucleus surface, of numerous small-scale features. | large-scale dust jets in the coma of 67p/churyumov-gerasimenko as seen by the osiris instrument onboard rosetta |
comet interceptor (comet-i) was selected in june 2019 as the first esa f-class mission. in 2029+, comet-i will hitch a ride to a sun-earth l2 quasi-halo orbit, as a co-passenger of esa's m4 ariel mission. it will then remain idle at the l2 point until the right departure conditions are met to intercept a yet-to-be-discovered long period comet (or interstellar body). the fact that comet-i target is thus unidentified becomes a key aspect of the trajectory and mission design. the paper first analyses the long period comet population and concludes that 2 to 3 feasible targets a year should be expected. yet, comet-i will only be able to access some of these, depending mostly on the angular distance between the earth and the closest nodal point to the earth's orbit radius. a preliminary analysis of the transfer trajectories has been performed to assess the trade-off between the accessible region and the transfer time for a given spacecraft design, including a fully chemical, a fully electric and a hybrid propulsion system. the different earth escape options also play a paramount role to enhance comet-i capability to reach possible long period comet targets. particularly, earth-leading intercept configurations have the potential to benefit the most from lunar swing-by departures. finally, a preliminary monte carlo analysis shows that comet-i has a 95-99% likelihood of successfully visit a pristine newly-discovered long period comet in less than 6 years of mission timespan. | esa f-class comet interceptor: trajectory design to intercept a yet-to-be-discovered comet |
context. the rosetta encounter with comet 67p/churyumov-gerasimenko provides a unique opportunity for an in situ, up-close investigation of ion-neutral chemistry in the coma of a weakly outgassing comet far from the sun.aims: observations of primary and secondary ions and modeling are used to investigate the role of ion-neutral chemistry within the thin coma.methods: observations from late october through mid-december 2014 show the continuous presence of the solar wind 30 km from the comet nucleus. these and other observations indicate that there is no contact surface and the solar wind has direct access to the nucleus. on several occasions during this time period, the rosetta/rosina/double focusing mass spectrometer measured the low-energy ion composition in the coma. organic volatiles and water group ions and their breakup products (masses 14 through 19), co2+ (masses 28 and 44) and other mass peaks (at masses 26, 27, and possibly 30) were observed. secondary ions include h3o+ and hco+ (masses 19 and 29). these secondary ions indicate ion-neutral chemistry in the thin coma of the comet. a relatively simple model is constructed to account for the low h3o+/h2o+ and hco+/co+ ratios observed in a water dominated coma. results from this simple model are compared with results from models that include a more detailed chemical reaction network.results: at low outgassing rates, predictions from the simple model agree with observations and with results from more complex models that include much more chemistry. at higher outgassing rates, the ion-neutral chemistry is still limited and high hco+/co+ ratios are predicted and observed. however, at higher outgassing rates, the model predicts high h3o+/h2o+ ratios and the observed ratios are often low. these low ratios may be the result of the highly heterogeneous nature of the coma, where co and co2 number densities can exceed that of water. | rosina/dfms and ies observations of 67p: ion-neutral chemistry in the coma of a weakly outgassing comet |
as a result of two years of observations the authors identified twilight class of bolides. unlike the nighttime observations of bright meteor traces that can be seen no more than ten seconds, twilight traces are observed from a few minutes to two hours. the paper considers the synchronous observation of the evening twilight fireball in the sky over kiev 25/06/2014 by dashkiev g.n. and steklov a.f. the basic distance between of photographing points was 8.55 km. on the assumption that the explosions of large meteoroids in the atmosphere and flash of brightness occurs at the height of their maximum braking in the range significantly less than the height of the homogeneous atmosphere, it follows that the thermal explosion of the meteoroid took place at the height substantially more than 15 km. moreover, flashings and disintegration of perhaps cometary material, began at height more than 25 km. the meteoroid does not flown to the surface of the earth and disintegrated into fine particles. preliminary estimate of initial mass of the fragment before the entry into the atmosphere is from 2 to 10 tons. on both photos are visible only dispersed trace of cometary nuclei fragments that disintegrated | observations of fragment of cometary nuclei in the atmosphere over kiev |
we present a high-resolution observation of the distant comet c/2014 un271 (bernardinelli-bernstein) using the hubble space telescope on 2022 january 8. the signal of the nucleus was successfully isolated by means of a nucleus extraction technique, with an apparent v-band magnitude measured to be 21.65 ± 0.11, corresponding to an absolute magnitude of 8.63 ± 0.11. the product of the visual geometric albedo with the effective radius squared is ${p}_{v}{r}_{{\rm{n}}}^{2}=157\pm 16$ km2. if the alma observation by lellouch et al. refers to a bare nucleus, we derived a visual geometric albedo of 0.033 ± 0.009. if dust contamination of the alma signal is present at the maximum allowed level (24%), we found an albedo of 0.044 ± 0.012 for the nucleus having an effective diameter of 119 ± 15 km. in either case, we confirm that c/2014 un271 is the largest long-period comet ever detected. judging from the measured surface brightness profile of the coma, whose logarithmic gradient varies azimuthally between ~1.0 and 1.7 as a consequence of solar radiation pressure, the mass production is consistent with steady-state production but not with impulsive ejection, as would be produced by an outburst. using aperture photometry, we estimated an enormous (albeit uncertain) mass-loss rate of ~103 kg s-1 at a heliocentric distance of ~20 au. | hubble space telescope detection of the nucleus of comet c/2014 un271 (bernardinelli-bernstein) |
starting from several monthly data sets of rosetta's cometary pressure sensor, we reconstruct the gas density in the coma around comet 67p/churyumov-gerasimenko. the underlying inverse gas model is constructed by fitting ten thousands of measurements to thousands of potential gas sources distributed across the entire nucleus surface. the ensuing self-consistent solution for the entire coma density and surface activity reproduces the temporal and spatial variations seen in the data for monthly periods with pearson correlation coefficients of 0.93 and higher. for different seasonal illumination conditions before and after perihelion, we observe a systematic shift of gas sources on the nucleus. | seasonal changes of the volatile density in the coma and on the surface of comet 67p/churyumov-gerasimenko |
context. debris discs have often been described as gas-poor discs as the gas-to-dust ratio is expected to be considerably lower than in primordial, protoplanetary discs. however, recent observations have confirmed the presence of a non-negligible amount of cold gas in the circumstellar (cs) debris discs around young main-sequence stars. this cold gas has been suggested to be related to the outgassing of planetesimals and cometary-like objects.aims: the goal of this paper is to investigate the presence of hot gas in the immediate surroundings of the cold-gas-bearing debris-disc central stars.methods: high-resolution optical spectra of all currently known cold-gas-bearing debris-disc systems, with the exception of β pic and fomalhaut, have been obtained from la palma (spain), la silla (chile), and la luz (mexico) observatories. to verify the presence of hot gas around the sample of stars, we have analysed the ca ii h&k and the na i d lines searching for non-photospheric absorptions of cs origin, usually attributed to cometary-like activity.results: narrow, stable ca ii and/or na i absorption features have been detected superimposed to the photospheric lines in 10 out of the 15 observed cold-gas-bearing debris-disc stars. features are found at the radial velocity of the stars, or slightly blue- or red-shifted, and/or at the velocity of the local interstellar medium (ism). some stars also present transient variable events or absorptions extended towards red wavelengths (red wings). these are the first detections of such ca ii features in 7 out of the 15 observed stars. although an ism origin cannot categorically be excluded, the results suggest that the stable and variable absorptions arise from relatively hot gas located in the cs close-in environment of the stars. this hot gas is detected in at least 80%, of edge-on cold-gas-bearing debris discs, while in only 10% of the discs seen close to face-on. we interpret this result as a geometrical effect, and suggest that the non-detection of hot gas absorptions in some face-on systems is due to the disc inclination and likely not to the absence of the hot-gas component. this gas is likely released in physical processes related in some way to the evaporation of exocomets, evaporation of dust grains, or grain-grain collisions close to the central star. the reduced spectra are only available at the cds (ascii files) and at the feros archive (fits files) via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?j/a+a/614/a3 | the co-existence of hot and cold gas in debris discs |
the highly favorable 2017 apparition of 2p/encke allowed the first comprehensive comparison of primary volatile abundances in a given comet across multiple apparitions. this apparition offered opportunities to address pressing questions in cometary science, including investigating evolutionary and/or heliocentric distance (r h) effects on volatile production, sampling the hypervolatiles co and ch4 in an ecliptic comet, and measuring volatile release at small r h. the faintness and frequently low geocentric velocity of ecliptic comets during most apparitions make our near-infrared observations of these hypervolatiles rare and of high scientific impact. we characterized the volatile composition of 2p/encke on three post-perihelion dates using the ishell spectrograph at the nasa infrared telescope facility on maunakea, hi. we detected fluorescent emission from nine primary volatiles (h2o, co, c2h6, ch3oh, ch4, h2co, nh3, ocs, and hcn) and three fragment species (oh*, nh2, and cn), and obtained a sensitive upper limit for c2h2. we report rotational temperatures, production rates, and mixing ratios (abundances relative to h2o). compared to mean abundances in comets observed to date in the near-infrared, mixing ratios of trace gases in 2p/encke were depleted for all species except h2co and nh3, which were “normal.” the detection of the hypervolatiles co and ch4 is particularly notable given the paucity of measurements in ecliptic comets. we observed significant differences in primary volatile composition compared to published pre-perihelion results from 2003 at larger r h. we discuss possible mechanisms for these differences and discuss these results in the context of findings from the rosetta mission and ground-based studies of comets. | a tale of “two” comets: the primary volatile composition of comet 2p/encke across apparitions and implications for cometary science |
exocomets are small bodies releasing gas and dust which orbit stars other than the sun. their existence was first inferred from the detection of variable absorption features in stellar spectra in the late 1980s using spectroscopy. more recently, they have been detected through photometric transits from space, and through far-ir/mm gas emission within debris disks. as (exo)comets are considered to contain the most pristine material accessible in stellar systems, they hold the potential to give us information about early stage formation and evolution conditions of extra solar systems. in the solar system, comets carry the physical and chemical memory of the protoplanetary disk environment where they formed, providing relevant information on processes in the primordial solar nebula. the aim of this paper is to compare essential compositional properties between solar system comets and exocomets to allow for the development of new observational methods and techniques. the paper aims to highlight commonalities and to discuss differences which may aid the communication between the involved research communities and perhaps also avoid misconceptions. the compositional properties of solar system comets and exocomets are summarized before providing an observational comparison between them. exocomets likely vary in their composition depending on their formation environment like solar system comets do, and since exocomets are not resolved spatially, they pose a challenge when comparing them to high fidelity observations of solar system comets. observations of gas around main sequence stars, spectroscopic observations of "polluted" white dwarf atmospheres and spectroscopic observations of transiting exocomets suggest that exocomets may show compositional similarities with solar system comets. the recent interstellar visitor 2i/borisov showed gas, dust and nuclear properties similar to that of solar system comets. this raises the tantalising prospect that observations of interstellar comets may help bridge the fields of exocomet and solar system comets. | exocomets from a solar system perspective |
the first computational model of solid-phase chemistry in cometary nuclear ices is presented. an astrochemical kinetics model, the model for astrophysical gas and ice chemical kinetics and layering, is adapted to trace the chemical evolution in multiple layers of cometary ice, over a representative period of 5 gyr. physical conditions are chosen appropriate for “cold storage” of the cometary nucleus in the outer solar system, prior to any active phase. the chemistry is simulated at a selection of static temperatures in the range 5-60 k, while the ice is exposed to the interstellar radiation field, inducing a photochemistry in the outer ice layers that produces significant formation of complex organic molecules. a treatment for the chemistry resulting from cosmic-ray bombardment of the ices is also introduced into the model, along with a new formulation for low-temperature photochemistry. production of simple and complex molecules to depth on the order of 10 m or more is achieved, with local fractional abundances comparable to observed values in many cases. the production of substantial amounts of o2 (and h2o2) is found, suggesting that long-term processing by high-energy cosmic rays of cometary ices in situ, over a period on the order of 1 gyr, may be sufficient to explain the large observed abundances of o2, if the overall loss of material from the comet is limited to a depth on the order of 10 m. entry into the inner solar system could produce a further enhancement in the molecular content of the nuclear ices that may be quantifiable using this modeling approach. | simulations of ice chemistry in cometary nuclei |
context. the presence or absence of ice in protoplanetary disks is of great importance to the formation of planets. by enhancing solid surface density and increasing sticking efficiency, ice catalyzes the rapid formation of planetesimals and decreases the timescale of giant planet core accretion.aims: in this paper, we analyze the composition of the outer disk around the herbig star hd 142527. we focus on the composition of water ice, but also analyze the abundances of previously proposed minerals.methods: we present new herschel far-infrared spectra and a re-reduction of archival data from the infrared space observatory (iso). we modeled the disk using full 3d radiative transfer to obtain the disk structure. also, we used an optically thin analysis of the outer disk spectrum to obtain firm constraints on the composition of the dust component.results: the water ice in the disk around hd 142527 contains a large reservoir of crystalline water ice. we determine the local abundance of water ice in the outer disk (i.e., beyond 130 au). the re-reduced iso spectrum differs significantly from that previously published, but matches the new herschel spectrum at their common wavelength range. in particular, we do not detect any significant contribution from carbonates or hydrous silicates, in contrast to earlier claims.conclusions: the amount of water ice detected in the outer disk requires ~80% of oxygen atoms. this is comparable to the water ice abundance in the outer solar system, comets, and dense interstellar clouds. the water ice is highly crystalline while the temperatures where we detect it are too low to crystallize the water on relevant timescales. we discuss the implications of this finding. | the abundance and thermal history of water ice in the disk surrounding hd 142527 from the digit herschel key program |
we report production rates for h2o and eight trace molecules (co, c2h6, ch4, ch3oh, nh3, h2co, hcn, c2h2) in the dynamically new, sun-grazing comet c/2012 s1 (ison), using high-resolution spectroscopy at keck ii and the nasa irtf on 10 pre-perihelion dates encompassing heliocentric distances rh = 1.21-0.34 au. measured water production rates spanned two orders of magnitude, consistent with a long-term heliocentric power law q(h2o) {{\propto }}{r}{{h}}(-3.1+/- 0.1). abundance ratios for co, c2h6, and ch4 with respect to h2o remained constant with rh and below their corresponding mean values measured among a dominant sample of oort cloud comets. ch3oh was also depleted for rh > 0.5 au, but was closer to its mean value for rh ≤ 0.5 au. the remaining four molecules exhibited higher abundance ratios within 0.5 au: for rh > 0.8 au, nh3 and c2h2 were consistent with their mean values while h2co and hcn were depleted. for rh < 0.5 au, all four were enriched, with nh3, h2co, and hcn increasing most. spatial profiles of gas emission in ison consistently peaked sunward of the dust continuum, which was asymmetric antisunward and remained singly peaked for all observations. nh3 within 0.5 au showed a broad spatial distribution, possibly indicating its release in the coma provided that optical depth effects were unimportant. the column abundance ratio nh2/h2o at 0.83 au was close to the “typical” nh/oh from optical wavelengths, but was higher within 0.5 au. establishing its production rate and testing its parentage (e.g., nh3) require modeling of coma outflow. this paper includes data collected at the w. m. keck observatory, maunakea, hi, usa, operated as a scientific partnership among caltech, ucla, and nasa, and made possible by the generous financial support of the w. m. keck foundation. | en route to destruction: the evolution in composition of ices in comet d/2012 s1 (ison) between 1.2 and 0.34 au from the sun as revealed at infrared wavelengths |
comet dust is primitive and shows significant diversity. our knowledge of the properties of primitive cometary particles has expanded significantly through microscale investigations of cosmic dust samples (anhydrous interplanetary dust particles (idps), chondritic porous (cp) idps and ultracarbonaceous antarctic micrometeorites, stardust and rosetta), as well as through remote sensing (spitzer ir spectroscopy). comet dust are aggregate particles of materials unequilibrated at submicrometre scales. we discuss the properties and processes experienced by primitive matter in comets. primitive particles exhibit a diverse range of: structure and typology; distribution of constituents; concentration and form of carbonaceous and refractory organic matter; mg- and fe-contents of the silicate minerals; sulfides; existence/abundance of type ii chondrule fragments; high-temperature calcium-aluminium inclusions and ameboid-olivine aggregates; and rarely occurring mg-carbonates and magnetite, whose explanation requires aqueous alteration on parent bodies. the properties of refractory materials imply there were disc processes that resulted in different comets having particular selections of primitive materials. the diversity of primitive particles has implications for the diversity of materials in the protoplanetary disc present at the time and in the region where the comets formed. this article is part of the themed issue 'cometary science after rosetta'. | cometary dust: the diversity of primitive refractory grains |
the formation of stars and planetary systems is a complex phenomenon that relies on the interplay of multiple physical processes. nonetheless, it represents a crucial stage for our understanding of the universe, and in particular of the conditions leading to the formation of key molecules (e.g. water) on comets and planets. herschel observations demonstrated that stars form in gaseous filamentary structures in which the main constituent is molecular hydrogen (h2). depending on its nuclear spin h2 can be found in two forms: `ortho' with parallel spins and `para' where the spins are anti-parallel. the relative ratio among these isomers, the ortho-to-para ratio (opr), plays a crucial role in a variety of processes related to the thermodynamics of star-forming gas and to the fundamental chemistry affecting the deuteration of water in molecular clouds, commonly used to determine the origin of water in solar system bodies. here, for the first time, we assess the evolution of the opr starting from the warm neutral medium by means of state-of-the-art 3d magnetohydrodynamic simulations of turbulent molecular clouds. our results show that star-forming clouds exhibit a low opr (≪0.1) already at moderate densities (∼1000 cm−3). we also constrain the cosmic-ray ionisation rate, finding that 10−16 s−1 is the lower limit required to explain the observations of diffuse clouds. our results represent a step forward in the understanding of the star and planet formation processes providing a robust determination of the chemical initial conditions for both theoretical and observational studies. | on the low ortho-to-para h2 ratio in star-forming filaments |
primordial black holes (pbhs) formed in the early universe are sources of neutrinos emitted via hawking radiation. such astrophysical neutrinos could be detected at earth and constraints on the abundance of comet-mass pbhs could be derived from the null observation of this neutrino flux. here, we consider non-rotating pbhs and improve constraints using super-kamiokande neutrino data, as well as we perform forecasts for next-generation neutrino (hyper-kamiokande, juno, dune) and dark matter (darwin, argo) detectors, which we compare. for pbhs less massive than ~ few × 1014 g, pbhs would have already evaporated by now, whereas more massive pbhs would still be present and would constitute a fraction of the dark matter of the universe. we consider monochromatic and extended (log-normal) mass distributions, and a pbh mass range spanning from 1012 g to ~ 1016 g. finally, we also compare our results with previous ones in the literature. | current and future neutrino limits on the abundance of primordial black holes |
aims: the aim of our work is to analyze the light curves of β pic which were recently observed by the transiting exoplanet survey satellite (tess) in sectors 32, 33, and 34 by searching for the signatures of exocomet transits.methods: we processed the β pic light curves from the mikulski archive for space telescopes database, applying the frequency analysis to remove harmonic signals due to the star's pulsations, and we used a simple 1d model to fit the profiles of the events that were found.results: we recovered events previously found by other authors in sectors 5 and 6 and found five new distinct aperiodic dipping events with asymmetric shapes resembling the expected profiles due to the passage of a comet-like body across the star disk. these dips are rather shallow, with the flux drop at a level of 0.03% and a duration of less than 1 day. no periodic transits were found in the sectors investigated.conclusions: the depth and duration of the identified dips are similar to the recently discovered transits in the β pic light curves from sector 5 of the tess observations as well as to those found in the light curves of kic 354116 and kic 1108472 from the kepler database. this indicates that aperiodic shallow dips are not likely an exceptional phenomenon, at least for the β pic system. | new exocomets of β pic |
understanding the chemical past of our sun and how life appeared on earth is no mean feat. the best strategy we can adopt is to study newborn stars located in an environment similar to the one in which our sun was born and assess their chemical content. in particular, hot corinos are prime targets because recent studies have shown correlations between interstellar complex organic molecules abundances from hot corinos and comets. the orion alma new generation survey aims to assess the number of hot corinos in the closest and best analog to our sun's birth environment, the omc-2/3 filament. in this context, we investigated the chemical nature of 19 solar-mass protostars and found that 26% of our sample sources show warm methanol emission indicative of hot corinos. compared to the perseus low-mass star-forming region, where the perseus alma chemistry survey detected hot corinos in ~60% of the sources, the hot corinos seem to be relatively scarce in the omc-2/3 filament. while this suggests that the chemical nature of protostars in orion and perseus is different, improved statistics is needed in order to consolidate this result. if the two regions are truly different, this would indicate that the environment is likely playing a role in shaping the chemical composition of protostars. | the chemical nature of orion protostars: are oranges different from peaches? oranges ii. |
we study the evolution of the plasma environment of comet 67p using measurements of the spacecraft potential from early september 2014 (heliocentric distance 3.5 au) to late march 2015 (2.1 au) obtained by the langmuir probe instrument. the low collision rate keeps the electron temperature high (∼5 ev), resulting in a negative spacecraft potential whose magnitude depends on the electron density. this potential is more negative in the northern (summer) hemisphere, particularly over sunlit parts of the neck region on the nucleus, consistent with neutral gas measurements by the cometary pressure sensor of the rosetta orbiter spectrometer for ion and neutral analysis. assuming constant electron temperature, the spacecraft potential traces the electron density. this increases as the comet approaches the sun, most clearly in the southern hemisphere by a factor possibly as high as 20-44 between september 2014 and january 2015. the northern hemisphere plasma density increase stays around or below a factor of 8-12, consistent with seasonal insolation change. | evolution of the plasma environment of comet 67p from spacecraft potential measurements by the rosetta langmuir probe instrument |
context. the electrostatic potential of a spacecraft, vs, is important for the capabilities of in situ plasma measurements. rosetta has been found to be negatively charged during most of the comet mission and even more so in denser plasmas.aims: our goal is to investigate how the negative vs correlates with electron density and temperature and to understand the physics of the observed correlation.methods: we applied full mission comparative statistics of vs, electron temperature, and electron density to establish vs dependence on cold and warm plasma density and electron temperature. we also used spacecraft-plasma interaction system (spis) simulations and an analytical vacuum model to investigate if positively biased elements covering a fraction of the solar array surface can explain the observed correlations.results: here, the vs was found to depend more on electron density, particularly with regard to the cold part of the electrons, and less on electron temperature than was expected for the high flux of thermal (cometary) ionospheric electrons. this behaviour was reproduced by an analytical model which is consistent with numerical simulations.conclusions: rosetta is negatively driven mainly by positively biased elements on the borders of the front side of the solar panels as these can efficiently collect cold plasma electrons. biased elements distributed elsewhere on the front side of the panels are less efficient at collecting electrons apart from locally produced electrons (photoelectrons). to avoid significant charging, future spacecraft may minimise the area of exposed bias conductors or use a positive ground power system. | a charging model for the rosetta spacecraft |
four interplanetary corotating interaction regions (cirs) were identified during 2016 june-september by the rosetta plasma consortium (rpc) monitoring in situ the plasma environment of the comet 67p/churyumov-gerasimenko (67p) at heliocentric distances of ∼3-3.8 au. the cirs, formed in the interface region between low- and high-speed solar wind streams with speeds of ∼320-400 km s-1 and ∼580-640 km s-1, respectively, are characterized by relative increases in solar wind proton density by factors of ∼13-29, in proton temperature by ∼7-29, and in magnetic field by ∼1-4 with respect to the pre-cir values. the cir boundaries are well defined with interplanetary discontinuities. out of 10 discontinuities, four are determined to be forward waves and five are reverse waves, propagating at ∼5-92 per cent of the magnetosonic speed at angles of ∼20°-87° relative to ambient magnetic field. only one is identified to be a quasi-parallel forward shock with magnetosonic mach number of ∼1.48 and shock normal angle of ∼41°. the cometary ionosphere response was monitored by rosetta from cometocentric distances of ∼4-30 km. a quiet time plasma density map was developed by considering dependences on cometary latitude, longitude, and cometocentric distance of rosetta observations before and after each of the cir intervals. the cirs lead to plasma density enhancements of ∼500-1000 per cent with respect to the quiet time reference level. ionospheric modelling shows that increased ionization rate due to enhanced ionizing (>12-200 ev) electron impact is the prime cause of the large cometary plasma density enhancements during the cirs. plausible origin mechanisms of the cometary ionizing electron enhancements are discussed. | cometary plasma response to interplanetary corotating interaction regions during 2016 june-september: a quantitative study by the rosetta plasma consortium |
we report the first in situ detection of metal ions in the upper atmosphere of mars resulting from the ablation of dust particles from comet siding spring. this detection was carried out by the neutral gas and ion mass spectrometer on board the mars atmosphere and volatile evolution mission. metal ions of na, mg, al, k, ti, cr, mn, fe, co, ni, cu, and zn, and possibly of si, and ca, were identified in the ion spectra collected at altitudes of ~185 km. the measurements revealed that na+ was the most abundant species, and that the remaining metals were depleted with respect to the ci (type 1 carbonaceous chondrites) abundance of na+. the temporal profile and abundance ratios of these metal ions suggest that the combined effects of dust composition, partial ablation, differential upward transport, and differences in the rates of formation and removal of these metal ions are responsible for the observed depletion. | metallic ions in the upper atmosphere of mars from the passage of comet c/2013 a1 (siding spring) |
aims.we carried out an investigation of the surface variegation of comet 67p/churyumov-gerasimenko, the detection of regions showing activity, the determination of active and inactive surface regions of the comet with spectral methods, and the detection of fallback material.methods: we analyzed multispectral data generated with optical, spectroscopic, and infrared remote imaging system (osiris) narrow angle camera (nac) observations via spectral techniques, reflectance ratios, and spectral slopes in order to study active regions. we applied clustering analysis to the results of the reflectance ratios, and introduced the new technique of activity thresholds to detect areas potentially enriched in volatiles.results: local color inhomogeneities are detected over the investigated surface regions. active regions, such as hapi, the active pits of seth and ma'at, the clustered and isolated bright features in imhotep, the alcoves in seth and ma'at, and the large alcove in anuket, have bluer spectra than the overall surface. the spectra generated with osiris nac observations are dominated by cometary emissions of around 700 nm to 750 nm as a result of the coma between the comet's surface and the camera. one of the two isolated bright features in the imhotep region displays an absorption band of around 700 nm, which probably indicates the existence of hydrated silicates. an absorption band with a center between 800-900 nm is tentatively observed in some regions of the nucleus surface. this absorption band can be explained by the crystal field absorption of fe2+, which is a common spectral feature seen in silicates. | variegation of comet 67p/churyumov-gerasimenko in regions showing activity |
the rosetta mission arrived at comet 67 p/churyumov-gerasimenko in summer 2014, after more than 10 years in space. all previous mission encounters with a comet have provided a snapshot of the cometary activity at a given heliocentric distance. in contrast, rosetta has escorted the comet nucleus for an extended period (>2 years) at a large range of cometo-centric and heliocentric distances, which has provided exceptional and unprecedented observing conditions to study, analyse and monitor 67 p during its passage to, through and away from perihelion. one of the biggest challenges of this mission is the development of an observation plan that adequately addresses the mission's science objectives while coping with a largely unknown and continuously evolving environment that constantly modifies the planning constraints. the rosetta science ground segment (rsgs), in support of the project scientist and the science working team, is in charge of translating the high level mission science objectives into a low level pointing and operations plan. we present here the high-level science planning process adopted during the comet escort phase. we describe the main science objectives addressed along the mission lifetime, the different groups involved in the science planning, and the approach followed to translate those requirements into a viable and scientifically valid operations plan. finally, we describe how the science planning scheme has evolved since arrival at the comet to react to the unexpected environment, largely reducing the planning lead times. | the science planning process on the rosetta mission |
solar heating of a cometary surface provides the energy necessary to sustain gaseous activity, through which dust is removed1,2. in this dynamical environment, both the coma3,4 and the nucleus5,6 evolve during the orbit, changing their physical and compositional properties. the environment around an active nucleus is populated by dust grains with complex and variegated shapes7, lifted and diffused by gases freed from the sublimation of surface ices8,9. the visible colour of dust particles is highly variable: carbonaceous organic material-rich grains10 appear red while magnesium silicate-rich11,12 and water-ice-rich13,14 grains appear blue, with some dependence on grain size distribution, viewing geometry, activity level and comet family type. we know that local colour changes are associated with grain size variations, such as in the bluer jets made of submicrometre grains on comet hale-bopp15 or in the fragmented grains in the coma16 of c/1999 s4 (linear). apart from grain size, composition also influences the coma's colour response, because transparent volatiles can introduce a substantial blueing in scattered light, as observed in the dust particles ejected after the collision of the deep impact probe with comet 9p/tempel 117. here we report observations of two opposite seasonal colour cycles in the coma and on the surface of comet 67p/churyumov-gerasimenko through its perihelion passage18. spectral analysis indicates an enrichment of submicrometre grains made of organic material and amorphous carbon in the coma, causing reddening during the passage. at the same time, the progressive removal of dust from the nucleus causes the exposure of more pristine and bluish icy layers on the surface. far from the sun, we find that the abundance of water ice on the nucleus is reduced owing to redeposition of dust and dehydration of the surface layer while the coma becomes less red. | an orbital water-ice cycle on comet 67p from colour changes |
context. on 12 november 2014 the european mission rosetta succeeded in delivering a lander, named philae, on the surface of one of the smallest, low-gravity and most primitive bodies of the solar system, the comet 67p/churyumov-gerasimenko (67p).aims: the aim of this paper is to provide a comprehensive geomorphological and spectrophotometric analysis of philae's landing site (agilkia) to give an essential framework for the interpretation of its in situ measurements.methods: osiris images, coupled with gravitational slopes derived from the 3d shape model based on stereo-photogrammetry were used to interpret the geomorphology of the site. we adopted the hapke model, using previously derived parameters, to photometrically correct the images in orange filter (649.2 nm). the best approximation to the hapke model, given by the akimov parameter-less function, was used to correct the reflectance for the effects of viewing and illumination conditions in the other filters. spectral analyses on coregistered color cubes were used to retrieve spectrophotometric properties.results: the landing site shows an average normal albedo of 6.7% in the orange filter with variations of ~15% and a global featureless spectrum with an average red spectral slope of 15.2%/100 nm between 480.7 nm (blue filter) and 882.1 nm (near-ir filter). the spatial analysis shows a well-established correlation between the geomorphological units and the photometric characteristics of the surface. in particular, smooth deposits have the highest reflectance a bluer spectrum than the outcropping material across the area.conclusions: the featureless spectrum and the redness of the material are compatible with the results by other instruments that have suggested an organic composition. the observed small spectral variegation could be due to grain size effects. however, the combination of photometric and spectral variegation suggests that a compositional differentiation is more likely. this might be tentatively interpreted as the effect of the efficient dust-transport processes acting on 67p. high-activity regions might be the original sources for smooth fine-grained materials that then covered agilkia as a consequence of airfall of residual material. more observations performed by osiris as the comet approaches the sun would help interpreting the processes that work at shaping the landing site and the overall nucleus. | geomorphology and spectrophotometry of philae's landing site on comet 67p/churyumov-gerasimenko |
virtis-m observations of the nucleus of comet 67p/churyumov-gerasimenko acquired from 2014 august to 2015 may have been analysed to investigate surface temporal variability at both seasonal and diurnal scales. the measured reflectance spectra are studied by means of comet spectral indicators (csi) such as slopes in the visible and infrared ranges, and 3.2 μm band area and band centre. csi maps derived from data acquired at different heliocentric distances (from 3.62 to 1.72 au) along the inbound leg of the comet's orbit are used to infer surface water ice abundance. we measure a global scale enrichment of water ice from 2014 august to 2015 may across the body of the comet, along with variability at small spatial scale, possibly related with the local insolation conditions. analysis of water ice diurnal variability is performed on 2014 august observations. water ice appears at the border of receding shadows in the neck of the comet (hapi), sublimating in less than 1 h, after exposure to sunlight. as similar variability is not observed in other regions of the comet, we interpreted this as the expression of a diurnal cycle of sublimation and re-condensation of water ice, triggered by sudden shadowing produced on the neck by the body and the head of the nucleus. | the global surface composition of 67p/churyumov-gerasimenko nucleus by rosetta/virtis. ii) diurnal and seasonal variability |
we use published models of the early solar system evolution with a slow, long-range and grainy migration of neptune to predict the orbital element distributions and the number of modern-day centaurs. the model distributions are biased by the outer solar system origins survey (ossos) simulator and compared with the ossos centaur detections. we find an excellent match to the observed orbital distribution, including the wide range of orbital inclinations which was the most troublesome characteristic to fit in previous models. a dynamical model, in which the original population of outer disk planetesimals was calibrated from jupiter trojans, is used to predict that ossos should detect 11 ± 4 centaurs with semimajor axes of a < 30 au, perihelion distances of q > 7.5 au, and diameter of d > 10 km (absolute magnitude hr< 13.7 for a 6% albedo). this is consistent with 15 actual ossos centaur detections with hr< 13.7. the population of centaurs is estimated to be 21,000 ± 8000 for d > 10 km. the inner scattered disk at 50 < a < 200 au should contain (2.0 ± 0.8) × 107 d > 10 km bodies and the oort cloud should contain (5.0 ± 1.9) × 108 d > 10 km comets. population estimates for different diameter cutoffs can be obtained from the size distribution of jupiter trojans (n(>d) ∝ d -2.1 for 5 < d < 100 km). we discuss model predictions for the large synoptic survey telescope observations of centaurs. | ossos. xix. testing early solar system dynamical models using ossos centaur detections |
the discovery of the second interstellar object 2i/borisov on 2019 august 30 raises the question of whether it was ejected recently from a nearby stellar system. here we compute the asymptotic incoming trajectory of 2i/borisov, based on both recent and pre-discovery data extending back to december 2018, using a range of force models that account for cometary outgassing. from gaia dr2 astrometry and radial velocities, we trace back in time the galactic orbits of 7.4 million stars to look for close encounters with 2i/borisov. the closest encounter we find took place 910 kyr ago with the m0v star ross 573, at a separation of 0.068 pc (90% confidence interval of 0.053-0.091 pc) with a relative velocity of 23 km s-1. this encounter is nine times closer than the closest past encounter identified for the first interstellar object 1i/`oumuamua. ejection of 2i/borisov via a three-body encounter in a binary or planetary system is possible, although such a large ejection velocity is unlikely to be obtained and ross 573 shows no signs of binarity. we also identify and discuss some other recent close encounters, recognizing that if 2i/borisov is more than about 10 myr old, our search would be unlikely to find its parent system. | a search for the origin of the interstellar comet 2i/borisov |
comets are small bodies thought to contain the most pristine material in the solar system. however, since their formation ≈4.5 gy ago, they have been altered by different processes. while not exposed to much electromagnetic radiation, they experience intense particle radiation. galactic cosmic rays and solar energetic particles have a broad spectrum of energies and interact with the cometary surface and subsurface; they are the main source of space weathering for a comet in the kuiper belt or in the oort cloud, and also affect the ice prior to the comet agglomeration. while low-energy particles interact only with the cometary surface, the most energetic ones deposit a significant amount of energy down to tens of meters. this interaction can modify the isotopic ratios in cometary ices and create secondary compounds through radiolysis, such as o2 and h2o2 (paper ii). in this paper, we model the energy deposition of energetic particles as a function of depth using a geant4 application modified to account for the isotope creation process. we quantify the energy deposited in cometary nucleus by galactic cosmic rays and solar energetic particles. the consequences of the energy deposition on the isotopic and chemical composition of cometary ices and their implication on the interpretation of cometary observations, notably of 67p/churyumov gerasimenko by the esa rosetta spacecraft, will be discussed in paper ii. | the effect of cosmic rays on cometary nuclei. i. dose deposition |
the wide angle camera of the osiris instrument on board the rosetta spacecraft is equipped with several narrow-band filters that are centered on the emission lines and bands of various fragment species. these are used to determine the evolution of the production and spatial distribution of the gas in the inner coma of comet 67p with time and heliocentric distance, here between 2.6 and 1.3 au pre-perihelion. our observations indicate that the emission observed in the oh, o i, cn, nh, and nh2 filters is mostly produced by dissociative electron impact excitation of different parent species. we conclude that co2 rather than h2o is a significant source of the [o i] 630 nm emission. a strong plume-like feature observed in the cn and o i filters is present throughout our observations. this plume is not present in oh emission and indicates a local enhancement of the co2/h2o ratio by as much as a factor of 3. we observed a sudden decrease in intensity levels after 2015 march, which we attribute to decreased electron temperatures in the first few kilometers above the surface of the nucleus. | changes in the physical environment of the inner coma of 67p/churyumov-gerasimenko with decreasing heliocentric distance |
most known trans-neptunian objects (tnos) that gravitationally scatter off the giant planets have orbital inclinations that are consistent with an origin from the classical kuiper belt; however, a small fraction of these “scattering tnos” have inclinations that are far too large (i > 45°) for this origin. these scattering outliers have previously been proposed to be interlopers from the oort cloud or evidence of an undiscovered planet. here we test these hypotheses using n-body simulations and the 69 centaurs and scattering tnos detected in the outer solar systems origins survey and its predecessors. we confirm that observed scattering objects cannot solely originate from the classical kuiper belt, and we show that both the oort cloud and a distant planet generate observable highly-inclined scatterers. although the number of highly-inclined scatterers from the oort cloud is ∼3 times less than observed, oort cloud enrichment from the sun’s galactic migration or birth cluster could resolve this. meanwhile, a distant, low-eccentricity 5 m ⊕ planet replicates the observed fraction of highly-inclined scatterers, but the overall inclination distribution is more excited than observed. furthermore, the distant planet generates a longitudinal asymmetry among detached tnos that is less extreme than often presumed and its direction reverses across the perihelion range spanned by known tnos. more complete models that explore the dynamical origins of the planet are necessary to further study these features. with well-characterized observational biases, our work shows that the orbital distribution of detected scattering bodies is a powerful constraint on the unobserved distant solar system. | ossos. xv. probing the distant solar system with observed scattering tnos |
comet 2i/borisov is the first true interstellar comet discovered. here, we present results from observational programs at two indian observatories, 2 m himalayan chandra telescope at the indian astronomical observatory, hanle (hct) and 1.2 m telescope at the mount abu infrared observatory (miro). two epochs of imaging and spectroscopy were carried out at the hct and three epochs of imaging at miro. we found cn to be the dominant molecular emission on both epochs, 2019 november 30 and december 22, at distances of rh = 2.013 and 2.031 au, respectively. the comet was inferred to be relatively depleted in carbon bearing molecules on the basis of low c2 and c3 abundances. we find the production rate ratio, q(c2)/q(cn) = 0.54 ± 0.18, pre-perihelion and q(c2)/q(cn) = 0.34 ± 0.12 post-perihelion. this classifies the comet as being moderately depleted in carbon chain molecules. using the results from spectroscopic observations, we believe the comet to have a chemically heterogeneous surface having variation in abundance of carbon chain molecules. from imaging observations, we infer a dust-to-gas ratio similar to carbon chain depleted comets of the solar system. we also compute the nucleus size to be in the range 0.18 km ≤ r ≤ 3.1 km. our observations show that 2i/borisov's behaviour is analogous to that of the solar system comets. | activity of the first interstellar comet 2i/borisov around perihelion: results from indian observatories |
on 31 august 2019, an interstellar comet was discovered as it passed through the solar system (2i/borisov). on the basis of initial imaging observations, 2i/borisov seemed to be similar to ordinary solar system comets1,2—an unexpected characteristic given the multiple peculiarities of the only known previous interstellar visitor, 1i/`oumuamua3-6. spectroscopic investigations of 2i/borisov identified the familiar cometary emissions from cn (refs. 7-9), c2 (ref. 10), o i (ref. 11), nh2 (ref. 12), oh (ref. 13), hcn (ref. 14) and co (refs. 14,15), revealing a composition similar to that of carbon monoxide-rich solar system comets. at temperatures greater than 700 kelvin, comets also show metallic vapours that are produced by the sublimation of metal-rich dust grains16. observation of gaseous metals had until very recently17 been limited to bright sunskirting and sungrazing comets18-20 and giant star-plunging exocomets21. here we report spectroscopic observations of atomic nickel vapour in the cold coma of 2i/borisov at a heliocentric distance of 2.322 astronomical units—equivalent to an equilibrium temperature of 180 kelvin. nickel in 2i/borisov seems to originate from a short-lived nickel-containing molecule with a lifetime of 340−200+260 seconds at 1 astronomical unit and is produced at a rate of 0.9 ± 0.3 × 1022 atoms per second, or 0.002 per cent relative to oh and 0.3 per cent relative to cn. the detection of gas-phase nickel in the coma of 2i/borisov is in line with the recent identification of this atom—as well as iron—in the cold comae of solar system comets17. | gaseous atomic nickel in the coma of interstellar comet 2i/borisov |
previously proposed mechanisms have difficulty explaining the disruption of comet c/2012 s1 (ison) as it approached the sun. we describe a novel cometary disruption mechanism whereby comet nuclei fragment and disperse through dynamic sublimation pressure, which induces differential stresses within the interior of the nucleus. when these differential stresses exceed its material strength, the nucleus breaks into fragments. we model the sublimation process thermodynamically and propose that it is responsible for the disruption of comet ison. we estimate the bulk unconfined crushing strength of comet ison's nucleus and the resulting fragments to be 0.5 pa and 1-9 pa, respectively, assuming typical jupiter family comet (jfc) albedos. however, if comet ison has an albedo similar to pluto, this strength estimate drops to 0.2 pa for the intact nucleus and 0.6-4 pa for its fragments. regardless of assumed albedo, these are similar to previous strength estimates of jfcs. this suggests that, if comet ison is representative of dynamically new comets, then low bulk strength is a primordial property of some comet nuclei, and not due to thermal processing during migration into the jupiter family. | dynamic sublimation pressure and the catastrophic breakup of comet ison |
we present a model to account for the observed debris discs around young white dwarfs and the presence of metal lines in their spectra. stellar evolution models predict that the mass-loss on the agb will be pulsed; furthermore, observations indicate that the bulk of the mass-loss occurs on the agb. in this case, if the progenitors of the white dwarfs had remnants of planetary formation like the sun's oort cloud or the kuiper belt and a planet lying within that cloud or nearby, we find that up to 2 per cent of the planetesimals will fall either into planet-crossing orbits or into chaotic regions after the mass-loss, depending on the location and mass of the planet (from mars to neptune). this yields a sufficient mass of comets that can be scattered towards the star, form a debris disc and pollute the atmosphere. | polluting white dwarfs with perturbed exo-comets |
in 2004, the rosetta spacecraft was sent to comet 67p/churyumov-gerasimenko for the first ever long-term investigation of a comet. after its arrival in 2014, the spacecraft spent more than two years in immediate proximity to the comet. during these two years, the rosina double focusing mass spectrometer (dfms) onboard rosetta discovered a coma with an unexpectedly complex chemical composition that included many oxygenated molecules. determining the exact cometary composition is an essential first step to understanding of the organic rich chemistry in star forming regions and protoplanetary disks that are ultimately conserved in cometary ices. in this study a joint approach of laboratory calibration and space data analysis was used to perform a detailed identification and quantification of cho-compounds in the coma of 67p/churyumov-gerasimenko. the goal was to derive the cho-compound abundances relative to water for masses up to 100 u. for this study, the may 2015 post-equinox period represent the best bulk abundances of comet 67p/churyumov-gerasimenko. a wide variety of cho-compounds were discovered and their bulk abundances were derived. finally, these results are compared to abundances of cho-bearing molecules in other comets, obtained mostly from ground-based observations and modelling. | cho-bearing molecules in comet 67p/churyumov-gerasimenko |
we present a study of comet c/2017 k2 (panstarrs) using pre-discovery archival data taken from 2013 to 2017. our measurements show that the comet has been marginally increasing in activity since at least 2013 may (heliocentric distance of {r}{{h}}=23.7 {au} pre-perihelion). we estimate the mass-loss rate during the period 2013-2017 as \overline{\dot{m}}≈ (2.4+/- 1.1)× {10}2 kg s-1, which requires a minimum active surface area of ∼10-102 km2 for sublimation of supervolatiles such as co and co2, by assuming a nominal cometary albedo {p}v=0.04+/- 0.02. the corresponding lower limit to the nucleus radius is a few kilometers. our monte carlo dust simulations show that dust grains in the coma are ≳ 0.5 {mm} in radius, with ejection speeds from ∼1 to 3 m s-1, and have been emitted in a protracted manner since 2013, confirming estimates by jewitt et al. the current heliocentric orbit is hyperbolic. our n-body backward dynamical integration of the orbit suggests that the comet is most likely (with a probability of ∼98%) from the oort spike. the calculated median reciprocal of the semimajor axis 1 myr ago was {a}med}-1=(3.61+/- 1.71)× {10}-5 au-1 (in a reference system centered on the solar-system barycenter). | pre-discovery observations and orbit of comet c/2017 k2 (panstarrs) |
context. the diffusion of volatile species on amorphous solid water ice affects the chemistry on dust grains in the interstellar medium as well as the trapping of gases enriching planetary atmospheres or present in cometary material.aims: the aim of the work is to provide diffusion coefficients of ch4 on amorphous solid water (asw) and to understand how they are affected by the asw structure.methods: ice mixtures of h2o and ch4 were grown in different conditions and the sublimation of ch4 was monitored via infrared spectroscopy or via the mass loss of a cryogenic quartz crystal microbalance. diffusion coefficients were obtained from the experimental data assuming the systems obey fick's law of diffusion. monte carlo simulations were used to model the different amorphous solid water ice structures investigated and were used to reproduce and interpret the experimental results.results: diffusion coefficients of methane on amorphous solid water have been measured to be between 10-12 and 10-13 cm2 s-1 for temperatures ranging between 42 k and 60 k. we show that diffusion can differ by one order of magnitude depending on the morphology of amorphous solid water. the porosity within water ice and the network created by pore coalescence enhance the diffusion of species within the pores. the diffusion rates derived experimentally cannot be used in our monte carlo simulations to reproduce the measurements.conclusions: we conclude that fick's laws can be used to describe diffusion at the macroscopic scale, while monte carlo simulations describe the microscopic scale where trapping of species in the ices (and their movement) is considered. | diffusion of ch4 in amorphous solid water |
in this series of papers, we present statistical maps of mirror-mode-like (mm) structures in the magnetosheaths of mars and venus and calculate the probability of detecting them in spacecraft data. we aim to study and compare them with the same tools and a similar payload at both planets. we consider their dependence on extreme ultraviolet (euv) solar flux levels (high and low) and, specific to mars, on mars year (my) as well as atmospheric seasons (four solar longitudes ls). we first use magnetic-field-only criteria to detect these structures and present ways to mitigate ambiguities in their nature. in line with many previous studies at earth, this technique has the advantage of using one instrument (a magnetometer) with good time resolution, facilitating comparisons between planetary and cometary environments. applied to the magnetometer data of the mars atmosphere and volatile evolution (maven) spacecraft from november 2014 to february 2021 (my32-my35), we detect events closely resembling mms lasting in total more than 170 000 s, corresponding to about 0.1 % of maven's total time spent in the martian plasma environment. we calculate mm-like occurrences normalised to the spacecraft's residence time during the course of the mission. detection probabilities are about 1 % at most for any given controlling parameter. in general, mm-like structures appear in two main regions: one behind the shock and the other close to the induced magnetospheric boundary, as expected from theory. detection probabilities are higher on average in low-solar-euv conditions, whereas high-solar-euv conditions see an increase in detections within the magnetospheric tail. we tentatively link the former tendency to two combining effects: the favouring of ion cyclotron waves the closer to perihelion due to plasma beta effects and, possibly, the non-gyrotropy of pickup ion distributions. this study is the first of two on the magnetosheaths of mars and venus. | statistical distribution of mirror-mode-like structures in the magnetosheaths of unmagnetised planets - part 1: mars as observed by the maven spacecraft |
in recent decades, several missions have detected signs of water and other volatiles in cold, permanently shadowed craters near the lunar poles. observations suggest that some of these volatiles could have been delivered by comet impacts and therefore, understanding the impact delivery mechanism becomes key to explaining the origin and distribution of lunar water. during impact, the constituent ices of a comet nucleus vaporize; a significant part of this vapor remains gravitationally bound to the moon, transforming the tenuous, collisionless lunar exosphere into a collisionally thick, transient atmosphere. here, we use numerical simulations to investigate the physical processes governing volatile transport in the transient atmosphere generated after a comet impact, with a focus on how these processes influence the accumulation of water in polar cold traps. it is observed that the transient atmosphere maintains a certain characteristic structure for at least several earth days after impact, during which time volatile transport occurs primarily through low-altitude winds that sweep over the lunar day-side. meanwhile, reconvergence of vapor antipodal to the point of impact results in preferential redistribution of water in the vicinity of the antipode. due to the quantity of vapor that remains gravitationally bound, the atmosphere is sufficiently dense that lower layers are shielded from photodestruction, prolonging the lifetime of water molecules and allowing greater amounts of water to reach cold traps. short-term ice deposition patterns are markedly non-uniform and the variations that arise in simulated volatile abundance between different cold traps could potentially explain variations that have been observed through remote sensing. | transport of water in a transient impact-generated lunar atmosphere |
exposure to coal mining dust poses a substantial health hazard to individuals due to the complex mixture of components released during the extraction process. this study aimed to assess the oxidative potential of residual coal mining dust on human lymphocyte dna and telomeres and to perform a chemical characterization of coal dust and urine samples. the study included 150 individuals exposed to coal dust for over ten years, along with 120 control individuals. the results revealed significantly higher levels of dna damage in the exposed group, as indicated by the standard comet assay, and oxidative damage, as determined by the fpg-modified comet assay. moreover, the exposed individuals exhibited significantly shorter telomeres compared to the control group, and a significant correlation was found between telomere length and oxidative dna damage. using the pixe method on urine samples, significantly higher concentrations of sodium (na), phosphorus (p), sulfur (s), chlorine (cl), potassium (k), iron (fe), zinc (zn), and bromine (br) were observed in the exposed group compared to the control group. furthermore, men showed shorter telomeres, greater dna damage, and higher concentrations of nickel (ni), calcium (ca), and chromium (cr) compared to exposed women. additionally, the study characterized the particles released into the environment through gc–ms analysis, identifying several compounds, including polycyclic aromatic hydrocarbons (pahs) such as fluoranthene, naphthalene, anthracene, 7h-benzo[c]fluorene, phenanthrene, pyrene, benz[a]anthracene, chrysene, and some alkyl derivatives. these findings underscore the significant health risks associated with exposure to coal mining dust, emphasizing the importance of further research and the implementation of regulatory measures to safeguard the health of individuals in affected populations. graphic abstract | the dangerous link between coal dust exposure and dna damage: unraveling the role of some of the chemical agents and oxidative stress |
the interaction between a comet and the impinging solar wind leads to modifications of the magnetic field in the environment of a comet. among those, one finds magnetic field pile-up and draping, which reveal properties of the interaction and are known from previous cometary spacecraft missions. this work studies the magnetic field configuration at comet 67p/churyumov-gerasimenko at 2.0 au. the data reveal a pile-up of the magnetic field and a draping signature nearly perpendicular to the original solar wind flow and the plane containing the solar wind flow and the interplanetary magnetic field. a comparison of the magnetic field data with a hybrid plasma simulation supports this idea of a plasma flow which is strongly deflected from the sun-comet direction and which is in line with other plasma observations by the rosetta plasma consortium. | magnetic field pile-up and draping at intermediately active comets: results from comet 67p/churyumov-gerasimenko at 2.0 au |
the upper silesian coal basin in poland is one of the major european hotspots of ch4 release. until now, no data concerning short-term ch4 emissions from coal mines have been accessible worldwide. they are available only on a yearly timescale. no values are provided on a higher temporal scale, that's why the measurements presented here are of great importance. this paper discusses short-term ch4 emissions from ventilation shafts of three mining fronts (mf) divided into two periods. the concentrations of ch4 in shafts varied from 0.05 to 0.4 %. the highest levels occurred in shaft iv (mf i) and shaft vi (mf ii): from 0.15 to 0.38 % (period 1). these values correspond to emission levels ranging from 27 to 75 m3/min (shaft iv) and from 18 to 40 m3/min (shaft vi). in period 2, the highest concentrations of ch4 occurred in shaft vi (mf ii and iii): from 0.2 to 0.4 %. the most significant ch4 emissions were recorded for shaft vi (mf ii) and ranged from 29 to 54 m3/min. presented data have been used to validate the measurements obtained in the comet campaign, which aimed at verifying the sensitivity of the test equipment operating from aircraft. during the test flights of halo in 2015, the comet team achieved a remarkable consistency of measurements conducted with airborne equipment (26 ± 3m3/min) and the emission data (24.34 m3/min), for shaft vi (mf ii). the analysed short-term data for individual shafts are more reliable and can improve ch4 flux estimates during the comet campaign in 2018. | polish underground coal mines as point sources of methane emission to the atmosphere |
the discovery of ’oumuamua (1i/2017 u1), the first interstellar interloper, suggests an abundance of free-floating small bodies whose ejection into galactic space cannot be explained by the current population of confirmed exoplanets. shortly after ’oumuamua’s discovery, observational results from the disk substructures at high angular resolution project (dsharp) illustrated the near ubiquity of ring/gap substructures within protoplanetary disks, strongly suggesting the existence of a vast population of as-yet undetected wide-separation planets that are capable of efficiently ejecting debris from their environments. these planets have a ≳ 5 au and masses on the order of neptune’s mass or larger, and they may accompany ∼50% of newly formed stars. we combine the dsharp results with statistical constraints from current time-domain surveys to quantify the population of detectable icy planetesimals ejected by disk-embedded giant planets through gravity assists. assessment of the expected statistical distribution of interstellar objects (isos) is critical to accurately plan for and interpret future detections. we show that the number density of isos implied by ’oumuamua is consistent with ’oumuamua itself having originated as an icy planetesimal ejected from a dsharp-type system via gravity assists, with the caveat that ’oumuamua’s lack of observed outgassing remains in strong tension with a cometary origin. under this interpretation, ’oumuamua’s detection points toward a large number of long-period giant planets in extrasolar systems, supporting the hypothesis that the observed gaps in protoplanetary disks are carved by planets. in the case that ’oumuamua is an ejected cometary planetesimal, we conclude that the large synoptic survey telescope (lsst) should detect up to a few isos per year of ’oumuamua’s size or larger and over 100 yr-1 for objects with r > 1 m. | hidden planets: implications from ’oumuamua and dsharp |
during the short period of our observations (from march 2013 to 2015), was fixed falling at least a dozen fragments of cometary nuclei, at least five of sufficiently large and dozens of smaller fragments of meteoroids. the results of our observations also showed that during the morning and evening twilight over kiev clearly visible the plume of aerosols of technical nature from the plants, factories and other production facilities. | several twilight bolides over kiev in 2013-2015 - fragments of comets nuclei |
the accelerometers of the sesame/casse instrument aboard rosetta's lander philae recorded surface waves produced during the mupus hammering phase. after presenting evidence that all feet of philae were in contact with the ground, we analyze group arrival time differences between the three feet of philae and obtain a rayleigh wave velocity between 79 m s-1 and 400 m s-1 , which translate into a shear modulus μ of 3.6 mpa ≤ μ ≤ 346 mpa, and a young's modulus e of 7.2 mpa ≤ e ≤ 980 mpa (with the lower bounds being better constrained than the upper bounds). mixture models of snow and regolith suggest a porosity below 0.74. from the frequency-dependent dispersion of the average signal we conclude that the above values are valid for a surface layer of 10 cm to 50 cm thickness, while rigidity is significantly reduced underneath this layer. our findings are consistent with the concept of a thin consolidated shell around a less rigid interior. | structure and elastic parameters of the near surface of abydos site on comet 67p/churyumov-gerasimenko, as obtained by sesame/casse listening to the mupus insertion phase |
recent measurements carried out at comet 67p/churyumov-gerasimenko (67p) with the rosetta probe revealed that molecular oxygen, o2, is the fourth most abundant molecule in comets. models show that o2 is likely of primordial nature, coming from the interstellar cloud from which our solar system was formed. however, gaseous o2 is an elusive molecule in the interstellar medium with only one detection towards quiescent molecular clouds, in the ρ oph a core. we perform a deep search for molecular oxygen, through the 21-01 rotational transition at 234 ghz of its 16o18o isotopologue, towards the warm compact gas surrounding the nearby class 0 protostar iras 16293-2422 b with the alma interferometer. we also look for the chemical daughters of o2, ho2, and h2o2. unfortunately, the h2o2 rotational transition is dominated by ethylene oxide c-c2h4o while ho2 is not detected. the targeted 16o18o transition is surrounded by two brighter transitions at ± 1 km s-1 relative to the expected 16o18o transition frequency. after subtraction of these two transitions, residual emission at a 3σ level remains, but with a velocity offset of 0.3-0.5 km s-1 relative to the source velocity, rendering the detection "tentative". we derive the o2 column density for two excitation temperatures tex of 125 and 300 k, as indicated by other molecules, in order to compare the o2 abundance between iras 16293 and comet 67p. assuming that 16o18o is not detected and using methanol ch3oh as a reference species, we obtain a [o2]/[ch3oh] abundance ratio lower than 2-5, depending on the assumed tex, a three to four times lower abundance than the [o2]/[ch3oh] ratio of 5-15 found in comet 67p. such a low o2 abundance could be explained by the lower temperature of the dense cloud precursor of iras 16293 with respect to the one at the origin of our solar system that prevented efficient formation of o2 in interstellar ices. | linking interstellar and cometary o2: a deep search for 16o18o in the solar-type protostar iras 16293-2422 |
the isotopic ratios are diagnostics for the physico-chemical conditions governing molecular formation. in comets, 14n/15n ratios have been measured from hcn in three comets and from cn in more than 20 comets. those ratios are enriched in 15n compared to the sun by a factor of ∼3, have a small diversity and do not depend on the dynamical type of the comets. the origin of this high 15n-fractionation is still in debate because cn probably comes not only from hcn, but also from other materials (such as polymers or organic dusts) in the coma. consequently, an interpretation of the isotopic ratios in cometary cn is quite complicated due to the multiple possible parents of cn. in contrast with cn, the isotopic ratios of nitrogen in nh3 give us a much clearer interpretation than in cn because nh3 is directly incorporated in the nuclear ices. to estimate the 14n/15n ratios in nh3, 14n/15n ratios have been determined from high-resolution spectra of nh2 in the optical wavelength region. nh2 is indeed a dominant photodissociation product of nh3. those ratios were also found to be enriched in 15n compared to the sun by a factor of ∼3. in this paper, we present 14n/15n ratios in nh2 for an additional sample of 16 comets. our sample includes short-period comets as well as long-period comets. we found that the 14n/15n ratios in cometary nh2 also show a small dispersion and do not depend on the dynamical origin of the comets. | nitrogen isotopic ratios of nh2 in comets: implication for 15n-fractionation in cometary ammonia |
the ratios of the three stable oxygen isotopes 16o, 17o, and 18o on the earth and, as far as we know in the solar system, show variations on the order of a few per cent at most, with a few outliers in meteorites. however, in the interstellar medium there are some highly fractionated oxygen isotopic ratios in some specific molecules. the goal of this work is to investigate the oxygen isotopic ratios in different volatile molecules found in the coma of comet 67p/churyumov-gerasimenko and compare them with findings from interstellar clouds in order to assess commonalities and differences. to accomplish this goal, we analysed data from the rosina instrument on rosetta during its mission around the comet. 16o/18o ratios could be determined for o2, methanol, formaldehyde, carbonyl sulfide, and sulfur monoxide/dioxide. for o2 the 16o/17o ratio is also available. some ratios are strongly enriched in the heavy isotopes, especially for sulfur-bearing molecules and formaldehyde, whereas for methanol the ratios are compatible with the ones in the solar system. o2 falls in-between, but its oxygen isotopic ratios clearly differ from water, which likely rules out an origin of o2 from water, be it by radiolysis, dismutation during sublimation, or the eley-rideal process from water ions hitting the nucleus as postulated in the literature. | molecule-dependent oxygen isotopic ratios in the coma of comet 67p/churyumov-gerasimenko |
the orbital architecture of the solar system is thought to have been sculpted by a dynamical instability among the giant planets. during the instability a primordial outer disk of planetesimals was destabilized and ended up on planet-crossing orbits. most planetesimals were ejected into interstellar space, but a fraction were trapped on stable orbits in the kuiper belt and oort cloud. we use a suite of n-body simulations to map out the diversity of planetesimals' dynamical pathways. we focus on two processes: tidal disruption from very close encounters with a giant planet, and loss of surface volatiles from repeated passages close to the sun. we show that the rate of tidal disruption is more than a factor of 2 higher for ejected planetesimals than for surviving objects in the kuiper belt or oort cloud. ejected planetesimals are preferentially disrupted by jupiter and surviving ones by neptune. given that the gas giants contracted significantly as they cooled but the ice giants did not, taking into account the thermal evolution of the giant planets decreases the disruption rate of ejected planetesimals. the frequency of volatile loss and extinction is far higher for ejected planetesimals than for surviving ones and is not affected by the giant planets' contraction. even if all interstellar objects were ejected from solar system-like systems, our analysis suggests that their physical properties should be more diverse than those of solar system small bodies as a result of their divergent dynamical histories. this is consistent with the characteristics of the two currently known interstellar objects. | survivor bias: divergent fates of the solar system's ejected versus persisting planetesimals |
with an increasing number of parameters and pre-training data, generative large language models (llms) have shown remarkable capabilities to solve tasks with minimal or no task-related examples. notably, llms have been successfully employed as evaluation metrics in text generation tasks. within this context, we introduce the eval4nlp 2023 shared task that asks participants to explore prompting and score extraction for machine translation (mt) and summarization evaluation. specifically, we propose a novel competition setting in which we select a list of allowed llms and disallow fine-tuning to ensure a focus on prompting. we present an overview of participants' approaches and evaluate them on a new reference-free test set spanning three language pairs for mt and a summarization dataset. notably, despite the task's restrictions, the best-performing systems achieve results on par with or even surpassing recent reference-free metrics developed using larger models, including gemba and comet-kiwi-xxl. finally, as a separate track, we perform a small-scale human evaluation of the plausibility of explanations given by the llms. | the eval4nlp 2023 shared task on prompting large language models as explainable metrics |
aims: the rosetta space probe accompanied comet 67p/churyumov-gerasimenko for more than two years, obtaining an unprecedented amount of unique data of the comet nucleus and inner coma. this has enabled us to study its activity almost continuously from 4 au inbound to 3.6 au outbound, including the perihelion passage at 1.24 au. this work focuses identifying the source regions of faint jets and outbursts and on studying the spectrophotometric properties of some outbursts. we use observations acquired with the osiris/nac camera during july-october 2015, that is, close to perihelion.methods: we analyzed more than 2000 images from nac color sequences acquired with 7-11 filters covering the 250-1000 nm wavelength range. the osiris images were processed with the osiris standard pipeline up to level 3, that is, converted in radiance factor, then corrected for the illumination conditions. for each color sequence, color cubes were produced by stacking registered and illumination-corrected images.results: more than 200 jets of different intensities were identified directly on the nucleus. some of the more intense outbursts appear spectrally bluer than the comet dark terrain in the visible-to-near-infrared region. we attribute this spectral behavior to icy grains mixed with the ejected dust. some of the jets have an extremely short lifetime. they appear on the cometary surface during the color sequence observations, and vanish in less than some few minutes after reaching their peak. we also report a resolved dust plume observed in may 2016 at a resolution of 55 cm pixel-1, which allowed us to estimate an optical depth of ~0.65 and an ejected mass of ~2200 kg, assuming a grain bulk density of ~800 kg m-3. we present the results on the location, duration, and colors of active sources on the nucleus of 67p from the medium-resolution (i.e., 6-10 m pixel-1) images acquired close to perihelion passage. the observed jets are mainly located close to boundaries between different morphological regions. some of these active areas were observed and investigated at higher resolution (up to a few decimeter per pixel) during the last months of operations of the rosetta mission.conclusions: these observations allow us to investigate the link between morphology, composition, and activity of cometary nuclei. jets depart not only from cliffs, but also from smooth and dust-covered areas, from fractures, pits, or cavities that cast shadows and favor the recondensation of volatiles. this study shows that faint jets or outbursts continuously contribute to the cometary activity close to perihelion passage, and that these events are triggered byillumination conditions. faint jets or outbursts are not associated with a particular terrain type or morphology. | linking surface morphology, composition, and activity on the nucleus of 67p/churyumov-gerasimenko |
we present chemical abundances and radial velocities of six h ii regions in the extremely metal-poor star-forming dwarf galaxy ddo 68. they are derived from deep spectra in the wavelength range 3500-10 000 å, acquired with the multi-object double spectrograph at the large binocular telescope (lbt). in the three regions where the [o iii] λ4363 å line was detected, we inferred the abundance of he, n, o, ne, ar, and s through the `direct' method. we also derived the oxygen abundances of all the six regions adopting indirect-method calibrations. we confirm that ddo 68 is an extremely metal-poor galaxy, and a strong outlier in the luminosity-metallicity relation defined by star-forming galaxies. with the direct method, we find indeed an oxygen abundance of 12+log(o/h) = 7.14 ± 0.07 in the northernmost region of the galaxy and, although with large uncertainties, an even lower 12 + log(o/h) = 6.96 ± 0.09 in the `tail'. this is, at face value, the most metal-poor direct abundance detection of any galaxy known. we derive a radial oxygen gradient of -0.06 ± 0.03 dex kpc-1 (or -0.30 dex r_{25}^{-1}) with the direct method, and a steeper gradient of -0.12 ± 0.03 dex kpc-1 (or -0.59 dex r_{25}^{-1}) from the indirect method. for the α-element to oxygen ratios, we obtain values in agreement with those found in other metal-poor star-forming dwarfs. for nitrogen, instead, we infer much higher values, leading to log(n/o)∼- 1.4, at variance with the suggested existence of a tight plateau at -1.6 in extremely metal-poor dwarfs. the derived helium mass fraction ranges from y = 0.240 ± 0.005 to y = 0.25 ± 0.02, compatible with standard big bang nucleosynthesis. finally, we measured h ii region radial velocities in the range 479-522 km s-1 from the tail to the head of the `comet', consistent with the rotation derived in the h i. | chemical abundances and radial velocities in the extremely metal-poor galaxy ddo 68 |
polarization arising from aligned dust grains presents a unique opportunity to study magnetic fields in the diffuse interstellar medium and molecular clouds. polarization from circumstellar regions, accretion disks and comet atmospheres can also be related to aligned dust.to reliably trace magnetic fields quantitative theory of grain alignment is required. formulating the theory that would correspond to observations was one of the longstanding problems in astrophysics. lately this problem has been successfully addressed, and in this review we summarize some of the most important theoretical advances in the theory of grain alignment by radiative torques (rats) that act on realistic irregular dust grains. we discuss an analytical model of rats and the ways to make rat alignment more efficient, e.g. through paramagnetic relaxation when grains have inclusions with strong magnetic response. for very small grains for which rat alignment is inefficient, we also discuss paramagnetic relaxation and a process termed resonance relaxation. we provide an extensive analysis of the observational tests of grain alignment theory. | grain alignment: role of radiative torques and paramagnetic relaxation |
we test the hypothesis that an annually repeatable ca emission excess in mercury's exosphere at a true anomaly angle (taa) of 25° ± 5° is due to particles from comet 2p/encke impacting the surface. by simulating the dynamical evolution of encke particles under planetary perturbations and poynting-robertson drag, we find that millimeter-sized grains ejected 1-2 ×104 years ago encounter mercury at taa = 350°-30°. the timing of the excess emission is consistent with a major dust release episode ≲20 kyr ago, possibly due to encke progenitor breakup. the emission mechanism is likely the direct injection of impact-liberated ca into sunlight rather than nightside surface adsorption for subsequent release at dawn. the timing of dust release from the comet depends on this mechanism; a 10 kyr age is implied by the direct-injection scenario. | the meteoroid stream of comet encke at mercury: implications for mercury surface, space environment, geochemistry, and ranging observations of the exosphere |
comets are made of volatile and refractory material and naturally experience various degrees of sublimation as they orbit around the sun. this gas release, accompanied by dust, represents what is traditionally described as activity. although the basic principles are well established, most details remain elusive, especially regarding the mechanisms by which dust is detached from the surface and subsequently accelerated by the gas flows surrounding the nucleus. during its 2 years rendez-vous with comet 67p/churyumov-gerasimenko, esa's rosetta has observed cometary activity with unprecedented details, in both the inbound and outbound legs of the comet's orbit. this trove of data provides a solid ground on which new models of activity can be built. in this chapter, we review how activity manifests at close distance from the surface, establish a nomenclature for the different types of observed features, discuss how activity is at the same time transforming and being shaped by the topography, and finally address several potential mechanisms. | local manifestations of cometary activity |
we analyze the first color and polarization images of comet ison (c/2012 s1) taken during two measurement campaigns of the hubble space telescope (hst) on utc 2013 april 10 and may 8, when the phase angles of comet ison were α≈13.7° and 12.2°, respectively. we model the particles in the coma using highly irregular agglomerated debris particles. even though the observations were made over a small range of phase angle, the data still place significant constraints on the material properties of the cometary coma. the different photo-polarimetric responses are indicative of spatial chemical heterogeneity of coma in comet ison. for instance, at small projected distances to the nucleus (<500 km), our modeling suggests the cometary particles are composed predominantly of small, highly absorbing particles, such as amorphous carbon and/or organics material heavily irradiated with uv radiation; whereas, at longer projected distances (>1000 km), the refractive index of the particles is consistent with organic matter slightly processed with uv radiation, tholins, mg-fe silicates, and/or mg-rich silicates contaminated with ~10% (by volume) amorphous carbon. the modeling suggests low relative abundances of particles with low material absorption in the visible, i.e., im(m)≤0.02. such particles were detected unambiguously in other comets in the vicinity of nucleus through very strong negative polarization near backscattering (p≈-6%) and very low positive polarization (p≈3-5%) at side scattering. these materials were previously attributed to mg-rich silicates forming a refractory surface layer on the surface of cometary nuclei (zubko et al., 2012). the absence of such particles in comet ison could imply an absence of such a layer on its nucleus. | comet c/2012 s1 (ison) coma composition at ~4 au from hst observations |
we carried out photometric and spectroscopic observations of comet 29p/schwassmann-wachmann 1 at the 6-m bta telescope (sao ras, russia) and the 1.6-m telescope of the national laboratory for astrophysics (lna, brazil) on february 20, 2012, and on may 31, 2011, respectively. the spectra revealed the presence of co+ and n2+ emissions in the cometary coma at a distance of 5.25 au from the sun. the ratio [n2+]/[co+] within the projected slit is 0.013. the images obtained through bvr filters showed a bright, dust coma, indicating a high level of activity. we estimated a colour index and a colour excess for the comet. the parameter afρ, which is used as an indicator of a cometary activity, was measured to be 2584±50 cm in the reference optical aperture of ρ=104 km. the dust production constituted 33 kg/s and 9.3·103 kg/s, it was obtained using different methods. we also investigated the morphology of the comet using image enhancement techniques and found two jets in the coma. | photometric and spectroscopic analysis of comet 29p/schwassmann-wachmann 1 activity |
context. the cometary ionosphere is immersed in fast flowing solar wind. a polarisation electric field may arise for comets much smaller than the gyroradius of pickup ions because ions and electrons respond differently to the solar wind electric field.aims: a situation similar to that found at a low activity comet has been modelled for barium releases in the earth's ionosphere. we aim to use such a model and apply it to the case of comet 67p churyumov-gerasimenko, the target of the rosetta mission. we aim to explain the significant tailward acceleration of cometary ions through the modelled electric field.methods: we obtained analytical solutions for the polarisation electric field of the comet ionosphere using a simplified geometry. this geometry is applicable to the comet in the inner part of the coma as the plasma density integrated along the magnetic field line remains rather constant. we studied the range of parameters for which a significant tailward electric field is obtained and compare this with the parameter range observed.results: observations of the local plasma density and magnetic field strength show that the parameter range of the observations agree very well with a significant polarisation electric field shielding the inner part of the coma from the solar wind electric field.conclusions: the same process gives rise to a tailward directed electric field with a strength of the order of 10% of the solar wind electric field. using a simple cloud model we have shown that the polarisation electric field, which arises because of the small size of the comet ionosphere as compared to the pick up ion gyroradius, can explain the observed significant tailward acceleration of cometary ions and is consistent with the observed lack of influence of the solar wind electric field in the inner coma. | size of a plasma cloud matters. the polarisation electric field of a small-scale comet ionosphere |
the present study aims to evaluate the potential genotoxic and associated factors among coal miners, divided by degree of exposure. blood and buccal smears were collected from 158 workers, who actively participate in different activities in coal mining, and 48 individuals living in the same city but do not have participation in coal mining activities (control group). the workers were divided into three different groups, according to the level of contact with coal extraction. a questionnaire intended to identify factors associated with dna damage was performed in participants. the results regarding oral mucosa micronucleus test showed a significant difference ( p < 0.001) of the worker groups 1 and 2 in relation to the control group, where the group 1 has a higher degree of exposure to coal than group 2. for the lymphocyte micronucleus test and comet assay, there was no significant difference between the exposed groups and control group. there is an association between the outcome and the fact of living in the municipality of the mining company and the exposure to radiation in the last 12 months. besides, the multivariate analysis showed an association of the tail moment with radiation exposure in the last 12 months. thus, the findings of this study reveal genotoxicity in oral mucosa cells of workers exposed to coal and that workers with higher degree of contact with coal have a more pronounced response. | genotoxicity in brazilian coal miners and its associated factors |
context. ultracarbonaceous antarctic micrometeorites (ucamms) represent a small fraction of interplanetary dust particles reaching the earth's surface and contain large amounts of an organic component not found elsewhere. they are most probably sampling a contribution from the outer regions of the solar system to the local interplanetary dust particle (idp) flux.aims: we characterize ucamms composition focusing on the organic matter, and compare the results to the insoluble organic matter (iom) from primitive meteorites, idps, and the earth.methods: we acquired synchrotron infrared microspectroscopy (μftir) and μraman spectra of eight ucamms from the concordia/csnsm collection, as well as n/c atomic ratios determined with an electron microprobe.results: the spectra are dominated by an organic component with a low aliphatic ch versus aromatic c=c ratio, and a higher nitrogen fraction and lower oxygen fraction compared to carbonaceous chondrites and idps. the ucamms carbonyl absorption band is in agreement with a ketone or aldehyde functional group. some of the ir and raman spectra show a c≡n band corresponding to a nitrile. the absorption band profile from 1400 to 1100 cm-1 is compatible with the presence of c-n bondings in the carbonaceous network, and is spectrally different from that reported in meteorite iom. we confirm that the silicate-to-carbon content in ucamms is well below that reported in idps and meteorites. together with the high nitrogen abundance relative to carbon building the organic matter matrix, the most likely scenario for the formation of ucamms occurs via physicochemical mechanisms taking place in a cold nitrogen rich environment, like the surface of icy parent bodies in the outer solar system. the composition of ucamms provides an additional hint of the presence of a heliocentric positive gradient in the c/si and n/c abundance ratios in the solar system protoplanetary disc evolution. part of the equipment used in this work has been financed by the french insu-cnrs program "physique et chimie du milieu interstellaire" (pcmi). | dome c ultracarbonaceous antarctic micrometeorites. infrared and raman fingerprints |
organic matter has been continuously delivered by meteorites and comets to mars since its formation, and possibly formed in situ by abiogenic and/or biogenic processes. this organic matter may be preserved from the harsh oxidizing environment of mars in specific locations. together with water, organic molecules are necessary to the emergence of life as we know it. since the first martian landers, scientists have been searching for organics and until today, only one positive detection has been made by a gas chromatography mass spectrometer (gcms) instrument onboard the curiosity rover. in this article we investigate a complementary approach to guide the search for organic matter using chemcam, the first laser-induced breakdown spectroscopy (libs) instrument on mars. this experimental study focuses on the analysis of carbon and nitrogen libs signatures in organoclay samples and allows the determination of the critical level (lc) and limit of detection (lod) of these elements with libs under mars-like conditions, giving new insights into the search of organic matter on mars. | analysis of carbon and nitrogen signatures with laser-induced breakdown spectroscopy; the quest for organics under mars-like conditions |
the effect of ionizing radiation on solid methane has previously been an area of interest in the astrophysics community. in the late 1980s this interest was further boosted by the possibility of using solid methane as a moderating medium in spallation neutron sources. here we present test results of solid methane moderators commissioned at the isis neutron source, and compare them with a model based on the theory of thermal explosion. good agreement between the moderator test data and our model suggests that the process of radiolysis defect recombination happens at two different temperature ranges: the ;lower temperature; recombination process occurs at around 20 k, with the ;higher temperature; process taking place between 50 and 60 k. we discuss consequences of this mechanism for the designing and operation of solid methane moderators used in advanced neutron sources. we also discuss the possible role of radiolysis defect recombination processes in cryo-volcanism on comets, and suggest an application based on this phenomenon. | solid methane in neutron radiation: cryogenic moderators and cometary cryo volcanism |
high levels of dust have been detected in the immediate vicinity of many stars, both young and old. a promising scenario to explain the presence of this short-lived dust is that these analogues to the zodiacal cloud (or exozodis) are refilled in situ through cometary activity and sublimation. as the reservoir of comets is not expected to be replenished, the presence of these exozodis in old systems has yet to be adequately explained. it was recently suggested that mean-motion resonances with exterior planets on moderately eccentric (ep ≳ 0.1) orbits could scatter planetesimals on to cometary orbits with delays of the order of several 100 myr. theoretically, this mechanism is also expected to sustain continuous production of active comets once it has started, potentially over gyr time-scales. we aim here to investigate the ability of this mechanism to generate scattering on to cometary orbits compatible with the production of an exozodi on long time-scales. we combine analytical predictions and complementary numerical n-body simulations to study its characteristics. we show, using order of magnitude estimates, that via this mechanism, low-mass discs comparable to the kuiper belt could sustain comet scattering at rates compatible with the presence of the exozodis which are detected around solar-type stars, and on gyr time-scales. we also find that the levels of dust detected around vega could be sustained via our proposed mechanism if an eccentric jupiter-like planet were present exterior to the system's cold debris disc. | inner mean-motion resonances with eccentric planets: a possible origin for exozodiacal dust clouds |
the southern hemisphere of the 67p/churyumov-gerasimenko comet has become visible from rosetta only since 2015 march. it was illuminated during the perihelion passage and therefore it contains the regions that experienced the strongest heating and erosion rates, thus exposing the sub-surface most pristine material. in this work we investigate, thanks to the osiris images, the geomorphology, the spectrophotometry and some transient events of two southern hemisphere regions: anhur and part of bes. bes is dominated by outcropping consolidated terrain covered with fine particle deposits, while anhur appears strongly eroded with elongated canyon-like structures, scarp retreats, different kinds of deposits and degraded sequences of strata indicating a pervasive layering. we discovered a new 140 m long and 10 m high scarp formed in the anhur-bes boundary during/after the perihelion passage, close to the area where exposed co2 and h2o ices were previously detected. several jets have been observed originating from these regions, including the strong perihelion outburst, an active pit and a faint optically thick dust plume. we identify several areas with a relatively bluer slope (i.e. a lower spectral slope value) than their surroundings, indicating a surface composition enriched with some water ice. these spectrally bluer areas are observed especially in talus and gravitational accumulation deposits where freshly exposed material had fallen from nearby scarps and cliffs. the investigated regions become spectrally redder beyond 2 au outbound when the dust mantle became thicker, masking the underlying ice-rich layers. | the highly active anhur-bes regions in the 67p/churyumov-gerasimenko comet: results from osiris/rosetta observations |
we present a statistical analysis of the distribution of large-scale topographic features on comet 67p/churyumov-gerasimenko. we observe that the cumulative cliff height distribution across the surface follows a power law with a slope equal to -1.69 ± 0.02. when this distribution is studied independently for each region, we find a good correlation between the slope of the power law and the orbital erosion rate of the surface. for instance, the northern hemisphere topography is dominated by structures on the 100 m scale, while the southern hemisphere topography, illuminated at perihelion, is dominated by 10 m scale terrain features. our study suggests that the current size of a cliff is controlled not only by material cohesion but also by the dominant erosional process in each region. this observation can be generalized to other comets, where we argue that primitive nuclei are characterized by the presence of large cliffs with a cumulative height-power index equal to or above -1.5, while older, eroded cometary surfaces have a power index equal to or below -2.3. in effect, our model shows that a measure of the topography provides a quantitative assessment of a comet's erosional history, that is, its evolutionary age. | constraints on cometary surface evolution derived from a statistical analysis of 67p's topography |
context. there remain many open questions relating to the depletion of elements into dust, e.g., exactly how are c and o incorporated into dust in dense clouds and, in particular, what drives the disappearance of oxygen in the denser interstellar medium?aims: this work is, in part, an attempt to explain the apparently anomalous incorporation of o atoms into dust in dense clouds.methods: we re-visit the question of the depletion of the elements incorporated into the carbonaceous component of interstellar dust, i.e., c, h, o, n and s, in the light of recent analyses of the organics in comets, meteorites and interplanetary dust particles.results: we find that oxygen could be combined with ≈10-20 % of the carbon in the dust in dense regions in the form of a difficult to observe, organic carbonate, (-o-o>c =o), which could explain the unaccounted for 170-255 ppm oxygen depletion.conclusions: we conclude that, while c, o and n atoms are depleted into an amorphous a-c:h:o:n phase, we posit that a significant fraction of c and o atoms could be sequestered into an organic carbonate, which provides a viable solution to the oxygen depletion problem. further, the thermal or photolytic decomposition of this carbonate may have a bearing on the formation of co2 in the ism. | the essential elements of dust evolution. a viable solution to the interstellar oxygen depletion problem? |
context. solar wind charge-changing reactions are of paramount importance to the physico-chemistry of the atmosphere of a comet. the esa/rosetta mission to comet 67p/churyumov-gerasimenko (67p) provides a unique opportunity to study charge-changing processes in situ.aims: to understand the role of these reactions in the evolution of the solar wind plasma and interpret the complex in situ measurements made by rosetta, numerical or analytical models are necessary.methods: we used an extended analytical formalism describing solar wind charge-changing processes at comets along solar wind streamlines. the model is driven by solar wind ion measurements from the rosetta plasma consortium-ion composition analyser (rpc-ica) and neutral density observations from the rosetta spectrometer for ion and neutral analysis-comet pressure sensor (rosina-cops), as well as by charge-changing cross sections of hydrogen and helium particles in a water gas.results: a mission-wide overview of charge-changing efficiencies at comet 67p is presented. electron capture cross sections dominate and favor the production of he and h energetic neutral atoms (enas), with fluxes expected to rival those of h+ and he2+ ions.conclusions: neutral outgassing rates are retrieved from local rpc-ica flux measurements and match rosina estimates very well throughout the mission. from the model, we find that solar wind charge exchange is unable to fully explain the magnitude of the sharp drop in solar wind ion fluxes observed by rosetta for heliocentric distances below 2.5 au. this is likely because the model does not take the relative ion dynamics into account and to a lesser extent because it ignores the formation of bow-shock-like structures upstream of the nucleus. this work also shows that the ionization by solar extreme-ultraviolet radiation and energetic electrons dominates the source of cometary ions, although solar wind contributions may be significant during isolated events. | solar wind charge exchange in cometary atmospheres. iii. results from the rosetta mission to comet 67p/churyumov-gerasimenko |
investigations of the low-temperature radiation-induced transformations in the c2h2-h2o system are relevant to the chemistry occurring in interstellar and cometary ices. in this work, we applied a matrix isolation technique to study the radiation-driven evolution of this system at molecular level in order to get new mechanistic insight. the 1:1 c2h2ṡ ṡ ṡh2o complexes were prepared in various solid noble-gas matrices (ar, kr, xe) and these icy matrices were subjected to x-ray irradiation at 5 k. decomposition of initial complex and formation of products were monitored by fourier-transform infrared (ftir) spectroscopy. it was found that complexation with h2o resulted in significant enhancement of the radiolytic decay of c2h2 molecules and provided new channels for its radiation-induced transformations. ketene (both isolated h2cco and in the form of h2cco-h2 pair), ketenyl radical (hcco), carbon monoxide (co), and methane (ch4) were observed as main products of the c2h2ṡ ṡ ṡh2o radiolysis. in addition, vinyl alcohol (ch2choh) was detected in an ar matrix. the mechanistic interpretation (reaction routes leading to formation of these products) is discussed on the basis of consideration of kinetic dependences and matrix effects. conversion of c2h2ṡ ṡ ṡh2o to ch4 is a prominent example of water-mediated cleavage of the c≡c triple bond, which may occur under prolonged irradiation in rigid media. possible astrochemical implications of the obtained results are highlighted. | radiation-induced chemistry in the c2h2-h2o system at cryogenic temperatures: a matrix isolation study |
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