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when rubble-pile asteroid 2008 tc3 impacted earth on 7 october 2008, the recovered rock fragments indicated that such asteroids can contain exogenic material1,2. however, spacecraft missions to date have only observed exogenous contamination on large, monolithic asteroids that are impervious to collisional disruption3,4. here, we report the presence of metre-scale exogenic boulders on the surface of near-earth asteroid (101955) bennu—the 0.5-km-diameter, rubble-pile target of the osiris-rex mission5 that has been spectroscopically linked to the cm carbonaceous chondrite meteorites6. hyperspectral data indicate that the exogenic boulders have the same distinctive pyroxene composition as the howardite-eucrite-diogenite (hed) meteorites that come from (4) vesta, a 525-km-diameter asteroid that has undergone differentiation and extensive igneous processing7-9. delivery scenarios include the infall of vesta fragments directly onto bennu or indirectly onto bennu's parent body, where the latter's disruption created bennu from a mixture of endogenous and exogenic debris. our findings demonstrate that rubble-pile asteroids can preserve evidence of inter-asteroid mixing that took place at macroscopic scales well after planetesimal formation ended. accordingly, the presence of hed-like material on the surface of bennu provides previously unrecognized constraints on the collisional and dynamical evolution of the inner main belt. | exogenic basalt on asteroid (101955) bennu |
we study the generation of primordial black holes (pbh) in a single field inflection point model of inflation wherein the effective potential is expanded up to the sextic order and the inversion symmetry is imposed such that only even powers are retained in the potential. such a potential allows the existence of an inflection point which leads to a dynamical phase of ultra slow roll evolution, thereby causing an enhancement of the primordial perturbation spectrum at smaller scales. working with a quasi-inflection point in the potential, we find that pbhs can be produced in our scenario in the asteroid-mass window with a nearly monochromatic mass fraction which can account for the total dark matter in the universe. for different choices of quasi-inflection points and other parameters of our model, we can also generate pbhs in higher mass windows but the primordial spectrum of curvature perturbations becomes strongly tilted at the cmb scales. moreover, we study the effects of a reheating epoch after the end of inflation on the pbhs mass fraction and find that an epoch of a matter dominated reheating can shift the mass fraction to a larger mass window as well as increase their fractional contribution to the total dark matter even for the case of a monochromatic mass fraction. | primordial black holes dark matter from inflection point models of inflation and the effects of reheating |
the relative differences in the degree of hydration should be reflected in any classification scheme for aqueously altered meteorites. here we report the bulk mineralogies and degree of hydration in 37 different carbonaceous chondrites: renazzo-like (cr), mighei-like (cm), and ungrouped (type 2) samples. this is achieved by quantifying the modal abundances of all major (phases present in abundances >1 wt.%) minerals using position sensitive detector x-ray diffraction (psd-xrd). from these modal abundances, a classification scheme is constructed that is based on the normalized fraction of phyllosilicate (total phyllosilicate/total anhydrous silicate + total phyllosilicate). samples are linearly ranked from type 3.0 - corresponding to a phyllosilicate fraction of <0.05, to type 1.0 - corresponding to a total phyllosilicate fraction of >0.95. powdered meteorite samples from any hydrated carbonaceous chondrite group can be ranked on this single classification scale. the resulting classifications for crs exhibit a range from type 2.8 to 1.3, while for cms the range is 1.7-1.2. the primary manifestation of aqueous alteration is the production of phyllosilicate, which ceased when the fluid supply was exhausted, leading to the preservation of anhydrous silicates in all samples. the variability in hydration indicates that either accretion of ices was heterogeneous or fluid was mobilized. from the bulk mineral abundances of the most hydrated samples, we infer that the initial mass fraction of h2o inside of their parent body(ies) asteroids was <20 wt.%. bulk carbonaceous chondrite mineralogy evolved towards increasingly oxidizing assemblages as the extent of bulk hydration increased. this is consistent with the escape of reducing h2 gas that is predicted to have been produced from water during hydration reactions. | classification of hydrous meteorites (cr, cm and c2 ungrouped) by phyllosilicate fraction: psd-xrd modal mineralogy and planetesimal environments |
we present an experimental study on ion irradiation of carbonaceous chondrites, simulating solar wind irradiation on primitive asteroids, to better constrain the space weathering processes of low albedo objects. the irradiations were performed on pressed pellets of the cv allende, co frontier mountain 95002 and lancé, cm mighei, ci alais, and ungrouped tagish lake meteorites, as well as on some silicate samples (olivine and diopside). we used 40kev he+ with fluences up to 6 × 1016 ions/cm2 corresponding to timescales of 103-104 years for an object in the main belt. reflectance spectra were acquired ex situ before and after irradiations in the visible to mid-infrared range (0.4-16 μm). several spectral modifications are observed. in the mir range, we observe a shift of the phyllosilicates (near 3 and 10 μm) and silicates (near 10 μm) bands toward longer wavelength. in the visible-nir range, spectral darkening and reddening are observed for some samples, while others show spectral brightening and blueing. results are also compared with previous irradiation on ordinary and carbonaceous chondrites. we find that the spectral modifications in the visible range are correlated with the initial albedo/composition. we propose a model for space weathering effects on low albedo objects, showing that those with initial albedo between 5 and 9% shall not suffer spwe effects in the visible range. these experiments provide new clues on spectroscopic features modifications within the visible-infrared ranges that could be detected in situ by future sample return missions (hayabusa-2/jaxa and osiris-rex/nasa). | ion irradiation of carbonaceous chondrites: a new view of space weathering on primitive asteroids |
we study the prospects of future gravitational wave (gw) detectors in probing primordial black hole (pbh) binaries. we show that across a broad mass range from 10-5 m⊙ to 107 m⊙, future gw interferometers provide a potential probe of the pbh abundance that is more sensitive than any currently existing experiment. in particular, we find that galactic pbh binaries with masses as low as 10-5 m⊙ may be probed with et, aedge and lisa by searching for nearly monochromatic continuous gw signals. such searches could independently test the pbh interpretation of the ultrashort microlensing events observed by ogle. we also consider the possibility of observing gws from asteroid mass pbh binaries through graviton-photon conversion. | prospects for probing gravitational waves from primordial black hole binaries |
hayabusa2 arrived at the c-type asteroid ryugu in june 2018. during one and a half year of the ryugu-proximity operation, we succeeded in two rovers landing, one lander landing, two spacecraft touchdown/sample collection, one kinetic impact operation and two tiny reflective balls and one rover orbiting. among the two successful touchdowns, the second one succeeded in collecting subsurface material exposed by the kinetic impact operation. this paper describes the asteroid proximity operation activity of the hayabusa2 mission, and gives an overview of the achievements done so far. some important engineering and scientific activities, which have been done in synchronous with the spacecraft operations to tackle with unexpected ryugu environment, are also described. | hayabusa2 mission status: landing, roving and cratering on asteroid ryugu |
beyond-standard-model extensions of qcd could result in quark and gluon confinement occurring well above at temperature around the gev scale. these models can also alter the order of the qcd phase transition. therefore, the enhanced production of primordial black holes (pbhs) that can accompany the change in relativistic degrees of freedom at the qcd transition could favor the production of pbhs with mass scales smaller than the standard model qcd horizon scale. consequently, and unlike pbhs associated with a standard gev-scale qcd transition, such pbhs can account for all the dark matter abundance in the unconstrained asteroid-mass window. this links beyond-standard-model modifications of qcd physics over a broad range of unexplored temperature regimes (around 10 -103 tev ) with microlensing surveys searching for pbhs. additionally, we discuss implications of these models for gravitational wave experiments. we show that a first-order qcd phase transition at around 7 tev is consistent with the subaru hyper-suprime cam candidate event, while a transition of around 70 gev is consistent with ogle candidate events and could also account for the claimed nanograv gravitational wave signal. | signatures of a high temperature qcd transition in the early universe |
we investigate gravitational microlensing signals produced by a spatially extended object transiting in front of a finite-sized source star. the most interesting features arise for lens and source sizes comparable to the einstein radius of the setup. using this information, we obtain constraints from the subaru-hsc survey of m31 on the dark matter populations of nfw subhalos and boson stars of asteroid to earth masses. these lens profiles capture the qualitative behavior of a wide range of dark matter substructures. we find that dark matter fractions down to 5 ×10-3 may be probed, and that deviations from constraints on pointlike lenses (e.g., primordial black holes and machos) become visible for lenses of radius 0.1 r⊙ and larger, with the upper bound on lens masses weakening with increasing lens size. | subaru-hsc through a different lens: microlensing by extended dark matter structures |
the surface strength of small rubble-pile asteroids, which are aggregates of unconsolidated material under microgravity, is poorly constrained but critical to understanding surface evolution and geologic history of the asteroid. here we use images of an impact ejecta deposit and downslope avalanche adjacent to a 70-m-diameter impact crater on the rubble-pile asteroid (101955) bennu to constrain the asteroid's surface properties. we infer that the ejecta deposited near the crater must have been mobilized with velocities less than bennu's escape velocity (20 cm s-1); such low velocities can be explained only if the effective strength of the local surface is exceedingly low, nominally ≤2 pa. this value is four orders of magnitude below strength values commonly used for asteroid surfaces, but it is consistent with recent estimates of internal strength of rubble-pile asteroids and with the surface strength of another rubble-pile asteroid, ryugu. we find a downslope avalanche indicating a surface composed of material readily mobilized by impacts and that has probably been renewed multiple times since bennu's initial assembly. compared with stronger surfaces, very weak surfaces imply (1) more retention of material because of the low ejecta velocities and (2) lower crater-based age estimates—although the heterogeneous structure of rubble piles complicates interpretation. | low surface strength of the asteroid bennu inferred from impact ejecta deposit |
a major challenge for gravitational-wave (gw) detection in the μ hz band is engineering a test mass (tm) with sufficiently low acceleration noise. we propose a gw detection concept using asteroids located in the inner solar system as tms. our main purpose is to evaluate the acceleration noise of asteroids in the μ hz band. we show that a wide variety of environmental perturbations are small enough to enable an appropriate class of ∼10 km -diameter asteroids to be employed as tms. this would allow a sensitive gw detector in the band (few ) ×10-7 hz ≲fgw≲(few ) ×10-5 hz , reaching strain hc∼10-19 around fgw∼10 μ hz , sufficient to detect a wide variety of sources. to exploit these asteroid tms, human-engineered base stations could be deployed on multiple asteroids, each equipped with an electromagnetic transmitter/receiver to permit measurement of variations in the distance between them. we discuss a potential conceptual design with two base stations, each with a space-qualified optical atomic clock measuring the round-trip electromagnetic pulse travel time via laser ranging. trade space exists to optimize multiple aspects of this mission: for example, using a radio-ranging or interferometric link system instead of laser ranging. this motivates future dedicated technical design study. this mission concept holds exceptional promise for accessing this gw frequency band. | asteroids for μ hz gravitational-wave detection |
the composition of asteroids and their connection to meteorites provide insight into geologic processes that occurred in the early solar system. we present spectra of the nightingale crater region on near-earth asteroid bennu with a distinct infrared absorption around 3.4 micrometers. corresponding images of boulders show centimeters-thick, roughly meter-long bright veins. we interpret the veins as being composed of carbonates, similar to those found in aqueously altered carbonaceous chondrite meteorites. if the veins on bennu are carbonates, fluid flow and hydrothermal deposition on bennu’s parent body would have occurred on kilometer scales for thousands to millions of years. this suggests large-scale, open-system hydrothermal alteration of carbonaceous asteroids in the early solar system. | bright carbonate veins on asteroid (101955) bennu: implications for aqueous alteration history |
the concept of local climate zone (lcz) was developed to quantify the relationship between urban morphology and urban heat island (uhi) phenomenon. each lcz is supposed to represent homogeneous air temperature. however, there is inadequate data for verifying the air temperature differences between lcz classes. therefore, it is necessary to utilize alternative temperature data which allow more comprehensive assessment of the effect of lcz on local climatic conditions. land surface temperature (lst) acquired from satellite images can be used to establish the relationship between lst and lcz by providing continuous data on surface temperature. this paper aims to investigate how lst represents the uhi intensity determined by using an improved method of the world urban database and portal tool (wudapt) to develop the lcz map of the yangtze river delta (yrd) megaregion. the results show that lst in different yrd cities is generally consistent with the lcz classes with higher lst observed in built-up lcz classes. the diverse urban morphology and temporal vegetation variation are likely the reasons to inconsistencies in lcz 9, and lcz a to d. findings of this paper provide a better understanding of how urban morphology affects local climate and more accurate delineation of lcz classes. | investigating the relationship between local climate zone and land surface temperature using an improved wudapt methodology - a case study of yangtze river delta, china |
we investigate a possibility of primordial black hole (pbh) formation with a hierarchical mass spectrum in multiple phases of inflation. as an example, we find that one can simultaneously realize a mass spectrum that has recently attracted a lot of attention: stellar-mass pbhs (∼o (10 ) m⊙ ) as a possible source of binary black holes detected by ligo/virgo collaboration, asteroid-mass (∼o (10-12) m⊙) as a main component of dark matter, and earth-mass (∼o (10-5) m⊙) as a source of ultrashort-timescale events in optical gravitational lensing experiment microlensing data. the recent refined de sitter swampland conjecture may support such a multiphase inflationary scenario with hierarchical mass pbhs as a transition signal of each inflationary phase. | primordial black hole tower: dark matter, earth-mass, and ligo black holes |
during its approach to asteroid (101955) bennu, nasa's origins, spectral interpretation, resource identification, and security-regolith explorer (osiris-rex) spacecraft surveyed bennu's immediate environment, photometric properties, and rotation state. discovery of a dusty environment, a natural satellite, or unexpected asteroid characteristics would have had consequences for the mission's safety and observation strategy. here we show that spacecraft observations during this period were highly sensitive to satellites (sub-meter scale) but reveal none, although later navigational images indicate that further investigation is needed. we constrain average dust production in september 2018 from bennu's surface to an upper limit of 150 g s-1 averaged over 34 min. bennu's disk-integrated photometric phase function validates measurements from the pre-encounter astronomical campaign. we demonstrate that bennu's rotation rate is accelerating continuously at 3.63 ± 0.52 × 10-6 degrees day-2, likely due to the yarkovsky-o'keefe-radzievskii-paddack (yorp) effect, with evolutionary implications. | the operational environment and rotational acceleration of asteroid (101955) bennu from osiris-rex observations |
we present the discovery of asassn-18ey (maxi j1820+070), a new black hole low-mass x-ray binary (lmxb) discovered by the all-sky automated survey for supernovae (asas-sn). a week after asas-sn discovered asassn-18ey as an optical transient, it was detected as an x-ray transient by maxi/gcs. here, we analyze asas-sn and asteroid terrestrial-impact last alert system pre-outburst optical light curves, finding evidence of intrinsic variability for several years prior to the outburst. while there was no long-term rise leading to the outburst, as has been seen in several other systems, the start of the outburst in the optical preceded that in the x-rays by 7.20 ± 0.97 days. we analyze the spectroscopic evolution of asassn-18ey from pre-maximum to >100 days post-maximum. the spectra of asassn-18ey exhibit broad, asymmetric, double-peaked hα emission. the bowen blend (λ ≈ 4650 å) in the post-maximum spectra shows highly variable double-peaked profiles, likely arising from irradiation of the companion by the accretion disk, typical of low-mass x-ray binaries. the optical and x-ray luminosities of asassn-18ey are consistent with black hole low-mass x-ray binaries, both in outburst and quiescence. | asassn-18ey: the rise of a new black hole x-ray binary |
we describe a methodology of estimating the size-frequency distribution (sfd) of near-earth asteroids (neas). we estimate the completion versus size of present surveys based on the re-detection ratio, that is, the fraction of all detections over a recent period that are re-detections of already discovered objects rather than new discoveries. the re-detection ratio is a robust measure of completion, but must be corrected for the obvious bias caused by differences in ease of discovery due to specific orbital geometries. we do this with a computer survey simulation using a large set of synthetic orbital elements matching as best possible the distribution of the real nea population. once suitably "calibrated" to match re-detections of the real survey, the completion estimate versus size derived from the simulation can be extended both to large size where few if any new detections are recorded, and to small sizes beyond where re-detection numbers are statistically significant, thereby providing an estimate of the population and survey completion over the entire range from the largest neas down to the smallest sizes detected (∼3 m diameter). here we update our previous population estimates and survey progress, using discoveries by surveys from august, 2012 through july, 2014. we estimate that there are 990 ± 20 neas larger than 1 km in diameter (absolute magnitude h ⩽ 17.75), of which about 90% have been discovered as of august, 2014. we confirm a "dip" in the sfd, in the range from a few tens to a few hundreds of meters diameter, which may be due to the transition from larger "rubble pile" bodies to smaller "monolithic" bodies. we compare our population estimate at the smallest sizes with recent ones based on bolide frequency and find excellent agreement, within estimated errors. the same survey simulation methodology can be used to investigate population and survey completion of various subset populations, for example earth-crossing asteroids (ecas, with orbits crossing 1 au heliocentric distance), potentially hazardous asteroids (phas, with orbits passing within 0.05 au of the earth's orbit), or interior to earth asteroids (ieos, with orbits entirely interior to the earth's orbit). lastly, we have investigated the population and completion of so-called "arm-target" asteroids, of size ∼10 m diameter in orbits passing within 0.03 au of the earth's orbit with very low earth-encounter velocity, <2.5 km/s. we find current ground-based surveys are remarkably efficient in detecting this subset of neas, and are currently about 1% complete, implying a total population of such bodies of only a few thousand. | the population of near-earth asteroids |
impact glasses found in lunar soils provide a possible window into the impact history of the inner solar system. however, their use for precise reconstruction of this history is limited by an incomplete understanding of the physical mechanisms responsible for their origin and distribution and possible relationships to local and regional geology. here, we report u-pb isotopic dates and chemical compositions of impact glasses from the chang'e-5 soil and quantitative models of impact melt formation and ejection that account for the compositions of these glasses. the predominantly local provenance indicated by their compositions, which constrains transport distances to <~150 kilometers, and the age-frequency distribution are consistent with formation mainly in impact craters 1 to 5 kilometers in diameter. based on geological mapping and impact cratering theory, we tentatively identify specific craters on the basaltic unit sampled by chang'e-5 that may have produced these glasses and compare their ages with the impact record of the asteroid belt. the chronology of chang'e-5 impact glasses suggests a dynamical link between lunar impacts and collisions in the asteroid belt. | constraining the formation and transport of lunar impact glasses using the ages and chemical compositions of chang'e-5 glass beads |
the earthcare satellite mission's objective is to retrieve profiles of aerosol and water cloud physical properties from measurements made by its cloud-profiling radar, backscattering lidar, and passive multi-spectral imager. these retrievals, together with other geophysical properties, are input into broadband (bb) radiative transfer (rt) models that predict radiances and fluxes commensurate with measurements made and inferred from earthcare's bb radiometer (bbr). the scientific goal is that modelled and "observed" bb top-of-atmosphere (toa) fluxes differ, on average, by less than ±10 w m−2. when sound synergistic retrievals from the acm-cap process (acm: atlid - backscattering lidar, cpr - cloud-profiling radar, and msi - multi-spectral imager; cap: clouds, aerosols, and precipitation) are available, they are acted on by the rt models. when they are not available, the rt models act on "composite" profiles of properties retrieved from measurements made by individual sensors. compositing is performed in the acm-com (com: composite) process. the majority of this report describes the rt models - and their products - that make up earthcare's acm-rt process. profiles of bb shortwave (sw) and longwave (lw) fluxes and heating rates (hrs) are computed by 1d rt models for each ∼ 1 km nadir column of inferred properties. three-dimensional rt models compute radiances for the bbr's three viewing directions, with the sw model also computing flux and hr profiles; the 3d lw model produces upwelling flux at just one level. all 3d rt products are averages over 5×21 km "assessment domains" that are constructed using msi data. some of acm-rt's products are passed forward to the "radiative closure assessment" process that quantifies, for each assessment domain, the likelihood that earthcare's goal has been achieved. as earthcare represents the first mission to make "operational" use of 3d rt models, emphasis is placed on differences between 1d and 3d rt results. for upwelling sw flux at 20 km altitude, 1d and 3d values can be expected to differ by more than earthcare's scientific goal of ±10 w m−2 at least 50 % of the time. | broadband radiative quantities for the earthcare mission: the acm-com and acm-rt products |
the extraterrestrial materials returned from asteroid (162173) ryugu consist predominantly of low-temperature aqueously formed secondary minerals and are chemically and mineralogically similar to ci (ivuna-type) carbonaceous chondrites. here, we show that high-temperature anhydrous primary minerals in ryugu and ci chondrites exhibit a bimodal distribution of oxygen isotopic compositions: 16 o-rich (associated with refractory inclusions) and 16 o-poor (associated with chondrules). both the 16 o-rich and 16 o-poor minerals probably formed in the inner solar protoplanetary disk and were subsequently transported outward. the abundance ratios of the 16 o-rich to 16 o-poor minerals in ryugu and ci chondrites are higher than in other carbonaceous chondrite groups but are similar to that of comet 81p/wild2, suggesting that ryugu and ci chondrites accreted in the outer solar system closer to the accretion region of comets. c-type asteroid ryugu formed in the outer solar system close to the accretion region of 81p/wild2 comet. | oxygen isotopes of anhydrous primary minerals show kinship between asteroid ryugu and comet 81p/wild2 |
burn markers are observed in many records of the cretaceous-paleogene asteroid impact and mass extinction event. these materials could be derived from wildfires on land or from sedimentary rocks hit by the asteroid. we present a detailed record of molecular burn markers (polycyclic aromatic hydrocarbons [pahs]) from the chicxulub crater and in ocean sediments distant from the impact site. pah features indicate rapid heating and a fossil carbon source and are consistent with sedimentary carbon ejected from the impact crater and dispersed by the atmosphere. target rock-derived soot immediately contributed to global cooling and darkening that curtailed photosynthesis and caused widespread extinction. pah evidence indicates wildfires were present but less influential on global climate and extinction. | organic matter from the chicxulub crater exacerbated the k-pg impact winter |
current practice for asteroid close proximity maneuvers requires extremely accurate characterization of the environmental dynamics and precise spacecraft positioning prior to the maneuver. this creates a delay of several months between the spacecraft's arrival and the ability to safely complete close proximity maneuvers. in this work we develop an adaptive integrated guidance, navigation, and control system that can complete these maneuvers in environments with unknown dynamics, with initial conditions spanning a large deployment region, and without a shape model of the asteroid. the system is implemented as a policy optimized using reinforcement meta-learning. the lander is equipped with an optical seeker that locks to either a terrain feature, reflected light from a targeting laser, or an active beacon, and the policy maps observations consisting of seeker angles and lidar range readings directly to engine thrust commands. the policy implements a recurrent network layer that allows the deployed policy to adapt real time to both environmental forces acting on the agent and internal disturbances such as actuator failure and center of mass variation. we validate the guidance system through simulated landing maneuvers in a six degrees-of-freedom simulator. the simulator randomizes the asteroid's characteristics such as solar radiation pressure, density, spin rate, and nutation angle, requiring the guidance and control system to adapt to the environment. we also demonstrate robustness to actuator failure, sensor bias, and changes in the lander's center of mass and inertia tensor. finally, we suggest a concept of operations for asteroid close proximity maneuvers that is compatible with the guidance system. | terminal adaptive guidance via reinforcement meta-learning: applications to autonomous asteroid close-proximity operations |
evidence is mounting that the small bodies of our solar system, such as comets and asteroids, have at least partially inherited their chemical composition from the first phases of the solar system formation. it then appears that the molecular complexity of these small bodies is most likely related to the earliest stages of star formation. it is therefore important to characterize and to understand how the chemical evolution changes with solar-type protostellar evolution. we present here the large program `astrochemical surveys at iram' (asai). its goal is to carry out unbiased millimetre line surveys between 80 and 272 ghz of a sample of 10 template sources, which fully cover the first stages of the formation process of solar-type stars, from pre-stellar cores to the late protostellar phase. in this paper, we present an overview of the surveys and results obtained from the analysis of the 3 mm band observations. the number of detected main isotopic species barely varies with the evolutionary stage and is found to be very similar to that of massive star-forming regions. the molecular content in o- and c-bearing species allows us to define two chemical classes of envelopes, whose composition is dominated by either (a) a rich content in o-rich complex organic molecules, associated with hot corino sources, or (b) a rich content in hydrocarbons, typical of warm carbon-chain chemistry sources. overall, a high chemical richness is found to be present already in the initial phases of solar-type star formation. | astrochemical evolution along star formation: overview of the iram large program asai |
the osiris-rex camera suite (ocams) will acquire images essential to collecting a sample from the surface of bennu. during proximity operations, these images will document the presence of satellites and plumes, record spin state, enable an accurate model of the asteroid's shape, and identify any surface hazards. they will confirm the presence of sampleable regolith on the surface, observe the sampling event itself, and image the sample head in order to verify its readiness to be stowed. they will document bennu's history as an example of early solar system material, as a microgravity body with a planetesimal size-scale, and as a carbonaceous object. ocams is fitted with three cameras. the mapcam will record color images of bennu as a point source on approach to the asteroid in order to connect bennu's ground-based point-source observational record to later higher-resolution surface spectral imaging. the samcam will document the sample site before, during, and after it is disturbed by the sample mechanism. the polycam, using its focus mechanism, will observe the sample site at sub-centimeter resolutions, revealing surface texture and morphology. while their imaging requirements divide naturally between the three cameras, they preserve a strong degree of functional overlap. ocams and the other spacecraft instruments will allow the osiris-rex mission to collect a sample from a microgravity body on the same visit during which it was first optically acquired from long range, a useful capability as humanity reaches out to explore near-earth, main-belt and jupiter trojan asteroids. | ocams: the osiris-rex camera suite |
spacecraft missions have observed regolith blankets of unconsolidated subcentimetre particles on stony asteroids1-3. telescopic data have suggested the presence of regolith blankets also on carbonaceous asteroids, including (101955) bennu4 and (162173) ryugu5. however, despite observations of processes that are capable of comminuting boulders into unconsolidated materials, such as meteoroid bombardment6,7 and thermal cracking8, bennu and ryugu lack extensive areas covered in subcentimetre particles7,9. here we report an inverse correlation between the local abundance of subcentimetre particles and the porosity of rocks on bennu. we interpret this finding to mean that accumulation of unconsolidated subcentimetre particles is frustrated where the rocks are highly porous, which appears to be most of the surface10. the highly porous rocks are compressed rather than fragmented by meteoroid impacts, consistent with laboratory experiments11,12, and thermal cracking proceeds more slowly than in denser rocks. we infer that regolith blankets are uncommon on carbonaceous asteroids, which are the most numerous type of asteroid13. by contrast, these terrains should be common on stony asteroids, which have less porous rocks and are the second-most populous group by composition13. the higher porosity of carbonaceous asteroid materials may have aided in their compaction and cementation to form breccias, which dominate the carbonaceous chondrite meteorites14. | fine-regolith production on asteroids controlled by rock porosity |
meteorites provide a unique insight into early solar system processes. however, to fully interpret this record requires that these meteorites are related back to their source asteroids and ultimately to the original planetesimal population that formed early in solar system history. as a first step in this process an assessment has been undertaken of the likely number of distinct source asteroids sampled by meteorites and related extraterrestrial materials. the results of this survey indicate that there are between 95 and 148 parent bodies represented in our sample collections. this number has been steadily increasing as new "anomalous" meteorites are characterized. attempts to link these parent bodies to identified asteroidal sources has so far been of limited success, due to the non-unique reflectance spectra of almost all known asteroids. asteroid (4) vesta and the hed (howardites, eucrite, diogenite) meteorites are the best example of a relatively non-disputed asteroid-meteorite linkage. as part of this study the "parent body" concept has been examined and was found to be a widely, but loosely, used term in the literature to designate "a body that supplies meteorites to earth." this concept could be rendered more meaningful by discriminating between primary and secondary parent bodies. a primary parent body is the source asteroid from which the meteorite is ultimately derived, and a secondary parent body is an asteroid derived through impact or break-up of the primary body. a clear example of this usage is provided by (4) vesta, with the main asteroid being the primary parent body and the vestoids representing secondary parent bodies. the concept of primary vs. secondary parent bodies may have important implications for early solar system evolution. chondritic parent bodies are known to have accreted between 1 and 4 myr after cais. this timing difference may reflect the fact that their source asteroids, particularly those of the carbonaceous chondrites, are secondary bodies, with the original cai-bearing primary bodies destroyed during early collisional processing. the number of primary parent bodies represented by meteorites (95-148) appears low when compared to the estimated number of asteroids in the main belt (>100,000 with diameters exceeding ∼2 km). a range of potential reasons may explain this apparent mismatch: (i) meteorites provide an unrepresentative sampling of the main belt, (ii) the belt may only contain a limited number of primary parent bodies, (iii) meteorites may be preferentially derived from the ∼120 identified asteroid families, (iv) loosely consolidated types are filtered by earth's atmosphere, (v) multiple, near-identical, "clone" parent bodies may be present in the belt. at present, it is not possible to determine which of these potential mechanisms are dominant and all may be operating to a greater or lesser extent. based on classical accretion models the meteorite record appears to be highly unrepresentative of the primordial asteroid population. in contrast, pebble accretion models suggest that these first-generation bodies may have been relatively large, in which case meteorites may provide a more unbiased record of early solar system processes. | linking asteroids and meteorites to the primordial planetesimal population |
due to high metal values and raised difficulties in finding superficial deposits, the exploration for mineral resources needs to reach greater depths. to achieve this objective, integrating aster and 3d inversion of aeromagnetic can be a useful tool in imaging deep magnetic mineralization hosting-structures. in this study, we present such integrated approach for deep mineral exploration in some promising zones of gabal (g) semna region, eastern desert (ed) of egypt. aeromagnetic data was used to delineate the structures and their relation to ore deposits and aster data was utilized for mapping lithological units and the hydrothermal zones. the 3d inversion was employed to imagine the depth and geometry of the magnetized ore bodies in the zones that identified by processing aster data. the 3d magnetic inversion revealed that mafic rocks and metavolcanics exhibit high-amplitude magnetic anomalies. high amplitude (positive) magnetic anomalies are joined with the large susceptibilities metavolcanics. alteration zones in the mafic to basic bedrock and alongside major fault systems are high potential zones that revealed by aster image analysis. aster and aeromagnetic data were used to map and alteration zones surface geology that can host mineralization and the 3d inversion was applied to outline the subsurface extent of these promising areas. this approach can be broadly used for deep mineral exploration in the egyptian ed and identical areas around the world. | integration of aster satellite imagery and 3d inversion of aeromagnetic data for deep mineral exploration |
we propose and study the new (generalized) e-type α-attractor models of inflation, in order to include formation of primordial black holes (pbhs). the inflaton potential has a near-inflection point where slow-roll conditions are violated, thus leading to large scalar perturbations collapsing to pbhs later. an ultra-slow roll (short) phase exists between two (longer) phases of slow-roll inflation. we numerically investigate the phases of inflation, derive the power spectrum of scalar perturbations and calculate the pbhs masses. for certain values of the parameters, the asteroid-size pbhs can be formed with the masses of 1017 ÷ 1019 g, beyond the hawking evaporation limit and in agreement with current cosmic microwave background observations. those pbhs are a candidate for (part of) dark matter in the present universe, while the gravitational waves induced by the pbhs formation may be detectable by the future space-based gravitational interferometers. | e-models of inflation and primordial black holes |
constraining the inflationary epoch is one of the aims of modern cosmology. in order to fully exploit current and future small-scale observations, it is necessary to devise tools to directly relate them to the early universe's dynamics. we present here a novel reverse engineering approach able to connect fundamental late-time observables to consistent inflationary dynamics and, eventually, to the inflaton potential. employing this procedure, we are able to describe which conditions can give rise to a raised plateau in the power spectrum of curvature perturbations at small scales, which are not constrained by cmb observations. within this new phenomenologically driven approach, we find that inflation can generate a raised plateau in the spectrum of curvature perturbations that potentially connects three fundamental observables; a dominant component of the dark matter in the form of asteroid-mass/atomic-size primordial black holes, detectable signals in stochastic gravitational waves, and a subdominant fraction of stellar-mass primordial black holes mergers. | primordial black hole dark matter from inflation: the reverse engineering approach |
comets have three known reservoirs: the roughly spherical oort cloud (for long-period comets), the flattened kuiper belt (for ecliptic comets), and, surprisingly, the asteroid belt (for main-belt comets). comets in the oort cloud were thought to have formed in the region of the giant planets and then placed in quasi-stable orbits at distances of thousands or tens of thousands of au through the gravitational effects of the planets and the galaxy. the planets were long assumed to have formed in place. however, the giant planets may have undergone two episodes of migration. the first would have taken place in the first few million years of the solar system, during or shortly after the formation of the giant planets, when gas was still present in the protoplanetary disk around the sun. the grand tack (walsh et al. in nature 475:206-209, 2011) models how this stage of migration could explain the low mass of mars and deplete, then repopulate the asteroid belt, with outer-belt asteroids originating between, and outside of, the orbits of the giant planets. the second stage of migration would have occurred later (possibly hundreds of millions of years later) due to interactions with a remnant disk of planetesimals, i.e., a massive ancestor of the kuiper belt. safronov (evolution of the protoplanetary cloud and formation of the earth and the planets, 1969) and fernández and ip (icarus 58:109-120, 1984) proposed that the giant planets would have migrated as they interacted with leftover planetesimals; jupiter would have moved slightly inward, while saturn and (especially) uranus and neptune would have moved outward from the sun. malhotra (nature 365:819-821, 1993) showed that pluto's orbit in the 3:2 resonance with neptune was a natural outcome if neptune captured pluto into resonance while it migrated outward. building on this work, tsiganis et al. (nature 435:459-461, 2005) proposed the nice model, in which the giant planets formed closer together than they are now, and underwent a dynamical instability that led to a flood of comets and asteroids throughout the solar system (gomes et al. in nature 435:466-469, 2005b). in this scenario, it is somewhat a matter of luck whether an icy planetesimal ends up in the kuiper belt or oort cloud (brasser and morbidelli in icarus 225:40-49, 2013), as a trojan asteroid (morbidelli et al. in nature 435:462-465, 2005; nesvorný and vokrouhlický in astron. j. 137:5003-5011, 2009; nesvorný et al. in astrophys. j. 768:45, 2013), or as a distant "irregular" satellite of a giant planet (nesvorný et al. in astron. j. 133:1962-1976, 2007). comets could even have been captured into the asteroid belt (levison et al. in nature 460:364-366, 2009). the remarkable finding of two "inner oort cloud" bodies, sedna and 2012 vp_{113}, with perihelion distances of 76 and 81 au, respectively (brown et al. in astrophys. j. 617:645-649, 2004; trujillo and sheppard in nature 507:471-474, 2014), along with the discovery of other likely inner oort cloud bodies (chen et al. in astrophys. j. lett. 775:8, 2013; brasser and schwamb in mon. not. r. astron. soc. 446:3788-3796, 2015), suggests that the sun formed in a denser environment, i.e., in a star cluster (brasser et al. in icarus 184:59-82, 2006, 191:413-433, 2007, 217:1-19, 2012b; kaib and quinn in icarus 197:221-238, 2008). the sun may have orbited closer or further from the center of the galaxy than it does now, with implications for the structure of the oort cloud (kaib et al. in icarus 215:491-507, 2011). | origin and evolution of the cometary reservoirs |
spherex is a proposed smex mission selected for phase a. spherex will carry out the first all-sky spectral survey and provide for every 6.2" pixel a spectra between 0.75 and 4.18 $\mu$m [with r$\sim$41.4] and 4.18 and 5.00 $\mu$m [with r$\sim$135]. the spherex team has proposed three specific science investigations to be carried out with this unique data set: cosmic inflation, interstellar and circumstellar ices, and the extra-galactic background light. it is readily apparent, however, that many other questions in astrophysics and planetary sciences could be addressed with the spherex data. the spherex team convened a community workshop in february 2016, with the intent of enlisting the aid of a larger group of scientists in defining these questions. this paper summarizes the rich and varied menu of investigations that was laid out. it includes studies of the composition of main belt and trojan/greek asteroids; mapping the zodiacal light with unprecedented spatial and spectral resolution; identifying and studying very low-metallicity stars; improving stellar parameters in order to better characterize transiting exoplanets; studying aliphatic and aromatic carbon-bearing molecules in the interstellar medium; mapping star formation rates in nearby galaxies; determining the redshift of clusters of galaxies; identifying high redshift quasars over the full sky; and providing a nir spectrum for most erosita x-ray sources. all of these investigations, and others not listed here, can be carried out with the nominal all-sky spectra to be produced by spherex. in addition, the workshop defined enhanced data products and user tools which would facilitate some of these scientific studies. finally, the workshop noted the high degrees of synergy between spherex and a number of other current or forthcoming programs, including jwst, wfirst, euclid, gaia, k2/kepler, tess, erosita and lsst. | science impacts of the spherex all-sky optical to near-infrared spectral survey: report of a community workshop examining extragalactic, galactic, stellar and planetary science |
ceres, the largest body in the asteroid belt (940 km diameter), provides a unique opportunity to study the interior structure of a volatile-rich dwarf planet. variations in a planetary body's subsurface rheology and density affect the rate of topographic relaxation. preferential attenuation of long wavelength topography (≥150 km) on ceres suggests that the viscosity of its crust decreases with increasing depth. we present finite element (fe) geodynamical simulations of ceres to identify the internal structures and compositions that best reproduce its topography as observed by the nasa dawn mission. we infer that ceres has a mechanically strong crust with maximum effective viscosity ∼1025 pa s. combined with density constraints, this rheology suggests a crustal composition of carbonates or phyllosilicates, water ice, and at least 30 volume percent (vol.%) low-density, high-strength phases most consistent with salt and/or clathrate hydrates. the inference of these crustal materials supports the past existence of a global ocean, consistent with the observed surface composition. meanwhile, we infer that the uppermost ≥60 km of the silicate-rich mantle is mechanically weak with viscosity <1021 pa s, suggesting the presence of liquid pore fluids in this region and a low temperature history that avoided igneous differentiation due to late accretion or efficient heat loss through hydrothermal processes. | the interior structure of ceres as revealed by surface topography |
random bombardment by comets, asteroids and associated fragments form and alter the lunar regolith and other rocky surfaces. the accumulation of impact craters over time is of fundamental use in evaluating the relative ages of geologic units. crater counts and radiometric ages from returned samples provide constraints with which to derive absolute model ages for unsampled units on the moon and other solar system objects. however, although studies of existing craters and returned samples offer insight into the process of crater formation and the past cratering rate, questions still remain about the present rate of crater production, the effect of early-stage jetting during impacts and the influence that distal ejecta have on the regolith. here we use lunar reconnaissance orbiter camera (lroc) narrow angle camera (nac) temporal (‘before and after’) image pairs to quantify the contemporary rate of crater production on the moon, to reveal previously unknown details of impact-induced jetting, and to identify a secondary impact process that is rapidly churning the regolith. from this temporal dataset, we detected 222 new impact craters and found 33 per cent more craters (with diameters of at least ten metres) than predicted by the standard neukum production and chronology functions for the moon. we identified broad reflectance zones associated with the new craters that we interpret as evidence of a surface-bound jetting process. we also observe a secondary cratering process that we estimate churns the top two centimetres of regolith on a timescale of 81,000 years—more than a hundred times faster than previous models estimated from meteoritic impacts (ten million years). | quantifying crater production and regolith overturn on the moon with temporal imaging |
the relative proportion of the two isotopes of nitrogen, 14n and 15n, varies dramatically across the solar system, despite little variation on earth. nasa's genesis mission directly sampled the solar wind and confirmed that the sun -- and, by inference, the protosolar nebula from which the solar system formed -- is highly depleted in the heavier isotope compared with the reference nitrogen isotopic composition, that of earth's atmosphere. in contrast, the inner planets, asteroids, and comets are enriched in 15n by tens to hundreds of per cent; organic matter in primitive meteorites records the highest 15n/14n isotopic ratios. the measurements indicate that the protosolar nebula, inner solar system, and cometary ices represent three distinct isotopic reservoirs, and that the 15n enrichment generally increases with distance from the sun. the 15n enrichments were probably not inherited from presolar material, but instead resulted from nitrogen isotope fractionation processes that occurred early in solar system history. improvements in analytical techniques and spacecraft observations have made it possible to measure nitrogen isotopic variability in the solar system at a level of accuracy that offers a window into the processing of early solar system material, large-scale disk dynamics and planetary formation processes. | nitrogen isotope variations in the solar system |
carbonaceous meteorites are thought to be fragments of c-type (carbonaceous) asteroids. samples of the c-type asteroid (162173) ryugu were retrieved by the hayabusa2 spacecraft. we measured the mineralogy and bulk chemical and isotopic compositions of ryugu samples. the samples are mainly composed of materials similar to those of carbonaceous chondrite meteorites, particularly the ci (ivuna-type) group. the samples consist predominantly of minerals formed in aqueous fluid on a parent planetesimal. the primary minerals were altered by fluids at a temperature of 37° ± 10°c, about 5.2−0.7+0.8 million (statistical) or 5.2−2.1+1.6 million (systematic) years after the formation of the first solids in the solar system. after aqueous alteration, the ryugu samples were likely never heated above ~100°c. the samples have a chemical composition that more closely resembles that of the sun’s photosphere than other natural samples do. | samples returned from the asteroid ryugu are similar to ivuna-type carbonaceous meteorites |
subduction zones represent the interface between earth's interior (crust and mantle) and exterior (atmosphere and oceans), where carbon and other volatile elements are actively cycled between earth reservoirs by plate tectonics. helium is a sensitive tracer of volatile sources and can be used to deconvolute mantle and crustal sources in arcs; however it is not thought to be recycled into the mantle by subduction processes. in contrast, carbon is readily recycled, mostly in the form of carbon-rich sediments, and can thus be used to understand volatile delivery via subduction. further, carbon is chemically-reactive and isotope fractionation can be used to determine the main processes controlling volatile movements within arc systems. here, we report helium isotope and abundance data for 42 deeply-sourced fluid and gas samples from the central volcanic zone (cvz) and southern volcanic zone (svz) of the andean convergent margin (acm). data are used to assess the influence of subduction parameters (e.g., crustal thickness, subduction inputs, and convergence rate) on the composition of volatiles in surface volcanic fluid and gas emissions. he isotopes from the cvz backarc range from 0.1 to 2.6 ra (n = 23), with the highest values in the puna and the lowest in the sub-andean foreland fold-and-thrust belt. atmosphere-corrected he isotopes from the svz range from 0.7 to 5.0 ra (n = 19). taken together, these data reveal a clear southeastward increase in 3he/4he, with the highest values (in the svz) falling below the nominal range associated with pure upper mantle helium (8 ± 1 ra), approaching the mean he isotope value for arc gases of (5.4 ± 1.9 ra). notably, the lowest values are found in the cvz, suggesting more significant crustal inputs (i.e., assimilation of 4he) to the helium budget. the crustal thickness in the cvz (up to 70 km) is significantly larger than in the svz, where it is just ∼40 km. we suggest that crustal thickness exerts a primary control on the extent of fluid-crust interaction, as helium and other volatiles rise through the upper plate in the acm. we also report carbon isotopes from (n = 11) sites in the cvz, where δ13c varies between −15.3‰ and −1.2‰ [vs. vienna pee dee belemnite (vpdb)] and co2/3he values that vary by over two orders of magnitude (6.9 × 108-1.7 × 1011). in the svz, carbon isotope ratios are also reported from (n = 13) sites and vary between −17.2‰ and −4.1‰. co2/3he values vary by over four orders of magnitude (4.7 × 107-1.7 × 1012). low δ13c and co2/3he values are consistent with co2 removal (e.g., calcite precipitation and gas dissolution) in shallow hydrothermal systems. carbon isotope fractionation modeling suggests that calcite precipitation occurs at temperatures coincident with the upper temperature limit for life (122°c), suggesting that biology may play a role in c-he systematics of arc-related volcanic fluid and gas emissions. | the helium and carbon isotope characteristics of the andean convergent margin |
on 2018 february 4.41, the all-sky automated survey for supernovae (asas-sn) discovered asassn-18bt in the k2 campaign 16 field. with a redshift of z = 0.01098 and a peak apparent magnitude of b max = 14.31, asassn-18bt is the nearest and brightest sne ia yet observed by the kepler spacecraft. here we present the discovery of asassn-18bt, the k2 light curve, and prediscovery data from asas-sn and the asteroid terrestrial-impact last alert system. the k2 early-time light curve has an unprecedented 30-minute cadence and photometric precision for an sn ia light curve, and it unambiguously shows a ∼4 day nearly linear phase followed by a steeper rise. thus, asassn-18bt joins a growing list of sne ia whose early light curves are not well described by a single power law. we show that a double-power-law model fits the data reasonably well, hinting that two physical processes must be responsible for the observed rise. however, we find that current models of the interaction with a nondegenerate companion predict an abrupt rise and cannot adequately explain the initial, slower linear phase. instead, we find that existing published models with shallow 56ni are able to span the observed behavior and, with tuning, may be able to reproduce the asassn-18bt light curve. regardless, more theoretical work is needed to satisfactorily model this and other early-time sne ia light curves. finally, we use swift x-ray nondetections to constrain the presence of circumstellar material (csm) at much larger distances and lower densities than possible with the optical light curve. for a constant-density csm, these nondetections constrain ρ < 4.5 × 105 cm-3 at a radius of 4 × 1015 cm from the progenitor star. assuming a wind-like environment, we place mass loss limits of \dot{m}< 8× {10}-6 {m}⊙{yr}}-1 for v w = 100 km s-1, ruling out some symbiotic progenitor systems. this work highlights the power of well-sampled early-time data and the need for immediate multiband, high-cadence follow-up for progress in understanding sne ia. | seeing double: asassn-18bt exhibits a two-component rise in the early-time k2 light curve |
at least 25 {per cent} of white dwarfs show atmospheric pollution by metals, sometimes accompanied by detectable circumstellar dust/gas discs or (in the case of wd 1145+017) transiting disintegrating asteroids. delivery of planetesimals to the white dwarf by orbiting planets is a leading candidate to explain these phenomena. here, we study systems of planets and planetesimals undergoing planet-planet scattering triggered by the star's post-main-sequence mass loss, and test whether this can maintain high rates of delivery over the several gyr that they are observed. we find that low-mass planets (earth to neptune mass) are efficient deliverers of material and can maintain the delivery for gyr. unstable low-mass planetary systems reproduce the observed delayed onset of significant accretion, as well as the slow decay in accretion rates at late times. higher-mass planets are less efficient, and the delivery only lasts a relatively brief time before the planetesimal populations are cleared. the orbital inclinations of bodies as they cross the white dwarf's roche limit are roughly isotropic, implying that significant collisional interactions of asteroids, debris streams and discs can be expected. if planet-planet scattering is indeed responsible for the pollution of white dwarfs, many such objects, and their main-sequence progenitors, can be expected to host (currently undetectable) super-earth planets on orbits of several au and beyond. | unstable low-mass planetary systems as drivers of white dwarf pollution |
we derived 90% confidence limits (cls) on the interstellar number density ({ρ }{is}{cl}) of interstellar objects (isos; comets and asteroids) as a function of the slope of their size-frequency distribution (sfd) and limiting absolute magnitude. to account for gravitational focusing, we first generated a quasi-realistic iso population to ∼ 750 {au} from the sun and propagated it forward in time to generate a steady state population of isos with heliocentric distance < 50 {au}. we then simulated the detection of the synthetic isos using pointing data for each image and average detection efficiencies for each of three contemporary solar system surveys—pan-starrs1, the mt. lemmon survey, and the catalina sky survey. these simulations allowed us to determine the surveys’ combined iso detection efficiency under several different but realistic modes of identifying isos in the survey data. some of the synthetic detected isos had eccentricities as small as 1.01, which is in the range of the largest eccentricities of several known comets. our best cl of {ρ }{is}{cl}=1.4× {10}-4 {{au}}-3 implies that the expectation that extra-solar systems form like our solar system, eject planetesimals in the same way, and then distribute them throughout the galaxy, is too simplistic, or that the sfd or behavior of isos as they pass through our solar system is far from expectation. | an observational upper limit on the interstellar number density of asteroids and comets |
we present a real-time stamp classifier of astronomical events for the automatic learning for the rapid classification of events broker, alerce. the classifier is based on a convolutional neural network, trained on alerts ingested from the zwicky transient facility (ztf). using only the science, reference, and difference images of the first detection as inputs, along with the metadata of the alert as features, the classifier is able to correctly classify alerts from active galactic nuclei, supernovae (sne), variable stars, asteroids, and bogus classes, with high accuracy (~94%) in a balanced test set. in order to find and analyze sn candidates selected by our classifier from the ztf alert stream, we designed and deployed a visualization tool called sn hunter, where relevant information about each possible sn is displayed for the experts to choose among candidates to report to the transient name server database. from 2019 june 26 to 2021 february 28, we have reported 6846 sn candidates to date (11.8 candidates per day on average), of which 971 have been confirmed spectroscopically. our ability to report objects using only a single detection means that 70% of the reported sne occurred within one day after the first detection. alerce has only reported candidates not otherwise detected or selected by other groups, therefore adding new early transients to the bulk of objects available for early follow-up. our work represents an important milestone toward rapid alert classifications with the next generation of large etendue telescopes, such as the vera c. rubin observatory. | alert classification for the alerce broker system: the real-time stamp classifier |
the exploration of near-earth asteroids has revealed dynamic surfaces characterized by mobile, unconsolidated material that responds to local geophysical gradients, resulting in distinct morphologies and boulder distributions. the osiris-rex (origins, spectral interpretation, resource identification, and security-regolith explorer) mission confirmed that asteroid (101955) bennu is a rubble pile with an unconsolidated surface dominated by boulders. in this work, we documented morphologies indicative of mass movement on bennu and assessed the relationship to slope and other geologic features on the surface. we found globally distributed morphologic evidence of mass movement on bennu up to ~70° latitude and on spatial scales ranging from individual boulders (meter scale) to a single debris flow ~100 m long and several meters thick. the apparent direction of mass movement is consistent with the local downslope direction and dominantly moves from the midlatitudes toward the equator. mass movement appears to have altered the surface expression of large (≥30m diameter) boulders, excavating them in the midlatitudes and burying them in the equatorial region. up to a 10 ± 1 m depth of material may have been transported away from the midlatitudes, which would have deposited a layer ~5 ± 1 m thick in the equatorial region assuming a stagnated flow model. this mass movement could explain the observed paucity of small (<50-m diameter) craters and may have contributed material to bennu's equatorial ridge. models of changes in slope suggest that the midlatitude mass movement occurred in the past several hundred thousand years in regions that became steeper by several degrees. | global patterns of recent mass movement on asteroid (101955) bennu |
asteroid (101955) bennu is a dark asteroid on an earth-crossing orbit that is thought to have assembled from the fragments of an ancient collision. we use spatially resolved visible and near-infrared spectra of bennu to investigate its surface properties and composition. in addition to a hydrated phyllosilicate band, we detect a ubiquitous 3.4-micrometer absorption feature, which we attribute to a mix of organic and carbonate materials. the shape and depth of this absorption feature vary across bennu’s surface, spanning the range seen among similar main-belt asteroids. the distribution of the absorption feature does not correlate with temperature, reflectance, spectral slope, or hydrated minerals, although some of those characteristics correlate with each other. the deepest 3.4-micrometer absorptions occur on individual boulders. the variations may be due to differences in abundance, recent exposure, or space weathering. | widespread carbon-bearing materials on near-earth asteroid (101955) bennu |
the dwarf planet (1) ceres, the largest object in the main asteroid belt with a mean diameter of about 950 kilometres, is located at a mean distance from the sun of about 2.8 astronomical units (one astronomical unit is the earth-sun distance). thermal evolution models suggest that it is a differentiated body with potential geological activity. unlike on the icy satellites of jupiter and saturn, where tidal forces are responsible for spewing briny water into space, no tidal forces are acting on ceres. in the absence of such forces, most objects in the main asteroid belt are expected to be geologically inert. the recent discovery of water vapour absorption near ceres and previous detection of bound water and oh near and on ceres (refs 5, 6, 7) have raised interest in the possible presence of surface ice. here we report the presence of localized bright areas on ceres from an orbiting imager. these unusual areas are consistent with hydrated magnesium sulfates mixed with dark background material, although other compositions are possible. of particular interest is a bright pit on the floor of crater occator that exhibits probable sublimation of water ice, producing haze clouds inside the crater that appear and disappear with a diurnal rhythm. slow-moving condensed-ice or dust particles may explain this haze. we conclude that ceres must have accreted material from beyond the ‘snow line’, which is the distance from the sun at which water molecules condense. | sublimation in bright spots on (1) ceres |
we perform streaming-instability simulations at hill density and beyond to demonstrate that planetesimal formation is not completed when pebble accumulations exceed the local hill density. we find that hill density is not a sufficient criterion for further gravitational collapse of a pebble cloud into a planetesimal, but that additionally the accumulated mass has to be large enough to overcome turbulent diffusion. a toomre analysis of the system indicates that linear self-gravity modes play no role on the scale of our numerical simulation. we nevertheless find that self-gravity, by vertically contracting the pebble layer, increases the strength of turbulence, which is either an indication of kelvin-helmholtz instability or a boost of the streaming instability. we furthermore determine the bonnor-ebert central density to which a pebble cloud of a given mass has to be compressed before it would be able to continue contraction against internal diffusion. as the equivalent "solid body" size of the pebble cloud scales with the central density to the power of -1/6, it is much easier to have a pebble cloud of 100 km equivalent size to collapse than one of 10 km for the same level of turbulent diffusion. this can explain the lack of small bodies in the solar system and predicts small objects will form at large pebble-to-gas ratios, so either in the outskirts of the solar nebula or at late times of generally reduced gas mass. | testing the jeans, toomre, and bonnor-ebert concepts for planetesimal formation: 3d streaming-instability simulations of diffusion-regulated formation of planetesimals |
landing on an asteroid is a highly valuable endeavor accompanied by significant challenges, making it a promising area for further research. this paper aims to present a flexible lander specifically designed for asteroid landing missions while also studying the guidance and control techniques involved in its descent phase. firstly, the paper briefly introduces the mission simulation model and asteroid environment from previous work. it then provides a detailed exposition on the guidance model which facilitates trajectory planning and the attitude control model used for managing the lander's attitude. the parametric trajectory is subsequently applied to obtain the position-attitude coupling trajectory that allows for the flexible lander's trajectory planning. the generating set search method is used to optimize the parametric trajectory under the appropriate constraints, ensuring high efficiency. in tracking the proposed trajectory, an open-loop control is applied for position tracking, while a closed-loop attitude controller is proposed for attitude control. an attitude control technique involving a combination of the nonsingular sliding mode control method with rbf to capture perturbations and unmodeled dynamics is adopted, and its global stability is proven using a lyapunov-based approach. monte carlo simulation is conducted to test the performance of the proposed trajectory planning technique with results showing that real-time trajectory generation and refreshing are achieved. the feasibility and accuracy of the proposed guidance and control technique are further verified with another set of monte carlo simulation. finally, the paper highlights one successful scenario including the descent and touching phases of asteroid landing with the flexible lander, illustrating its feasibility further. | position-attitude coupling guidance and control for asteroid landing with a flexible lander |
the tmt detailed science case describes the transformational science that the thirty meter telescope will enable. planned to begin science operations in 2024, tmt will open up opportunities for revolutionary discoveries in essentially every field of astronomy, astrophysics and cosmology, seeing much fainter objects much more clearly than existing telescopes. per this capability, tmt's science agenda fills all of space and time, from nearby comets and asteroids, to exoplanets, to the most distant galaxies, and all the way back to the very first sources of light in the universe. more than 150 astronomers from within the tmt partnership and beyond offered input in compiling the new 2015 detailed science case. the contributing astronomers represent the entire tmt partnership, including the california institute of technology (caltech), the indian institute of astrophysics (iia), the national astronomical observatories of the chinese academy of sciences (naoc), the national astronomical observatory of japan (naoj), the university of california, the association of canadian universities for research in astronomy (acura) and us associate partner, the association of universities for research in astronomy (aura). cover image: artist's rendition of the tmt international observatory on mauna kea opening in the late evening before beginning operations. | thirty meter telescope detailed science case: 2015 |
since darpa grand challenges (rural) in 2004/05 and urban challenges in 2007, autonomous driving has been the most active field of ai applications. almost at the same time, deep learning has made breakthrough by several pioneers, three of them (also called fathers of deep learning), hinton, bengio and lecun, won acm turin award in 2019. this is a survey of autonomous driving technologies with deep learning methods. we investigate the major fields of self-driving systems, such as perception, mapping and localization, prediction, planning and control, simulation, v2x and safety etc. due to the limited space, we focus the analysis on several key areas, i.e. 2d and 3d object detection in perception, depth estimation from cameras, multiple sensor fusion on the data, feature and task level respectively, behavior modelling and prediction of vehicle driving and pedestrian trajectories. | autonomous driving with deep learning: a survey of state-of-art technologies |
"liciacube - the light italian cubesat for imaging of asteroids" is managed by the italian space agency (asi) and will be part of the nasa dart mission, with the aim of i) documenting the dart impact's effects on the secondary member of the (65803) didymos binary asteroid system, ii) characterizing the shape of the target, and iii) performing dedicated scientific investigations on it. dart probe will be launched at the end of 2021 and liciacube will be hosted as piggyback during the interplanetary cruise, then released 10 days before the impact, and autonomously guided along its fly-by trajectory. the liciacube payload is composed by leia, a narrow fov camera, and luke, a wide fov imager with an rgb bayer pattern filter, that will collect and transmit to earth several unique images of the effects of the dart impact on the asteroid, such as the formation and the development of the plume potentially determined by the impact. liciacube will be the first deep space mission developed and autonomously managed by an italian team: the design, integration and test of the cubesat have been assigned by asi to the aerospace company argotec, while the liciacube ground segment has a complex architecture based on the argotec mission control center, antennas of the nasa deep space network and data archiving and processing, managed at the asi space science data center. the liciacube team includes a wide italian scientific community, involved in the definition of all the aspects of the mission: trajectory design; mission definition (and real-time orbit determination during operations); impact, plume and imaging simulation and modelling, in preparation of a suitable framework for the analysis and interpretation of in-situ data. the major technological mission challenge, i.e. the autonomous targeting and imaging of such a small body during a fast fly-by, to be accomplished with the limited resources of a cubesat, is affordable thanks to a strong synergy of all the mentioned teams in support of the engineering tasks. | liciacube - the light italian cubesat for imaging of asteroids in support of the nasa dart mission towards asteroid (65803) didymos |
during slow-roll inflation, non-perturbative transitions can produce bubbles of metastable vacuum. these bubbles expand exponentially during inflation to super-horizon size, and later collapse into black holes when the expansion of the universe is decelerating. estimating the rate for these transitions during a time-dependent slow-roll phase requires the development of new techniques. our results show that in a broad class of models, the inflationary fine-tuning that gives rise to small density fluctuations causes these bubbles to appear only during a time interval that is short compared to the inflationary hubble time. as a result, despite the fact that the final mass of the black hole is exponentially sensitive to the moment bubbles form during inflation, the resulting primordial black hole mass spectrum can be nearly monochromatic. if the transition occurs near the middle of inflation, the mass can fall in the "asteroid" range $10^{17}-10^{22}$ g in which all known observations are compatible with black holes comprising 100% of dark matter. | monochromatic mass spectrum of primordial black holes |
primordial black holes (pbhs) can generically form in inflationary setups through the collapse of enhanced cosmological perturbations, providing us access to the early universe through their associated observational signatures. in the current work we propose a new mechanism of pbh production within non-canonical inflation, using a class of steep-deformed inflationary potentials compatible with natural values for the non-canonical exponents. in particular, requiring significant pbh production we extract constraints on the non-canonical exponents. additionally, we find that our scenario can lead to the formation of asteroid-mass pbhs, which can account for the totality of the dark matter, as well as to production of solar-mass pbhs within the ligo-virgo detection band. finally, we find that the enhanced cosmological perturbations which collapse to form pbhs can produce a stochastic gravitational-wave (gw) background induced by second-order gravitational interactions. very interestingly, we obtain a gw signal detectable by future gw experiments, in particular by ska, lisa and bbo. | primordial black holes and gravitational waves from non-canonical inflation |
soon after their formation, the terrestrial planets experienced intense impact bombardment by comets, leftover planetesimals from primary accretion, and asteroids. this temporal interval in solar system evolution, termed late accretion, thermally and chemically modified solid planetary surfaces and may have impeded life's emergence on the hadean (pre-3850 ma) earth. the sources and tempo of bombardment, however, remain obscure. here we present a timeline that relates variably retentive radiometric ages documented from asteroidal meteorites to new dynamical models that invoke an early episode of planetesimal-driven giant planet migration after the dispersal of the protoplanetary disk. reconciliation of geochronological data with dynamical models shows that such giant planet migration should lead to an intense ∼30 myr influx of comets to the entire solar system manifested in radiometric age data. the absence of wholesale crustal reset ages after ∼4450 ma for the most resilient chronometers from earth, moon, mars, 4 vesta, and various meteorite parent bodies confines the onset of giant planet migration to ca. 4480 ma. waning impacts continue to strike the inner planets through a protracted monotonic decline in impactor flux, in agreement with predictions from crater chronology. new global 3d thermal analytical bombardment models derived from our revised impact mass-production functions show also that persistent niches for prebiotic chemistry leading to the emergence of life on the early hadean earth could endure late accretion since at least about 4400 million years ago. | onset of giant planet migration before 4480 million years ago |
stochastic inflation resolves primordial perturbations nonlinearly, probing their probability distribution deep into its non-gaussian tail. the strongest perturbations collapse into primordial black holes. in typical black-hole-producing single-field inflation, the strongest stochastic kicks occur during a period of constant roll. in this paper, i solve the stochastic constant-roll system, drawing the stochastic kicks from a numerically computed power spectrum, beyond the usual de sitter approximation. the perturbation probability distribution is an analytical function of the integrated curvature power spectrum σk2 and the second slow-roll parameter ε2. with a large ε2, stochastic effects can reduce the height of the curvature power spectrum required to form asteroid mass black holes from 10-2 to 10-3. i compare these results to studies with the nonstochastic δ n formalism. | stochastic constant-roll inflation and primordial black holes |
hydrogen occurs at the near percent level in the most hydrated chondrites (ci and cm) attesting to the presence of water in the asteroid-forming regions. their h abundances and isotopic signatures are powerful proxies for deciphering the distribution of h in the protoplanetary disk and the origin of earth's water. here, we report h contents and isotopic compositions for a set of carbonaceous and ordinary chondrites, including previously analyzed and new samples analyzed after the powdered samples were degassed under vacuum at 120 °c for 48 hours to remove adsorbed atmospheric water. by comparing our results to literature data, we reveal that the h budgets of both h-poor and h-rich carbonaceous chondrites are largely affected by atmospheric moisture, and that their precise quantification requires a specific pre-degassing procedure to correct for terrestrial contamination. our results show that indigenous h contents of ci carbonaceous chondrites usually considered the most hydrated meteorites might be almost a factor of two lower than those previously reported, with uncontaminated d/h ratios differing significantly from that of earth's oceans. without pre-degassing, the h concentrations of h-poor samples (e.g., cvs chondrites) are also affected by terrestrial contamination. after correction for contamination, it appears that the amount of water in chondrites is not controlled by the matrix modal abundance, suggesting that the different chondritic parent bodies accreted variable amounts of water-ice grains. our results also imply that (i) thermal metamorphism play an important role in determining the h content of both cv and ordinary chondrites but without affecting drastically their h isotopic composition since no clear d enrichment is observed with the increase of petrographic type and (ii) the d enrichment of ordinary chondrite organics does not result from the loss of isotopically light h2 induced by metal oxidation but is rather linked to the persistence of a thermally resistant d-rich component. | hydrogen in chondrites: influence of parent body alteration and atmospheric contamination on primordial components |
ceres is the largest body in the asteroid belt with a radius of approximately 470 km. in part due to its large mass, ceres more closely approaches hydrostatic equilibrium than major asteroids. pre-dawn mission shape observations of ceres revealed a shape consistent with a hydrostatic ellipsoid of revolution. the dawn spacecraft framing camera has been imaging ceres since march 2015, which has led to high-resolution shape models of the dwarf planet, while the gravity field has been globally determined to a spherical harmonic degree 14 (equivalent to a spatial wavelength of 211 km) and locally to 18 (a wavelength of 164 km). we use these shape and gravity models to constrain ceres' internal structure. we find a negative correlation and admittance between topography and gravity at degree 2 and order 2. low admittances between spherical harmonic degrees 3 and 16 are well explained by airy isostatic compensation mechanism. different models of isostasy give crustal densities between 1,200 and 1,400 kg/m3 with our preferred model giving a crustal density of 1,287+70-87 kg/m3. the mantle density is constrained to be 2,434+5-8 kg/m3. we compute isostatic gravity anomaly and find evidence for mascon-like structures in the two biggest basins. the topographic power spectrum of ceres and its latitude dependence suggest that viscous relaxation occurred at the long wavelengths (>246 km). our density constraints combined with finite element modeling of viscous relaxation suggests that the rheology and density of the shallow surface are most consistent with a rock, ice, salt and clathrate mixture. | constraints on ceres' internal structure and evolution from its shape and gravity measured by the dawn spacecraft |
frozen ground covers a vast area of the earth's surface and it has important ecohydrological implications for cold regions under changing climate. however, it is challenging to characterize the simultaneous transfer of mass and energy in frozen soils. within the modeling framework of simultaneous transfer of mass, momentum, and energy in unsaturated soil (stemmus), the complexity of the soil heat and mass transfer model varies from the basic coupled model (termed bcm) to the advanced coupled heat and mass transfer model (acm), and, furthermore, to the explicit consideration of airflow (acm-air). the impact of different model complexities on understanding the mass, momentum, and energy transfer in frozen soil was investigated. the model performance in simulating water and heat transfer and surface latent heat flux was evaluated over a typical tibetan plateau meadow site. results indicate that the acm considerably improved the simulation of soil moisture, temperature, and latent heat flux. the analysis of the heat budget reveals that the improvement of soil temperature simulations by acm is attributed to its physical consideration of vapor flow and the thermal effect on water flow, with the former mainly functioning above the evaporative front and the latter dominating below the evaporative front. the contribution of airflow-induced water and heat transport (driven by the air pressure gradient) to the total mass and energy fluxes is negligible. nevertheless, given the explicit consideration of airflow, vapor flow and its effects on heat transfer were enhanced during the freezing-thawing transition period. | understanding the mass, momentum, and energy transfer in the frozen soil with three levels of model complexities |
context. the gaia spacecraft of the european space agency (esa) has been securing observations of solar system objects (ssos) since the beginning of its operations. data release 2 (dr2) contains the observations of a selected sample of 14,099 ssos. these asteroids have been already identified and have been numbered by the minor planet center repository. positions are provided for each gaia observation at ccd level. as additional information, complementary to astrometry, the apparent brightness of ssos in the unfiltered g band is also provided for selected observations.aims: we explain the processing of sso data, and describe the criteria we used to select the sample published in gaia dr2. we then explore the data set to assess its quality.methods: to exploit the main data product for the solar system in gaia dr2, which is the epoch astrometry of asteroids, it is necessary to take into account the unusual properties of the uncertainty, as the position information is nearly one-dimensional. when this aspect is handled appropriately, an orbit fit can be obtained with post-fit residuals that are overall consistent with the a-priori error model that was used to define individual values of the astrometric uncertainty. the role of both random and systematic errors is described. the distribution of residuals allowed us to identify possible contaminants in the data set (such as stars). photometry in the g band was compared to computed values from reference asteroid shapes and to the flux registered at the corresponding epochs by the red and blue photometers (rp and bp).results: the overall astrometric performance is close to the expectations, with an optimal range of brightness g 12 - 17. in this range, the typical transit-level accuracy is well below 1 mas. for fainter asteroids, the growing photon noise deteriorates the performance. asteroids brighter than g 12 are affected by a lower performance of the processing of their signals. the dramatic improvement brought by gaia dr2 astrometry of ssos is demonstrated by comparisons to the archive data and by preliminary tests on the detection of subtle non-gravitational effects. | gaia data release 2. observations of solar system objects |
we propose a new scenario for the formation of asteroid-mass primordial black holes (pbhs). our mechanism is based on the annihilation of the string-wall network associated with the breaking of a u(1) global symmetry into a discrete zn symmetry. if the potential has multiple local minima (n > 1) the network is stable, and the annihilation is guaranteed by a bias among the different vacua. the collapse of the string-wall network is accompanied by catastrogenesis, a large production of pseudo-goldstone bosons (pgbs) - e.g. axions, alps, or majorons - gravitational waves, and pbhs. if pgbs rapidly decay into products that thermalize, as predicted e.g. in the high-quality qcd axion and heavy majoron models, they do not contribute to the dark matter population, but we show that pbhs can constitute 100% of the dark matter. the gravitational wave background produced by catastrogenesis with heavy unstable axions, alps, or majorons could be visible in future interferometers. | primordial black hole dark matter from catastrogenesis with unstable pseudo-goldstone bosons |
the field of mobile and wearable computing is undergoing a revolutionary integration of machine learning. devices can now diagnose diseases, predict heart irregularities, and unlock the full potential of human cognition. however, the underlying algorithms powering these predictions are not immune to biases with respect to sensitive attributes (e.g., gender, race), leading to discriminatory outcomes. the goal of this work is to explore the extent to which the mobile and wearable computing community has adopted ways of reporting information about datasets and models to surface and, eventually, counter biases. our systematic review of papers published in the proceedings of the acm interactive, mobile, wearable and ubiquitous technologies (imwut) journal from 2018-2022 indicates that, while there has been progress made on algorithmic fairness, there is still ample room for growth. our findings show that only a small portion (5%) of published papers adheres to modern fairness reporting, while the overwhelming majority thereof focuses on accuracy or error metrics. to generalize these results across venues of similar scope, we analyzed recent proceedings of acm mobicom, mobisys, and sensys, ieee pervasive, and ieee transactions on mobile computing computing, and found no deviation from our primary result. in light of these findings, our work provides practical guidelines for the design and development of mobile and wearable technologies that not only strive for accuracy but also fairness. | beyond accuracy: a critical review of fairness in machine learning for mobile and wearable computing |
the formation of asteroids, comets, and planets occurs in the interior of protoplanetary disks during the early phase of star formation. consequently, the chemical composition of the disk might shape the properties of the emerging planetary system. in this context, it is crucial to understand whether and what organic molecules are synthesized in the disk. in this letter, we report the first detection of formic acid (hcooh) toward the tw hydrae protoplanetary disk. the observations of the trans-hcooh 6(1,6)-5(1,5) transition were carried out at 129 ghz with atacama large millimeter/submillimeter array (alma). we measured a disk-averaged gas-phase t-hcooh column density of ∼(2-4) × 1012 cm-2, namely as large as that of methanol. hcooh is the first organic molecule containing two oxygen atoms detected in a protoplanetary disk, a proof that organic chemistry is very active, albeit difficult to observe, in these objects. specifically, this simplest acid stands as the basis for synthesis of more complex carboxylic acids used by life on earth. this letter makes use of the following alma data: ads/jao.alma#2015.1.00845.s (pi c. favre). alma is a partnership of eso (representing its member states), nsf (usa) and nins (japan), together with nrc (canada), nsc and asiaa (taiwan), and kasi (republic of korea), in cooperation with the republic of chile. the joint alma observatory is operated by eso, aui/nrao and naoj. | first detection of the simplest organic acid in a protoplanetary disk |
white dwarfs that have accreted rocky planetary bodies provide unique insights regarding the bulk composition of exoplanetary material. the analysis presented here uses observed pollutant abundances to constrain both where in the planetary system the pollutant bodies originated, and the geological and collisional history of the pollutant bodies. at least 1, but possibly up to 9, of the 17 systems analysed have accreted a body dominated by either core-like or mantle-like material. the approximately even spread in the core mass fraction of the pollutants and the lack of crust-rich pollutants in the 17 systems studied here suggest that the pollutants are often the fragments produced by the collision of larger differentiated bodies. the compositions of many pollutants exhibit trends related to elemental volatility, which we link to the temperatures and, thus, the locations at which these bodies formed. our analysis found that the abundances observed in 11 of the 17 systems considered are consistent with the compositions of nearby stars in combination with a trend related to elemental volatility. the even spread and large range in the predicted formation location of the pollutants suggests that pollutants arrive in white dwarf atmospheres with a roughly equal efficiency from a wide range of radial locations. ratios of elements with different condensation temperatures such as ca/mg, na/mg, and o/mg distinguish between different formation temperatures, whilst pairs of ratios of siderophilic and lithophilic elements such as fe/mg, ni/mg and al/mg, ca/mg distinguish between temperature-dependent trends and geological trends. | polluted white dwarfs: constraints on the origin and geology of exoplanetary material |
we make the case for studying the complexity of approximately simulating (sampling) quantum systems for reasons beyond that of quantum computational supremacy, such as diagnosing phase transitions. we consider the sampling complexity as a function of time t due to evolution generated by spatially local quadratic bosonic hamiltonians. we obtain an upper bound on the scaling of t with the number of bosons n for which approximate sampling is classically efficient. we also obtain a lower bound on the scaling of t with n for which any instance of the boson sampling problem reduces to this problem and hence implies that the problem is hard, assuming the conjectures of aaronson and arkhipov [proceedings of the forty-third annual acm symposium on theory of computing (acm press, new york, new york, usa, 2011), p. 333]. this establishes a dynamical phase transition in sampling complexity. further, we show that systems in the anderson-localized phase are always easy to sample from at arbitrarily long times. we view these results in light of classifying phases of physical systems based on parameters in the hamiltonian. in doing so, we combine ideas from mathematical physics and computational complexity to gain insight into the behavior of condensed matter, atomic, molecular, and optical systems. | dynamical phase transitions in sampling complexity |
without a protective atmosphere, space-exposed surfaces of airless solar system bodies gradually experience an alteration in composition, structure and optical properties through a collective process called space weathering. the return of samples from near-earth asteroid (162173) ryugu by hayabusa2 provides the first opportunity for laboratory study of space-weathering signatures on the most abundant type of inner solar system body: a c-type asteroid, composed of materials largely unchanged since the formation of the solar system. weathered ryugu grains show areas of surface amorphization and partial melting of phyllosilicates, in which reduction from fe3+ to fe2+ and dehydration developed. space weathering probably contributed to dehydration by dehydroxylation of ryugu surface phyllosilicates that had already lost interlayer water molecules and to weakening of the 2.7 µm hydroxyl (-oh) band in reflectance spectra. for c-type asteroids in general, this indicates that a weak 2.7 µm band can signify space-weathering-induced surface dehydration, rather than bulk volatile loss. | a dehydrated space-weathered skin cloaking the hydrated interior of ryugu |
this paper proposes a revised definition for the entanglement cost of a quantum channel n . in particular, it is defined here to be the smallest rate at which entanglement is required, in addition to free classical communication, in order to simulate n calls to n , such that the most general discriminator cannot distinguish the n calls to n from the simulation. the most general discriminator is one who tests the channels in a sequential manner, one after the other, and this discriminator is known as a quantum tester [chiribella et al., phys. rev. lett. 101, 060401 (2008), 10.1103/physrevlett.101.060401] or one who is implementing a quantum costrategy [gutoski and watrous, in proceedings of the thirty-ninth annual acm symposium on theory of computing stoc '07 (acm press, new york, 2007), pp. 565-574]. as such, the proposed revised definition of entanglement cost of a quantum channel leads to a rate that cannot be smaller than the previous notion of a channel's entanglement cost [berta et al., ieee trans. inf. theory 59, 6779 (2013), 10.1109/tit.2013.2268533], in which the discriminator is limited to distinguishing parallel uses of the channel from the simulation. under this revised notion, i prove that the entanglement cost of certain teleportation-simulable channels is equal to the entanglement cost of their underlying resource states. then i find single-letter formulas for the entanglement cost of some fundamental channel models, including dephasing, erasure, three-dimensional werner-holevo channels, and epolarizing channels (complements of depolarizing channels), as well as single-mode pure-loss and pure-amplifier bosonic gaussian channels. these examples demonstrate that the resource theory of entanglement for quantum channels is not reversible. finally, i discuss how to generalize the basic notions to arbitrary resource theories. | entanglement cost and quantum channel simulation |
there are four known sources of dust in the inner solar system: jupiter family comets, asteroids, halley type comets, and oort cloud comets. here we combine the mass, velocity, and radiant distributions of these cosmic dust populations from an astronomical model with a chemical ablation model to estimate the injection rates of na and fe into the earth's upper atmosphere, as well as the flux of cosmic spherules to the surface. comparing these parameters to lidar observations of the vertical na and fe fluxes above 87.5 km, and the measured cosmic spherule accretion rate at south pole, shows that jupiter family comets contribute (80 ± 17)% of the total input mass (43 ± 14 t d-1), in good accord with cosmic background explorer and planck observations of the zodiacal cloud. | sources of cosmic dust in the earth's atmosphere |
most comets are volatile-rich bodies that have recently entered the inner solar system following long-term storage in the kuiper belt and the oort cloud reservoirs. these reservoirs feed several distinct, short-lived "small body" populations. here, we present new measurements of the optical colors of cometary and comet-related bodies including long-period (oort cloud) comets, damocloids (probable inactive nuclei of long-period comets) and centaurs (recent escapees from the kuiper belt and precursors to the jupiter family comets). we combine the new measurements with published data on short-period comets, jovian trojans and kuiper belt objects to examine the color systematics of the comet-related populations. we find that the mean optical colors of the dust in short-period and long-period comets are identical within the uncertainties of measurement, as are the colors of the dust and of the underlying nuclei. these populations show no evidence for scattering by optically small particles or for compositional gradients, even at the largest distances from the sun, and no evidence for ultrared matter. consistent with earlier work, ultrared surfaces are common in the kuiper belt and on the centaurs, but not in other small body populations, suggesting that this material is hidden or destroyed upon entry to the inner solar system. the onset of activity in the centaurs and the disappearance of the ultrared matter in this population begin at about the same perihelion distance (∼10 au), suggesting that the two are related. blanketing of primordial surface materials by the fallback of sub-orbital ejecta, for which we calculate a very short timescale, is the likely mechanism. the same process should operate on any mass-losing body, explaining the absence of ultrared surface material in the entire comet population. the data presented herein were obtained at the w. m. keck observatory, which is operated as a scientific partnership among the california institute of technology, the university of california and the national aeronautics and space administration. the observatory was made possible by the generous financial support of the w. m. keck foundation. | color systematics of comets and related bodies |
the mass extinction of life 66 million years ago at the cretaceous/paleogene boundary, marked by the extinctions of dinosaurs and shallow marine organisms, is important because it led to the macroevolution of mammals and appearance of humans. the current hypothesis for the extinction is that an asteroid impact in present-day mexico formed condensed aerosols in the stratosphere, which caused the cessation of photosynthesis and global near-freezing conditions. here, we show that the stratospheric aerosols did not induce darkness that resulted in milder cooling than previously thought. we propose a new hypothesis that latitude-dependent climate changes caused by massive stratospheric soot explain the known mortality and survival on land and in oceans at the cretaceous/paleogene boundary. the stratospheric soot was ejected from the oil-rich area by the asteroid impact and was spread globally. the soot aerosols caused sufficiently colder climates at mid-high latitudes and drought with milder cooling at low latitudes on land, in addition to causing limited cessation of photosynthesis in global oceans within a few months to two years after the impact, followed by surface-water cooling in global oceans in a few years. the rapid climate change induced terrestrial extinctions followed by marine extinctions over several years. | global climate change driven by soot at the k-pg boundary as the cause of the mass extinction |
information about the materials and conditions involved in planetary formation and differentiation in the early solar system is recorded in iron isotope ratios. samples from earth, the moon, mars and the asteroid vesta reveal significant variations in iron isotope ratios, but the sources of these variations remain uncertain. here we present experiments that demonstrate that under the conditions of planetary core formation expected for the moon, mars and vesta, iron isotopes fractionate between metal and silicate due to the presence of nickel, and enrich the bodies' mantles in isotopically light iron. however, the effect of nickel diminishes at higher temperatures: under conditions expected for earth's core formation, we infer little fractionation of iron isotopes. from our experimental results and existing conceptual models of magma ocean crystallization and mantle partial melting, we find that nickel-induced fractionation can explain iron isotope variability found in planetary samples without invoking nebular or accretionary processes. we suggest that near-chondritic iron isotope ratios of basalts from mars and vesta, as well as the most primitive lunar basalts, were achieved by melting of isotopically light mantles, whereas the heavy iron isotope ratios of terrestrial ocean floor basalts are the result of melting of near-chondritic earth mantle. | non-chondritic iron isotope ratios in planetary mantles as a result of core formation |
in all inflationary scenarios of primordial black hole (pbh) formation, amplified scalar perturbations inevitably accompany an induced stochastic gravitational wave background (isgwb) at smaller scales. in this paper, we study the isgwb originating from the inflationary model, introduced in our previous paper [n. bhaumik and r. k. jain, primordial black holes dark matter from inflection point models of inflation and the effects of reheating, j. cosmol. astropart. phys. 01 (2020) 037, 10.1088/1475-7516/2020/01/037] wherein pbhs can be produced with a nearly monochromatic mass fraction in the asteroid mass window accounting for the total dark matter in the universe. we numerically calculate the isgwb in our scenario for frequencies ranging from nano-hz to khz that covers the observational scales corresponding to future space-based gravitational wave (gw) observatories such as ipta, lisa, decigo, and einstein telescope. interestingly, we find that ultralight pbhs (mpbh∼10-20 m⊙ ), which shall completely evaporate by today with an exceedingly small contribution to dark matter, would still generate an isgwb that may be detected by a future design of the ground-based advanced ligo detector. using a model-independent approach, we obtain a stringent lower mass limit for ultralight pbhs which would be valid for a large class of ultra slow roll inflationary models. further, we extend our formalism to study the imprints of a reheating epoch on both the isgwb and the derived lower mass bound. we find that any noninstantaneous reheating leads to an even stronger lower bound on the pbh mass and an epoch of a prolonged matter-dominated reheating shifts the isgwb spectrum to smaller frequencies. in particular, we show that an epoch of an early matter-dominated phase leads to a secondary amplification of the isgwb at a much smaller scale corresponding to the smallest comoving scale leaving the horizon during inflation or the end of the inflation scale. finally, we discuss the prospects of the isgwb detection by the proposed and upcoming gw observatories. | small scale induced gravitational waves from primordial black holes, a stringent lower mass bound, and the imprints of an early matter to radiation transition |
noble gas isotopes are powerful tracers of the origins of planetary volatiles, and the accretion and evolution of the earth. the compositions of magmatic gases provide insights into the evolution of the earth’s mantle and atmosphere. despite recent analytical progress in the study of planetary materials and mantle-derived gases, the possible dual origin of the planetary gases in the mantle and the atmosphere remains unconstrained. evidence relating to the relationship between the volatiles within our planet and the potential cosmochemical end-members is scarce. here we show, using high-precision analysis of magmatic gas from the eifel volcanic area (in germany), that the light xenon isotopes identify a chondritic primordial component that differs from the precursor of atmospheric xenon. this is consistent with an asteroidal origin for the volatiles in the earth’s mantle, and indicates that the volatiles in the atmosphere and mantle originated from distinct cosmochemical sources. furthermore, our data are consistent with the origin of eifel magmatism being a deep mantle plume. the corresponding mantle source has been isolated from the convective mantle since about 4.45 billion years ago, in agreement with models that predict the early isolation of mantle domains. xenon isotope systematics support a clear distinction between mid-ocean-ridge and continental or oceanic plume sources, with chemical heterogeneities dating back to the earth’s accretion. the deep reservoir now sampled by the eifel gas had a lower volatile/refractory (iodine/plutonium) composition than the shallower mantle sampled by mid-ocean-ridge volcanism, highlighting the increasing contribution of volatile-rich material during the first tens of millions of years of terrestrial accretion. | chondritic xenon in the earth’s mantle |
asteroids' and comets' geodesy is a challenging yet important task for planetary science and spacecraft operations, such as esa's hera mission tasked to look at the aftermath of the recent nasa dart spacecraft's impact on dimorphos. here we present a machine learning approach based on so-called geodesynets which learns accurate density models of irregular bodies using minimal prior information. geodesynets are a three-dimensional, differentiable function representing the density of a target irregular body. we investigate six different bodies, including the asteroids bennu, eros, and itokawa and the comet churyumov-gerasimenko, and validate on heterogeneous and homogeneous ground-truth density distributions. induced gravitational accelerations and inferred body shape are accurate, resulting in a relative acceleration error of less than 1%, also close to the surface. with a shape model, geodesynets can even learn heterogeneous density fields and thus provide insight into the body's internal structure. this adds a powerful tool to consolidated approaches like spherical harmonics, mascon models, and polyhedral gravity. | geodesy of irregular small bodies via neural density fields |
the recent discovery by pan-starrs1 of 1i/2017 u1 (‘oumuamua), on an unbound and hyperbolic orbit, offers a rare opportunity to explore the planetary formation processes of other stars and the effect of the interstellar environment on a planetesimal surface. 1i/‘oumuamua’s close encounter with the inner solar system in 2017 october was a unique chance to make observations matching those used to characterize the small-body populations of our own solar system. we present near-simultaneous g‧, r‧, and j photometry and colors of 1i/‘oumuamua from the 8.1 m frederick c. gillett gemini-north telescope and gri photometry from the 4.2 m william herschel telescope. our g‧r‧j observations are directly comparable to those from the high-precision colours of the outer solar system origins survey (col-ossos), which offer unique diagnostic information for distinguishing between outer solar system surfaces. the j-band data also provide the highest signal-to-noise measurements made of 1i/‘oumuamua in the near-infrared. substantial, correlated near-infrared and optical variability is present, with the same trend in both near-infrared and optical. our observations are consistent with 1i/‘oumuamua rotating with a double-peaked period of 8.10 ± 0.42 hr and being a highly elongated body with an axial ratio of at least 5.3:1, implying that it has significant internal cohesion. the color of the first interstellar planetesimal is at the neutral end of the range of solar system g - r and r - j solar-reflectance colors: it is like that of some dynamically excited objects in the kuiper belt and the less-red jupiter trojans. | col-ossos: colors of the interstellar planetesimal 1i/‘oumuamua |
asteroid (101955) bennu, the target of nasa's osiris-rex sample return mission, is a d ≈ 0.5 km diameter low albedo near-earth object. it has a spectral signature consistent with primitive carbonaceous chondrites, and an orbit similar to that of the earth. a plausible evolution scenario for bennu is that it migrated inward across the inner main belt from a low albedo family by yarkovsky thermal forces over many hundreds of myr. eventually, it entered a resonance that took it into the terrestrial planet region, where a combination of planetary encounters and resonances took it to its current orbit over a few myr to tens of myr. when it departed the main belt, bennu probably had an eccentricity 0.1 < e < 0.2 and an inclination 1 ° < i < 6 ° . several low albedo families have the appropriate dynamical, color, albedo, and broad spectral characteristics to produce bennu: clarissa, erigone, eulalia, new polana, and sulamitis. here we used a suite of numerical simulations to determine the ages of the families above, how bennu reached its current orbit, and the most probable source family for bennu. specifically, we tracked test bennu-like asteroids evolving in semimajor axis by the coupled yarkovsky/yorp effects, incorporating a new formalism for how yorp torques modify the spin vector evolution of small asteroids. using results and insights provided by statler (statler, t.s. [2009]. icarus 202, 502-513), we assumed that modest shape changes to asteroids, produced by a variety of processes (e.g., crater formation, changes to asteroid rotational angular momentum by yorp), caused the test asteroids' spin rates, but not their obliquities, to undergo a random walk. this "stochastic yorp" mechanism slows down how often asteroids reach yorp endstates (i.e., spinning up so fast that the asteroid sheds mass, spinning down so much the asteroid enters into a tumbling rotation state). this new model allowed us to reproduce the semimajor axis distribution of observed family members from clarissa, erigone, eulalia, new polana, and sulamitis. in the process, we derived model family formation ages of ∼60 myr old, 130 ± 30 myr old, 830-100+370 myr old, 1400 ± 150 myr old, and 200 ± 40 myr, respectively. next, using a monte-carlo code to track millions of test asteroids from each of the families above to main belt escape routes capable of producing bennu-like orbits, we found the most likely parent families for bennu are eulalia and new polana. on average, more than twice as many 0.5 km objects from the new polana family reach bennu's orbit as those from the eulalia family. this corresponds to the new polana and eulalia families having a 70-4+8 % and 30-8+4 % probability of producing bennu, respectively. comparable runs to deduce the source of the hayabusa 2 target, the low albedo 0.87 km diameter near-earth object (162173) 1999 ju3, produced similar probabilities for both families. the former marco-polo-r target, the 1.9 km asteroid (175706) 1996 fg3, however, has a 85-83+4 % probability of coming from the eulalia family and a 15-4+83 % probability of coming from the new polana family. the reason for this switch is that 1996 fg3 may have been part of yarkovsky/yorp-produced wave of like-sized bodies that is only now reaching the terrestrial planet region. we suggest that the top-like shape of bennu is a byproduct of mass wasting and/or mass shedding events produced by yorp spin up during its long journey across the inner main belt. | in search of the source of asteroid (101955) bennu: applications of the stochastic yorp model |
asteroid surface exploration is challenging due to complex terrain topology and irregular gravity field. a hopping rover is considered as a promising mobility solution to explore the surface of small celestial bodies. conventional path planning tasks, such as traversing a given map to reach a known target, may become particularly challenging for hopping rovers if the terrain displays sufficiently complex 3-d structures. as an alternative to traditional path-planning approaches, this work explores the possibility of applying deep reinforcement learning (drl) to plan the path of a hopping rover across a highly irregular surface. the 3-d terrain of the asteroid surface is converted into a level matrix, which is used as an input of the reinforcement learning algorithm. a deep reinforcement learning architecture with good convergence and stability properties is presented to solve the rover path-planning problem. numerical simulations are performed to validate the effectiveness and robustness of the proposed method with applications to two different types of 3-d terrains. | path planning for asteroid hopping rovers with pre-trained deep reinforcement learning architectures |
dujmović et al. [\emph{j.~acm}~'20] recently proved that every planar graph is isomorphic to a subgraph of the strong product of a bounded treewidth graph and a path. analogous results were obtained for graphs of bounded euler genus or apex-minor-free graphs. these tools have been used to solve longstanding problems on queue layouts, non-repetitive colouring, $p$-centered colouring, and adjacency labelling. this paper proves analogous product structure theorems for various non-minor-closed classes. one noteable example is $k$-planar graphs (those with a drawing in the plane in which each edge is involved in at most $k$ crossings). we prove that every $k$-planar graph is isomorphic to a subgraph of the strong product of a graph of treewidth $o(k^5)$ and a path. this is the first result of this type for a non-minor-closed class of graphs. it implies, amongst other results, that $k$-planar graphs have non-repetitive chromatic number upper-bounded by a function of $k$. all these results generalise for drawings of graphs on arbitrary surfaces. in fact, we work in a more general setting based on so-called shortcut systems, which are of independent interest. this leads to analogous results for certain types of map graphs, string graphs, graph powers, and nearest neighbour graphs. | graph product structure for non-minor-closed classes |
on december 3rd, 2014, the japanese space agency (jaxa) launched successfully the hayabusa2 (hy2) spacecraft to its journey to near earth asteroid (162173) ryugu. aboard this spacecraft is a compact landing package, mascot (mobile asteroid surface scout), which was developed by the german aerospace centre (dlr) in collaboration with the centre national d'etudes spatiales (cnes). similar to the famous predecessor mission hayabusa, hayabusa2, will also study an asteroid and return samples to earth. this time, however, the target is a c-type asteroid which is considered to be more primitive than (25143) itokawa and provide insight into an even earlier stage of our solar system. | mascot—the mobile asteroid surface scout onboard the hayabusa2 mission |
the impact of an object on a granular solid is an ubiquitous phenomenon in nature, the scale of which ranges from the impact of a raindrop onto sand all the way to that of a large asteroid on a planet. despite the obvious relevance of these impact events, the study of the underlying physics mechanisms that guide them is relatively young, with most work concentrated in the past decade. upon impact, an object starts to interact with a granular bed and experiences a drag force from the sand. this ultimately leads to phenomena such as crater formation and the creation of a transient cavity that upon collapse may cause a jet to appear from above the surface of the sand. this review provides an overview of research that targets these phenomena, from the perspective of the analogous but markedly different impact of an object on a liquid. it successively addresses the drag an object experiences inside a granular bed, the expansion and collapse of the cavity created by the object leading to the formation of a jet, and the remarkable role played by the air that resides within the pores between the grains. | impact on granular beds |
the first definitely interstellar object 1i/'oumuamua (previously a/2017 u1) observed in our solar system provides the opportunity to directly study material from an other star system. can such objects be intercepted? the challenge of reaching the object within a reasonable timeframe is formidable due to its high heliocentric hyperbolic excess velocity of about 26 km/s; much faster than any vehicle yet launched. this paper presents a high-level analysis of potential near-term options for a mission to 1i/'oumuamua and potential similar objects. reaching 1i/'oumuamua via a spacecraft launched in a reasonable timeframe of 5-10 years (launch in 2022-2027) requires an earth departure hyperbolic excess velocity between 33 and 76 km/s for mission durations between 30 and 5 years, respectively. different mission durations and their velocity requirements are explored with respect to the launch date, assuming direct impulsive transfer to the intercept trajectory. in addition, missions using a powered jupiter gravity assist combined with a solar oberth manoeuvre are explored, using solid rocket engines and parker solar probe heat shield technology. for such a mission, a falcon heavy-class launcher would be able to launch a spacecraft of dozens of kilograms towards 1i/'oumuamua, if launched in 2021. an additional saturn gravity assist would allow for the launch of a new horizons-class spacecraft. further technology options are outlined, ranging from electric propulsion, and more advanced options such as laser electric propulsion, solar and laser sails. to maximize science return, decelerating the spacecraft at 'oumuamua is highly desirable, compared to the minimal science return from a flyby. electric and magnetic sails could be used for this purpose. it is concluded that although reaching the object is challenging, there seem to be feasible options based on current and near-term technology. | project lyra: sending a spacecraft to 1i/'oumuamua (former a/2017 u1), the interstellar asteroid |
samples of the carbonaceous asteroid ryugu were brought to earth by the hayabusa2 spacecraft. we analyzed 17 ryugu samples measuring 1 to 8 millimeters. carbon dioxide–bearing water inclusions are present within a pyrrhotite crystal, indicating that ryugu’s parent asteroid formed in the outer solar system. the samples contain low abundances of materials that formed at high temperatures, such as chondrules and calcium- and aluminum-rich inclusions. the samples are rich in phyllosilicates and carbonates, which formed through aqueous alteration reactions at low temperature, high ph, and water/rock ratios of <1 (by mass). less altered fragments contain olivine, pyroxene, amorphous silicates, calcite, and phosphide. numerical simulations, based on the mineralogical and physical properties of the samples, indicate that ryugu’s parent body formed ~2 million years after the beginning of solar system formation. | formation and evolution of carbonaceous asteroid ryugu: direct evidence from returned samples |
the induced gravitational wave (gw) background from enhanced primordial scalar perturbations is one of the most promising observational consequences of primordial black hole (pbh) formation from inflation. we investigate the induced gw spectrum ωigw from single-field inflation in the general ultra-slow-roll (usr) framework, restricting the peak frequency band to be inside 10-3-1 hz and saturating pbh abundance to comprise all dark matter (dm) in the ultralight asteroid-mass window. by invoking successful baryogenesis driven by usr inflation, we verify the viable parameter space for the specific density ratio between baryons and pbh dm observed today, the so-called "cosmic coincidence." we show that the cosmic coincidence requirement bounds the spectral index n uv in the high frequency limit, ωigw(f ≫ 1) ∝ f -2n uv , into 0 < n uv < 1, which implies that baryogenesis triggered by usr inflation for pbhs in the mass range of 10-16-10-12 m⊙ can be tested by upcoming advanced ligo and virgo data and next generation experiments such as lisa, einstein telescope, tianqin and decigo. | induced gravitational waves from the cosmic coincidence |
we study for the first time the possibility of probing long-range fifth forces utilizing asteroid astrometric data, via the fifth force-induced orbital precession. we examine nine near-earth object (neo) asteroids whose orbital trajectories are accurately determined via optical and radar astrometry. focusing on a yukawa-type potential mediated by a new gauge field (dark photon) or a baryon-coupled scalar, we estimate the sensitivity reach for the fifth force coupling strength and mediator mass in the mass range m ≃ (10-21-10-15) ev, near the "fuzzy" dark matter region. our estimated sensitivity is comparable to leading limits from equivalence principle tests, potentially exceeding these in a specific mass range. the fifth force-induced precession increases with the orbital semi-major axis in the small m limit, motivating the study of objects further away from the sun. we also demonstrate that precession tests are particularly strong in probing long-range forces which approximately conserve the equivalence principle. we discuss future prospects for extending our study to more than a million asteroids, including neos, main-belt asteroids, hildas, and jupiter trojans, as well as trans-neptunian objects and exoplanets. | novel constraints on fifth forces and ultralight dark sector with asteroidal data |
the solar system's dynamical state can be explained by an orbital instability among the giant planets. a recent model has proposed that the giant planet instability happened during terrestrial planet formation. this scenario has been shown to match the inner solar system by stunting mars' growth and preventing planet formation in the asteroid belt. here we present a large sample of new simulations of the "early instability" scenario. we use an n-body integration scheme that accounts for collisional fragmentation, and also perform a large set of control simulations that do not include an early giant planet instability. since the total particle number decreases slower when collisional fragmentation is accounted for, the growing planets' orbits are damped more strongly via dynamical friction and encounters with small bodies that dissipate angular momentum (eg: hit-and-run impacts). compared with simulations without collisional fragmentation, our fully evolved systems provide better matches to the solar system's terrestrial planets in terms of their compact mass distribution and dynamically cold orbits. collisional processes also tend to lengthen the dynamical accretion timescales of earth analogs, and shorten those of mars analogs. this yields systems with relative growth timescales more consistent with those inferred from isotopic dating. accounting for fragmentation is thus supremely important for any successful evolutionary model of the inner solar system. | the early instability scenario: terrestrial planet formation during the giant planet instability, and the effect of collisional fragmentation |
fast radio bursts (frbs) are newly discovered radio transient sources. their high dispersion measures indicate an extragalactic origin. however, due to the lack of observational data in other wavelengths, their progenitors still remain unclear. here we suggest that the collisions between neutron stars (nss) and asteroids/comets are promising mechanisms for frbs. during the impact process, a hot plasma fireball forms after the material of the small body penetrates into the ns surface. the ionized matter inside the fireball then expands along the magnetic field lines. coherent radiation from the thin shell at the top of the fireball will account for the observed frbs. our scenario can reasonably explain the main features of frbs, such as their durations, luminosities, and the event rate. we argue that for a single ns, frbs are not likely to happen repeatedly in a forseeable timespan since such impacts are of low probability. we predict that faint remnant x-ray emissions should be associated with frbs, but it may be too faint to be detected by detectors at work. | fast radio bursts: collisions between neutron stars and asteroids/comets |
we explore the role of dissipative effects during warm inflation leading to the small-scale enhancement of the power spectrum of curvature perturbations. in this paper, we specifically focus on non-canonical warm inflationary scenarios and study a model of warm higgs-g inflation, in which the standard model higgs boson drives inflation, with a galileon-like non-linear kinetic term. we show that in the galileon-dominated regime, the primordial power spectrum is strongly enhanced, leading to the formation of primordial black holes (pbh) with a wide range of the mass spectrum. interestingly, pbhs in the asteroid mass window $\sim (10^{17}$ -- $10^{23}$) g are generated in this model, which can explain the total abundance of the dark matter in the universe. in our analysis, we also calculate the secondary gravitational waves (gw) sourced by these small-scale overdense fluctuations and find that the induced gw spectrum can be detected in the future gw detectors, such as lisa, bbo, decigo, etc. our scenario thus provides a novel way of generating pbhs as dark matter and a detectable stochastic gw background from warm inflation. we also show that our scenario is consistent with the swampland and the trans-planckian censorship conjectures and, thus, remains in the viable landscape of uv complete theories. | primordial black holes dark matter and secondary gravitational waves from warm higgs-g inflation |
the conditions, timing, and setting for the origin of life on earth and whether life exists elsewhere in our solar system and beyond represent some of the most fundamental scientific questions of our time. although the bombardment of planets and satellites by asteroids and comets has long been viewed as a destructive process that would have presented a barrier to the emergence of life and frustrated or extinguished life, we provide a comprehensive synthesis of data and observations on the beneficial role of impacts in a wide range of prebiotic and biological processes. in the context of previously proposed environments for the origin of life on earth, we discuss how meteorite impacts can generate both subaerial and submarine hydrothermal vents, abundant hydrothermal-sedimentary settings, and impact analogues for volcanic pumice rafts and splash pools. impact events can also deliver and/or generate many of the necessary chemical ingredients for life and catalytic substrates such as clays as well. the role that impact cratering plays in fracturing planetary crusts and its effects on deep subsurface habitats for life are also discussed. in summary, we propose that meteorite impact events are a fundamental geobiological process in planetary evolution that played an important role in the origin of life on earth. we conclude with the recommendation that impact craters should be considered prime sites in the search for evidence of past life on mars. furthermore, unlike other geological processes such as volcanism or plate tectonics, impact cratering is ubiquitous on planetary bodies throughout the universe and is independent of size, composition, and distance from the host star. impact events thus provide a mechanism with the potential to generate habitable planets, moons, and asteroids throughout the solar system and beyond. | the role of meteorite impacts in the origin of life |
the initial galactic velocity vector for the recently discovered hyperbolic asteroid 1i/'oumuamua (a/2017 u1) is calculated for before its encounter with our solar system. the latest orbit (jpl-13) shows that 'oumuamua has eccentricity > 1 at 944\sigma\, significance (1.19936 +- 0.00021), i.e. clearly unbound. assuming no non-gravitational forces, the object's inbound galactic velocity was u, v, w = -11.457, -22.395, -7.746 (+-0.009, +-0.009, +-0.011) km/s (u towards galactic center), with total heliocentric speed 26.32 +- 0.01 km/s. when the velocity is compared to the local stars, 'oumuamua can be ruled out as co-moving with any of the dozen nearest systems, i.e. it does not appear to be associated with any local exo-oort clouds (most notably that of the alpha centauri triple system). 'oumuamua's velocity is within 5 km/s of the median galactic velocity of the stars in the solar neighborhood (<25 pc), and within 2 km/s of the mean velocity of the local m dwarfs. its velocity appears to be statistically "too" typical for a body whose velocity was drawn from the galactic velocity distribution of the local stars (i.e. less than 1 in 500 field stars in the solar neighborhood would have a velocity so close to the median uvw velocity). in the local standard of rest frame (circular galactic motion), 'oumuamua is remarkable for showing both negligible radial (u) and vertical (w) motion, while having a slightly sub-keplerian circular velocity (v; by ~11 km/s). these calculations strengthen the interpretation that a/2017 u1 has a distant extrasolar origin, but not among the very nearest stars. any formation mechanism for this interstellar asteroid should account for the coincidence of 'oumuamua's velocity being so close to the lsr. | kinematics of the interstellar vagabond 1i/‘oumuamua (a/2017 u1) |
we report on g-, r-, and i-band observations of the interstellar object 1i/‘oumuamua (1i) taken on 2017 october 29 from 04:28 to 08:40 utc by the apache point observatory (apo) 3.5 m telescope’s arctic camera. we find that 1i’s colors are g-r=0.41+/- 0.24 and r-i=0.23+/- 0.25, consistent with visible spectra and most comparable to the population of solar system c/d asteroids, trojans, or comets. we find no evidence of any cometary activity at a heliocentric distance of 1.46 au, approximately 1.5 months after 1i’s closest approach distance to the sun. significant brightness variability was seen in the r observations, with the object becoming notably brighter toward the end of the run. by combining our apo photometric time series data with the discovery channel telescope data of knight et al., taken 20 hr later on 2017 october 30, we construct an almost complete lightcurve with a most probable single-peaked lightcurve period of p≃ 4 {hr}. our results imply a double-peaked rotation period of 8.1 ± 0.02 hr, with a peak-to-trough amplitude of 1.5-2.1 mag. assuming that 1i’s shape can be approximated by an ellipsoid, the amplitude constraint implies that 1i has an axial ratio of 3.5-10.3, which is strikingly elongated. assuming that 1i is rotating above its critical break up limit, our results are compatible with 1i having modest cohesive strength and may have obtained its elongated shape during a tidal distortion event before being ejected from its home system. | apo time-resolved color photometry of highly elongated interstellar object 1i/‘oumuamua |
the composition and physical properties of martian regolith are dramatically better understood compared to just a decade ago, particularly through the use of x-ray diffraction by the curiosity rover. because there are no samples of this regolith on earth, researchers and engineers rely on terrestrial simulants to test future hardware and address fundamental science and engineering questions. even with eventual sample return, the amount of material brought back would not be enough for bulk studies. many existing mars simulants were designed 10 or 20 years ago based on a more rudimentary understanding of martian surface materials. here, we describe the mars global simulant (mgs-1), a new open standard designed as a high fidelity mineralogical analog to global basaltic regolith on mars, as represented by the rocknest windblown deposit at gale crater. we developed prototype simulants using the mgs-1 standard and characterized their basic physical properties, bulk chemistry, spectral properties, and volatile content. the mgs-1 based simulant compares favorably to rover and remote sensing observations from mars, and offers dramatic improvements over past simulants in many areas. modest amounts of simulant will be produced by the center for lunar & asteroid surface science (class) exolith lab to distribute to other researchers. by publishing the mineral recipe and production methods, we anticipate that other groups can re-create the simulant and modify it as they see fit, leading to a more sustainable model for simulant production and the possibility of extending the simulant for different regions on mars or for different applications. | mars global simulant mgs-1: a rocknest-based open standard for basaltic martian regolith simulants |
we report on the mineralogy, petrography, and in situ measured oxygen- and magnesium-isotope compositions of eight porphyritic chondrules (seven feo-poor and one feo-rich) from the renazzo-like carbonaceous (cr) chondrites graves nunataks 95229, grosvenor mountains 03116, pecora escarpment 91082, and queen alexandra range 99177, which experienced minor aqueous alteration and very mild thermal metamorphism. we find no evidence that these processes modified the oxygen- or al-mg isotope systematics of chondrules in these meteorites. olivine, low-ca pyroxene, and plagioclase within an individual chondrule have similar o-isotope compositions, suggesting crystallization from isotopically uniform melts. the only exceptions are relict grains in two of the chondrules; these grains are 16o-enriched relative to phenocrysts of the host chondrules. only the feo-rich chondrule shows a resolvable excesses of 26mg, corresponding to an inferred initial 26al/27al ratio [(26al/27al)0] of (2.5 ± 1.6) × 10-6 (±2se). combining these results with the previously reported al-mg isotope systematics of cr chondrules (nagashima et al., 2014, geochem. j. 48, 561), 7 of 22 chondrules (32%) measured show resolvable excesses of 26mg; the presence of excess 26mg does not correlate with the feo content of chondrule silicates. in contrast, virtually all chondrules in weakly metamorphosed (petrologic type 3.0-3.1) unequilibrated ordinary chondrites (uocs), ornans-like carbonaceous (co) chondrites, and the ungrouped carbonaceous chondrite acfer 094 show resolvable excesses of 26mg. the inferred (26al/27al)0 in cr chondrules with resolvable excesses of 26mg range from (1.0 ± 0.4) × 10-6 to (6.3 ± 0.9) × 10-6, which is typically lower than (26al/27al)0 in the majority of chondrules from uocs, cos, and acfer 094. based on the inferred (26al/27al)0, three populations of cr chondrules are recognized; the population characterized by low (26al/27al)0 (<3 × 10-6) is dominant. there are no noticeable trends with major and minor element or o-isotope compositions between these populations. the weighted mean (26al/27al)0 of 22 cr chondrules measured is (1.8 ± 0.3) × 10-6. an apparent agreement between the 26al-26mg ages (using weighted mean value) and the revised (using 238u/235u ratio for bulk cr chondrites of 137.7789 ± 0.0085) 207pb-206pb age of a set of chondrules from cr chondrites (amelin et al., 2002, science297, 1678) is consistent with the initial 26al/27al ratio in the cr chondrite chondrule-forming region at the canonical level (∼5.2 × 10-5), allowing the use of 26al-26mg systematics as a chronometer for cr chondrules. to prove chronological significance of 26al for cr chondrules, measurements of al-mg and u-pb isotope systematics on individual chondrules are required. the presence of several generations among cr chondrules indicates some chondrules that accreted into the cr chondrite parent asteroid avoided melting by later chondrule-forming events, suggesting chondrule-forming processes may have occurred on relatively limited spatial scales. accretion of the cr chondrite parent body occurred at >4.0-0.3+0.5 ma after the formation of cais with the canonical 26al/27al ratio, although rapid accretion after formation of the major population of cr chondrules is not required by our data. | distribution of 26al in the cr chondrite chondrule-forming region of the protoplanetary disk |
over the last decade, the solar wind has exhibited low densities and magnetic field strengths, representing anomalous states that have never been observed during the space age. as discussed by schwadron, blake, et al. (2014, https://doi.org/10.1002/2014sw001084), the cycle 23-24 solar activity led to the longest solar minimum in more than 80 years and continued into the "mini" solar maximum of cycle 24. during this weak activity, we observed galactic cosmic ray fluxes that exceeded theerobserved small solar energetic particle events. here we provide an update to the schwadron, blake, et al. (2014, https://doi.org/10.1002/2014sw001084) observations from the cosmic ray telescope for the effects of radiation (crater) on the lunar reconnaissance orbiter. the schwadron, blake, et al. (2014, https://doi.org/10.1002/2014sw001084) study examined the evolution of the interplanetary magnetic field and utilized a previously published study by goelzer et al. (2013, https://doi.org/10.1002/2013ja019404) projecting out the interplanetary magnetic field strength based on the evolution of sunspots as a proxy for the rate that the sun releases coronal mass ejections. this led to a projection of dose rates from galactic cosmic rays on the lunar surface, which suggested a ∼20% increase of dose rates from one solar minimum to the next and indicated that the radiation environment in space may be a worsening factor important for consideration in future planning of human space exploration. we compare the predictions of schwadron, blake, et al. (2014, https://doi.org/10.1002/2014sw001084) with the actual dose rates observed by crater in the last 4 years. the observed dose rates exceed the predictions by ∼10%, showing that the radiation environment is worsening more rapidly than previously estimated. much of this increase is attributable to relatively low-energy ions, which can be effectively shielded. despite the continued paucity of solar activity, one of the hardest solar events in almost a decade occurred in september 2017 after more than a year of all-clear periods. these particle radiation conditions present important issues that must be carefully studied and accounted for in the planning and design of future missions (to the moon, mars, asteroids, and beyond). | update on the worsening particle radiation environment observed by crater and implications for future human deep-space exploration |
asteroids and comets are the remnants of the swarm of planetesimals from which the planets ultimately formed, and they retain records of processes that operated prior to and during planet formation. they are also likely the sources of most of the water and other volatiles accreted by earth. in this review, we discuss the nature and probable origins of asteroids and comets based on data from remote observations, in situ measurements by spacecraft, and laboratory analyses of meteorites derived from asteroids. the asteroidal parent bodies of meteorites formed ≤ 4 ma after solar system formation while there was still a gas disk present. it seems increasingly likely that the parent bodies of meteorites spectroscopically linked with the e-, s-, m- and v-type asteroids formed sunward of jupiter's orbit, while those associated with c- and, possibly, d-type asteroids formed further out, beyond jupiter but probably not beyond saturn's orbit. comets formed further from the sun than any of the meteorite parent bodies, and retain much higher abundances of interstellar material. ci and cm group meteorites are probably related to the most common c-type asteroids, and based on isotopic evidence they, rather than comets, are the most likely sources of the h and n accreted by the terrestrial planets. however, comets may have been major sources of the noble gases accreted by earth and venus. possible constraints that these observations can place on models of giant planet formation and migration are explored. | water reservoirs in small planetary bodies: meteorites, asteroids, and comets |
the osiris-rex thermal emission spectrometer (otes) will provide remote measurements of mineralogy and thermophysical properties of bennu to map its surface, help select the osiris-rex sampling site, and investigate the yarkovsky effect. otes is a fourier transform spectrometer covering the spectral range 5.71-100 μm (1750-100 cm^{-1}) with a spectral sample interval of 8.66 cm^{-1} and a 6.5-mrad field of view. the otes telescope is a 15.2-cm diameter cassegrain telescope that feeds a flat-plate michelson moving mirror mounted on a linear voice-coil motor assembly. a single uncooled deuterated l-alanine doped triglycine sulfate (dlatgs) pyroelectric detector is used to sample the interferogram every two seconds. redundant ∼0.855 μm laser diodes are used in a metrology interferometer to provide precise moving mirror control and ir sampling at 772 hz. the beamsplitter is a 38-mm diameter, 1-mm thick chemical vapor deposited diamond with an antireflection microstructure to minimize surface reflection. an internal calibration cone blackbody target provides radiometric calibration. the radiometric precision in a single spectrum is ≤2.2 × 10^{-8} w cm^{-2} sr ^{-1}/cm^{-1} between 300 and 1350 cm^{-1}. the absolute integrated radiance error is <1% for scene temperatures ranging from 150 to 380 k. the overall otes envelope size is 37.5 × 28.9 × 52.2 cm, and the mass is 6.27 kg. the power consumption is 10.8 w average. otes was developed by arizona state university with moog broad reach developing the electronics. otes was integrated, tested, and radiometrically calibrated on the arizona state university campus in tempe, az. | the osiris-rex thermal emission spectrometer (otes) instrument |
we derive purely gravitational constraints on dark matter and cosmic neutrino profiles in the solar system using asteroid (101955) bennu. we focus on bennu because of its extensive tracking data and high-fidelity trajectory modeling resulting from the osiris-rex mission. we find that the local density of dark matter is bound by $\rho_{\rm dm}\lesssim 3.3\times 10^{-15}\;\rm kg/m^3 \simeq 6\times10^6\,\bar{\rho}_{\rm dm}$, in the vicinity of $\sim 1.1$ au (where $\bar{\rho}_{\rm dm}\simeq 0.3\;\rm gev/cm^3$). we show that high-precision tracking data of solar system objects can constrain cosmic neutrino overdensities relative to the standard model prediction $\bar{n}_{\nu}$, at the level of $\eta\equiv n_\nu/\bar{n}_{\nu}\lesssim 1.7 \times 10^{11}(0.1 \;{\rm ev}/m_\nu)$ (saturn), comparable to the existing bounds from katrin and other previous laboratory experiments (with $m_\nu$ the neutrino mass). these local bounds have interesting implications for existing and future direct-detection experiments. our constraints apply to all dark matter candidates but are particularly meaningful for scenarios including solar halos, stellar basins, and axion miniclusters, which predict or allow overdensities in the solar system. furthermore, introducing a dm-sm long-range fifth force with a strength $\tilde{\alpha}_d$ times stronger than gravity, bennu can set a constraint on $\rho_{\rm dm}\lesssim \bar{\rho}_{\rm dm}\left(6 \times 10^6/\tilde{\alpha}_d\right)$. these constraints can be improved in the future as the accuracy of tracking data improves, observational arcs increase, and more missions visit asteroids. | new constraints on dark matter and cosmic neutrino profiles through gravity |
studies of the lunar atmosphere have shown it to be a stable, low-density surface boundary exosphere for the last 3 billion years. however, substantial volcanic activity on the moon prior to 3 ga may have released sufficient volatiles to form a transient, more prominent atmosphere. here, we calculate the volume of mare basalt emplaced as a function of time, then estimate the corresponding production of volatiles released during the mare basalt-forming eruptions. results indicate that during peak mare emplacement and volatile release ∼3.5 ga, the maximum atmospheric pressure at the lunar surface could have reached ∼1 kpa, or ∼1.5 times higher than mars' current atmospheric surface pressure. this lunar atmosphere may have taken ∼70 million years to fully dissipate. most of the volatiles released by mare basalts would have been lost to space, but some may have been sequestered in permanently shadowed regions on the lunar surface. if only 0.1% of the mare water vented during these eruptions remains in the polar regions of the moon, volcanically-derived volatiles could account for all hydrogen deposits - suspected to be water - currently observed in the moon's permanently shadowed regions. future missions to such locations may encounter evidence of not only asteroidal, cometary, and solar wind-derived volatiles, but also volatiles vented from the interior of the moon. | lunar volcanism produced a transient atmosphere around the ancient moon |
the canadian space agency (csa) has contributed to the origins spectral interpretation resource identification security-regolith explorer (osiris-rex) spacecraft the osiris-rex laser altimeter (ola). the osiris-rex mission will sample asteroid 101955 bennu, the first b-type asteroid to be visited by a spacecraft. bennu is thought to be primitive, carbonaceous, and spectrally most closely related to ci and/or cm meteorites. as a scanning laser altimeter, the ola instrument will measure the range between the osiris-rex spacecraft and the surface of bennu to produce digital terrain maps of unprecedented spatial scales for a planetary mission. the digital terrain maps produced will measure ∼7 cm per pixel globally, and ∼3 cm per pixel at specific sample sites. in addition, ola data will be used to constrain and refine the spacecraft trajectories. global maps and highly accurate spacecraft trajectory estimates are critical to infer the internal structure of the asteroid. the global and regional maps also are key to gain new insights into the surface processes acting across bennu, which inform the selection of the osiris-rex sample site. these, in turn, are essential for understanding the provenance of the regolith sample collected by the osiris-rex spacecraft. the ola data also are important for quantifying any hazards near the selected osiris-rex sample site and for evaluating the range of tilts at the sampling site for comparison against the capabilities of the sample acquisition device. | the osiris-rex laser altimeter (ola) investigation and instrument |
we optimize a six degrees of freedom hovering policy using reinforcement meta-learning. the policy maps flash lidar measurements directly to on/off spacecraft body-frame thrust commands, allowing hovering at a fixed position and attitude in the asteroid body-fixed reference frame. importantly, the policy does not require position and velocity estimates, and can operate in environments with unknown dynamics, and without an asteroid shape model or navigation aids. indeed, during optimization the agent is confronted with a new randomly generated asteroid for each episode, insuring that it does not learn an asteroid's shape, texture, or environmental dynamics. this allows the deployed policy to generalize well to novel asteroid characteristics, which we demonstrate in our experiments. moreover, our experiments show that the optimized policy adapts to actuator failure and sensor noise. although the policy is optimized using randomly generated synthetic asteroids, it is tested on two shape models from actual asteroids: bennu and itokawa. we find that the policy generalizes well to these shape models. the hovering controller has the potential to simplify mission planning by allowing asteroid body-fixed hovering immediately upon the spacecraft's arrival to an asteroid. this in turn simplifies shape model generation and allows resource mapping via remote sensing immediately upon arrival at the target asteroid. | six degree-of-freedom body-fixed hovering over unmapped asteroids via lidar altimetry and reinforcement meta-learning |
context. it is well known that the magnetic activity of solar-type stars decreases with age, but it is widely debated in the literature whether there is a smooth decline or if there is an early sharp drop until 1-2 gyr that is followed by a relatively inactive constant phase.aims: we revisited the activity-age relation using time-series observations of a large sample of solar twins whose precise isochronal ages and other important physical parameters have been determined.methods: we measured the ca ii h and k activity indices using ≈9000 harps spectra of 82 solar twins. in addition, the average solar activity was calculated through asteroids and moon reflection spectra using the same instrumentation. thus, we transformed our activity indices into the s mount wilson scale (smw), recalibrated the mount wilson absolute flux and photospheric correction equations as a function of teff, and then computed an improved bolometric flux normalized activity index log r'hk (teff) for the entire sample.results: new relations between activity and the age of solar twins were derived by assessing the chromospheric age-dating limits using log r'hk (teff). we measured an average solar activity of smw = 0.1712 ± 0.0017 during solar magnetic cycles 23-24 covered by harps observations, and we also inferred an average of smw = 0.1694 ± 0.0025 for cycles 10-24, anchored on a sunspot number correlation of s index versus. we also found a simple relation between the average and the dispersion of the activity levels of solar twins. this enabled us to predict the stellar variability effects on the age-activity diagram, and consequently, to estimate the chromospheric age uncertainties that are due to the same phenomena. the age-activity relation is still statistically significant up to ages around 6-7 gyr, in agreement with previous works using open clusters and field stars with precise ages.conclusions: our research confirms that ca ii h & k lines remain a useful chromospheric evolution tracer until stars reach ages of at least 6-7 gyr. we found evidence that for the most homogenous set of old stars, the chromospheric activity indices seem to continue to decrease after the solar age toward the end of the main sequence. our results indicate that a significant part of the scatter observed in the age-activity relation of solar twins can be attributed to stellar cycle modulations effects. the sun seems to have a normal activity level and variability for its age. based on observations collected at the european organisation for astronomical research in the southern hemisphere under eso programs 188.c-0265, 183.d-0729, 292.c-5004, 097.c-0571, 092.c-0721, 093.c-0409, 072.c-0488, 183.c-0972, 091.c-0936, 192.c-0852, 196.c-1006, 076.c-0155, 096.c-0499, 185.d-0056, 192.c-0224, 075.c-0332, 090.c-0421, 091.c-0034, 077.c-0364, 089.c-0415, 60.a-9036, 092.c-0832, 295.c-5035, 295.c-5031, 60.a-9700, 289.d-5015, 096.c-0210, 086.c-0284, 088.c-0323, 0100.d-0444, and 099.c-0491.tables 1 and 2 are only available at the cds via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?j/a+a/619/a73 | the solar twin planet search. the age-chromospheric activity relation |
extraterrestrial materials, including meteorites, interplanetary dust, and spacecraft-returned asteroidal and cometary samples, provide a record of the starting materials and early evolution of the solar system. we review how laboratory analyses of these materials provide unique information, complementary to astronomical observations, about a wide variety of stellar, interstellar and protoplanetary processes. presolar stardust grains retain the isotopic compositions of their stellar sources, mainly asymptotic giant branch stars and type ii supernovae. they serve as direct probes of nucleosynthetic and dust formation processes in stars, galactic chemical evolution, and interstellar dust processing. extinct radioactivities suggest that the sun's birth environment was decoupled from average galactic nucleosynthesis for some tens to hundreds of myr but was enriched in short-lived isotopes from massive stellar winds or explosions shortly before or during formation of the solar system. radiometric dating of meteorite components tells us about the timing and duration over which solar nebula solids were assembled into the building blocks of the planets. components of the most primitive meteoritical materials provide further detailed constraints on the formation, processing, and transport of material and associated timescales in the sun's protoplanetary disk as well as in other forming planetary systems. | astrophysics with extraterrestrial materials |
the periodic activity of the repeating fast radio burst (frb) 180916.j0158+65 was recently reported by the chime/frb collaboration team. from this source 28 bursts not only show a ∼16 day period with an active phase of ∼4.0 days, but they also exhibit a broken power law in differential energy distribution. in this letter, we suggest that frb 180916.j0158+65-like periodic frbs could provide a unique probe of extragalactic asteroid belts (eabs), based on our previously proposed pulsar-eab impact model, in which repeating frbs arise from an old-aged, slowly spinning, moderately magnetized pulsar traveling through an eab around another stellar-mass object. these two objects form a binary, and thus the observed period is in fact the orbital period. we show that this model can be used to well interpret all of the observed data of frb 180916.j0158+65. furthermore, we constrain the eab's physical properties and find that (1) the outer radius of the eab is at least an order of magnitude smaller than that of its analog in the solar system, (2) the differential size distribution of the eab's asteroids at small diameters (large diameters) is shallower (steeper) than that of solar system small objects, and (3) the two belts have a comparable mass. | periodic fast radio bursts as a probe of extragalactic asteroid belts |
the origins, spectral interpretation, resource identification, security-regolith explorer mission will return a sample to earth from asteroid (101955) bennu. digital terrain models (dtms) of the asteroid, and products enabled by them, are key to understanding the origin and evolution of the asteroid, providing geological and geophysical context for the sample, maximizing the amount of sample returned, navigating the spacecraft, and ensuring the safety of the spacecraft during sampling. the mission has two approaches for producing these dtms: a camera-based approach and a lidar-based approach. we provide an overview of the methods used for these two approaches and how they fit into the originally planned mission. we also discuss a summary of tests using these plans to evaluate the expected performance of the dtms and describe the data products derived from them. | digital terrain mapping by the osiris-rex mission |
this study examines the history of the paradigm concerning a lunar (or solar-system-wide) terminal cataclysm (also called "late heavy bombardment" or lhb), a putative, brief spike in impacts at ~3.9 ga ago, preceded by low impact rates. we examine origin of the ideas, why they were accepted, and why the ideas are currently being seriously revised, if not abandoned. the paper is divided into the following sections: overview of paradigm.pre-apollo views (1949–1969).initial suggestions of cataclysm (ca. 1974).ironies.alternative suggestions, megaregolith evolution (1970s).impact melt rocks "establish" cataclysm (1990).imbrium redux (ca. 1998).impact melt clasts (early 2000s).dating of front-side lunar basins?dynamical models "explain" the cataclysm (c. 2000s).asteroids as a test case.impact melts predating 4.0 ga ago (ca. 2008–present.).biological issues.growing doubts (ca. 1994–2014).evolving dynamical models (ca. 2001–present).connections to lunar origin.dismantling the paradigm (2015–2018)."megaregolith evolution model" for explaining the data.conclusions and new directions for future work. overview of paradigm. pre-apollo views (1949–1969). initial suggestions of cataclysm (ca. 1974). ironies. alternative suggestions, megaregolith evolution (1970s). impact melt rocks "establish" cataclysm (1990). imbrium redux (ca. 1998). impact melt clasts (early 2000s). dating of front-side lunar basins? dynamical models "explain" the cataclysm (c. 2000s). asteroids as a test case. impact melts predating 4.0 ga ago (ca. 2008–present.). biological issues. growing doubts (ca. 1994–2014). evolving dynamical models (ca. 2001–present). connections to lunar origin. dismantling the paradigm (2015–2018). "megaregolith evolution model" for explaining the data. conclusions and new directions for future work. the author hopes that this open-access discussion may prove useful for classroom discussions of how science moves forward through self-correction of hypotheses. | history of the terminal cataclysm paradigm: epistemology of a planetary bombardment that never (?) happened |
the planetary building blocks that formed in the terrestrial planet region were likely very dry, yet water is comparatively abundant on earth. here we review the various mechanisms proposed for the origin of water on the terrestrial planets. various in-situ mechanisms have been suggested, which allow for the incorporation of water into the local planetesimals in the terrestrial planet region or into the planets themselves from local sources, although all of those mechanisms have difficulties. comets have also been proposed as a source, although there may be problems fitting isotopic constraints, and the delivery efficiency is very low, such that it may be difficult to deliver even a single earth ocean of water this way. the most promising route for water delivery is the accretion of material from beyond the snow line, similar to carbonaceous chondrites, that is scattered into the terrestrial planet region as the planets are growing. two main scenarios are discussed in detail. first is the classical scenario in which the giant planets begin roughly in their final locations and the disk of planetesimals and embryos in the terrestrial planet region extends all the way into the outer asteroid belt region. second is the grand tack scenario, where early inward and outward migration of the giant planets implants material from beyond the snow line into the asteroid belt and terrestrial planet region, where it can be accreted by the growing planets. sufficient water is delivered to the terrestrial planets in both scenarios. while the grand tack scenario provides a better fit to most constraints, namely the small mass of mars, planets may form too fast in the nominal case discussed here. this discrepancy may be reduced as a wider range of initial conditions is explored. finally, we discuss several more recent models that may have important implications for water delivery to the terrestrial planets. | the delivery of water during terrestrial planet formation |
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