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32 páginas.- 3 figuras.- 2 tablas.- 260 referencias, Nearly half a century ago, two papers postulated the likelihood of lunar lava tube caves using mathematical models. Today, armed with an array of orbiting and fly-by satellites and survey instrumentation, we have now acquired cave data across our solar system-including the identification of potential cave entrances on the Moon, Mars, and at least nine other planetary bodies. These discoveries gave rise to the study of planetary caves. To help advance this field, we leveraged the expertise of an interdisciplinary group to identify a strategy to explore caves beyond Earth. Focusing primarily on astrobiology, the cave environment, geology, robotics, instrumentation, and human exploration, our goal was to produce a framework to guide this subdiscipline through at least the next decade. To do this, we first assembled a list of 198 science and engineering questions. Then, through a series of social surveys, 114 scientists and engineers winnowed down the list to the top 53 highest priority questions. This exercise resulted in identifying emerging and crucial research areas that require robust development to ultimately support a robotic mission to a planetary cave-principally the Moon and/or Mars. With the necessary financial investment and institutional support, the research and technological development required to achieve these necessary advancements over the next decade are attainable. Subsequently, we will be positioned to robotically examine lunar caves and search for evidence of life within Martian caves; in turn, this will set the stage for human exploration and potential habitation of both the lunar and Martian subsurface., The following funding sources are recognized for supporting several of the contributing authors: Human Frontiers Science Program grant #RGY0066/2018 (for AAB), NASA Innovative Advanced Concepts Grant #80HQTR19C0034 (HJ, UYW, and WLW), and European Research Council, ERC Consolidator Grant #818602 (AGF), the Spanish Ministry of Science and Innovation (project PID2019-108672RJ-I00) and the "Ramon y Cajal" post-doctoral contract (grant #RYC2019-026885-I (AZM)), and Contract #80NM0018D0004 between the Jet Propulsion Laboratory, California Institute of Technology and the National Aeronautics and Space Administration (AA, MJM, KU, and LK).

The gravity and shape data acquired by the Dawn spacecraft during its primary mission revealed that Ceres is partially differentiated with an interior structure consistent with a volatile-rich crust, a mantle of hydrated rock and isostatically compensated topography1–3. Detailed analyses showed that the mechanically strong crust overlays a weak, fluid-bearing upper mantle4. Previous studies, however, assumed that Ceres’s crust is a uniform layer. Here, we report findings from the new high-resolution gravity data from Dawn’s second extended mission (XM2), which reveal a complex crustal structure of Ceres. In the low-altitude regions probed by the Dawn spacecraft during the XM2 phase, we observe that gravity–topography admittance progressively shifts to a lower density solution at higher degrees, implying a radial density gradient across Ceres’s crust that is consistent with decreasing porosity with depth and/or increasing content of dense phases, such as rock and salts. That gradient brings a critical new constraint on the crustal freezing history, suggesting that the salts and silicates concentrated in the liquid phase while the crust was growing. Localized spectral analysis of the new data also shows evidence for a lower crustal density in the north polar region than in the south or near the equator, supporting impact-driven porosity variations for the observed latitudinal density differences5. On the local scale, the new data show evidence for density or rheological variations within the crust, in association with lobate landslides and ejecta deposits that were inferred to be ice-rich6,7 as well as an extensional fault system8. These inferences provide geophysical context for geological features on the surface and help us advance our understanding of the evolution of an ice-rich but heat-starved body, whose evolution was in part shaped by impacts. High-resolution gravity data from Dawn’s second extended mission could probe the global and local structure of Ceres’s crust. The results show significant spatial and vertical variations of crustal density and porosity, associated with ice features and ice-related processes driven from the interior, and impacts.

The Lunar Polar Hydrogen Mapper (LunaH-Map) mission will map the distribution of hydrogen around the lunar South Pole using a miniature neutron spectrometer. The mission builds upon a decade of lunar science, which has revealed both regional and more localized enrichments of water ice near the lunar poles. Localized enrichments are primarily within permanently shadowed regions (PSRs) and craters throughout the South Pole. The spatial extent of these regions is often below the resolution of previous neutron instruments that have flown on lunar missions. The neutron leakage spectrum from planetary surfaces is primarily sensitive to hydrogen abundance in the top meter of regolith, however, for neutron spectrometers with omnidirectional sensitivity, the spatial resolution is limited by the spacecraft orbital altitude above the surface. A low altitude measurement from a distance on the same scale of the PSRs could spatially isolate and constrain the hydrogen enrichments both within and around within those regions. A small spacecraft mission is ideally suited to acquire the low-altitude measurements required to localize hydrogen enrichments using neutron spectroscopy at the lunar South Pole. LunaH-Map will use a solid iodine ion propulsion system, X-Band radio communications through the NASA Deep Space Network, star tracker, Command & Data Handling System, and EPS systems from Blue Canyon Technologies, solar arrays from MMA Designs, LLC, mission design and navigation by KinetX. Spacecraft systems design, integration, qualification, test, and mission operations are performed by Arizona State University, AZ Space Technologies and Qwaltec.

International audience; Ceres, the most water-rich body in the inner solar system after Earth, has recently been recognized to have astrobiological importance. Chemical and physical measurements obtained by the Dawn mission enabled the quantification of key parameters, which helped to constrain the habitability of the inner solar system's only dwarf planet. The surface chemistry and internal structure of Ceres testify to a protracted history of reactions between liquid water, rock, and likely organic compounds. We review the clues on chemical composition, temperature, and prospects for long-term occurrence of liquid and chemical gradients. Comparisons with giant planet satellites indicate similarities both from a chemical evolution standpoint and in the physical mechanisms driving Ceres' internal evolution.

International audience; Ceres' regolith contains water ice that has receded in response to insolation-driven sublimation. Specially targeted, high spatial-resolution measurements of hydrogen by Dawn's Gamma Ray and Neutron Detector (GRaND) reveal elevated hydrogen concentrations in and around Occator, a young, 90 km diameter, complex crater located at 19.82°N where near-surface ice is not expected. The excess hydrogen can be explained by impact excavation of water-rich outer crustal materials and their emplacement in the crater floor and ejecta blanket. This is supported by thermophysical models that show water ice could survive at sub-meter depths, given Occator's relatively young age (∼20 Myr). We hypothesize that the regolith can be replenished with ice from large impacts and that this process partially controls the distribution and depth of near surface ice. This is supported by results from Occator and similarities in the global distribution of hydrogen and the pattern of large craters (20-100 km diameter).

2 páginas.- 1 figura.- 16 referencias, To the Editor — 2021 is the International Year of Caves and Karst. To honour this occasion, we wish to emphasize the vast potential embodied in planetary subsurfaces. While researchers have pondered the possibility of extraterrestrial caves for more than 50 years, we have now entered the incipient phase of planetary caves exploration....

Combined analyses of the surface elemental composition and mineralogy of Vesta and Ceres provide insights into their interior evolution, crustal formation, and regolith processes. Compositional data acquired by Dawn's Visible to Infrared Mapping Spectrometer (VIR) and Gamma Ray and Neutron Detector (GRaND) are sensitive to different depths and spatial scales. To compare these data sets, high-resolution maps of absorption band strengths from VIR are degraded to the broad spatial scales sampled by GRaND using a physics-based smoothing algorithm that accounts for the shape and topography of Vesta and Ceres. On Vesta, the distributions of elemental hydrogen and hydroxyl are similar, which implies that hydrogen is primarily in the form of hydroxyl, likely as phyllosilicates delivered by the infall of carbonaceous chondrite impactors. Small differences in the spatial patterns of hydroxyl and hydrogen imply that hydrogen is layered in some locations. In Vesta's dark hemisphere, hydrogen deposits are more extensive than hydroxyl, which indicates higher concentrations of hydrated minerals at depth. In contrast, the distributions of elemental hydrogen and hydrogen-bearing species (OH and NH_4^+) on Ceres are dissimilar. High concentrations of hydrogen in the Ceres’ polar regions (approaching 30 wt.% equivalent H_2O) indicate the presence of subsurface ice as predicted by ice stability theory. The concentration of iron follows a water-dilution trend when plotted as a function of regolith hydrogen content, consistent with the presence of subsurface water ice. The VIR and GRaND data jointly constrain aspects of Ceres’ surface chemistry and evolution. GRaND iron measurements place a firm upper bound on magnetite content, which supports graphitized carbon as an alternative to magnetite as a darkening agent. Lower-bounds on the concentration of carbon in carbonates implied by VIR, together with the ratio of carbonates to organics in carbonaceous chondrite meteorite analogs suggest high concentrations of carbon within Ceres’ regolith. GRaND neutron measurements permit elevated carbon concentrations, equal to or in excess of that found in CI chondrites (greater than a few wt.%). Organic matter, detected by VIR at Ernutet crater, might be widespread and may have been converted to graphite, e.g. via UV exposure, elsewhere on the surface. Furthermore, elevated concentrations of carbonaceous material can explain the difference between iron and hydrogen concentrations measured by GRaND and the CI carbonaceous chondrites, which are representative of the materials from which Ceres accreted. The elemental measurements indicate that ice and rock fractionated during Ceres’ evolution producing a crust that differs in composition from the whole body.

The surface mineralogy of dwarf planet Ceres appears to be dominated by products of rock–fluid interactions, such as phyllosilicates—some of which are NH4-bearing—and carbonates1–3. Elemental concentrations derived from the inferred mineral mixing fractions, however, do not match measurements of H, C, K and Fe on Ceres4. A complicating factor in assessing Ceres’s unique surface composition is the secular accretion of asteroids typical of chondritic compositions. Here we show that Ceres’s mineral and elemental data can be explained by the presence of carbonaceous chondritic-like materials (~50–60 vol%), possibly due to infalling asteroids, admixed with aqueously altered endogenic materials that contain higher-than-chondritic concentrations of carbon. We find that Ceres’s surface may contain up to 20 wt% of carbon, which is more than five times higher than in carbonaceous chondrites. The coexistence of phyllosilicates, magnetite, carbonates and a high carbon content implies rock–water alteration played an important role in promoting widespread carbon chemistry. These findings unveil pathways for the synthesis of organic matter, with implications for their transport across the Solar System. Infrared and neutron spectroscopic observations by Dawn give contrasting results on the elemental composition of Ceres’s surface, which can be reconciled by assuming that Ceres’s surface contains ~20 wt% of carbon, coming from impacts by carbonaceous asteroids and/or generated by extensive aqueous alteration.

The Psyche mission’s Oxidation-Reduction Working Group is focused on understanding, determining, and applying the redox state of (16) Psyche to understand the origin of a metal-rich world. The oxidation-reduction state of an asteroid, along with its temperature, parent body size, and composition, is a key parameter in determining the history of an asteroid. Determining the redox state from spacecraft data is most easily done by examining potential metal-oxide buffer pairs. The occurrence of Ni, Fe, C, Cr, P and Si, in that order, in the metal or sulfide phase of an asteroidal body indicates increasingly reduced conditions. Key observations by the Imager and Gamma-Ray and Neutron Spectrometer (GRNS) of Psyche can bracket the redox state using metal-oxide buffers. The presence of Fe,Ni metal can be confirmed by the ratios of Fe/O or Fe/Si and the concentration of Ni variability in metal across the asteroid can be determined by GRNS. The FeO concentration of silicates is complementary to the Ni concentration of metal and can be constrained using filters on the Imager. The presence of FeO in silicates from ground-based observations is one of the few measurements we already have of redox state, although available data permit a wide range of silicate compositions and mineralogies. The presence of C, P or Si concentrated in the metallic, Fe-rich portion of the asteroid, as measured by GRNS, or Ca-sulfide, determined by imaging, would indicate increasingly reducing conditions. Linkage to known types of meteorites, whether metal-rich chondrites, stony-irons or irons, expands the mineralogical, chemical and isotopic data not available from remote observations alone. Redox also controls both silicate and metal mineralogy, influencing differentiation, solidification, and subsolidus cooling, including the relative abundance of sulfur in the core and possible magnetic signatures. The redox state of Psyche, if a fully-differentiated metallic core, might constrain the location and timing of both the formation of Psyche and any oxidation it might have experienced.

Research on planetary caves requires cross-planetary-body investigations spanning multiple disciplines, including geology, climatology, astrobiology, robotics, human exploration and operations. The community determined that a roadmap was needed to establish a common framework for planetary cave research. This white paper is our initial conception

Ceres, a nearly 1000-km diameter body located in the Solar System’s main asteroid belt, has been classified under many categories: planet, comet, asteroid, minor planet and, presently, dwarf planet. No matter what the designation, Ceres has experienced major planetary processes. Its evolution has been controlled by water, making it a most unusual, interesting and accessible inner Solar System object that can inform the evolution of outer Solar System moons and other dwarf planets. Early telescopic observations suggested a hydroxylated mineralogy similar to carbonaceous chondrite meteorites and a size and mass indicating a bulk density that implied a water content of 17−27 wt%. Thermodynamic modeling of Ceres’ evolution indicated that thermal aqueous evolution likely occurred. The Dawn Mission produced a huge increase in our understanding of Ceres, confirming but vastly extending the early knowledge. Dawn, carrying multispectral cameras, a visible-infrared imaging spectrometer and a nuclear spectrometer, orbited Ceres between 2015–2018 (after orbiting Vesta) at a number of different altitudes, ultimately reaching 35 km from the surface at periapsis. Observations of almost the entire surface and gravity field mapping revealed multiple geological and internal features attributed to the effects of water. The surface displays cryovolcanic-like and flow structures, exposed phyllosilicates, carbonates, evaporites and water ice. The subsurface shows partial differentiation, decreasing viscosity with depth, and lateral density heterogeneity. Ceres appears to be geologically active today and possesses liquid water/brine pockets or even an extended liquid layer in the interior, confirming an “Ocean World” designation in today’s vernacular.

Ceres, the only dwarf planet in the inner Solar System, appears to be a relict ocean world. Data collected by NASA’s Dawn spacecraft provided evidence that global aqueous alteration within Ceres resulted in a chemically evolved body that remains volatile-rich1. Recent emplacement of bright deposits sourced from brines attests to Ceres being a persistently geologically active world2,3, but the surprising longevity of this activity at the 92-km Occator crater has yet to be explained. Here, we use new high-resolution Dawn gravity data to study the subsurface architecture of the region surrounding Occator crater, which hosts extensive young bright carbonate deposits (faculae). Gravity data and thermal modelling imply an extensive deep brine reservoir beneath Occator, which we argue could have been mobilized by the heating and deep fracturing associated with the Occator impact, leading to long-lived extrusion of brines and formation of the faculae. Moreover, we find that pre-existing tectonic cracks may provide pathways for deep brines to migrate within the crust, extending the regions affected by impacts and creating compositional heterogeneity. The long-lived hydrological system resulting from the impact might also occur for large impacts in icy moons, with implications for creation of transient habitable niches over time. High-resolution data of Ceres’s bright spots (faculae), obtained by Dawn’s second extended mission, suggest the existence of a deep brine-rich reservoir that emerged to the surface through long-lived cryovolcanic activity as a consequence of the impact that created Occator crater.

Some years ago, the consensus was that asteroid (16) Psyche was almost entirely metal. New data on density, radar properties, and spectral signatures indicate that the asteroid is something perhaps even more enigmatic: a mixed metal and silicate world. Here we combine observations of Psyche with data from meteorites and models for planetesimal formation to produce the best current hypotheses for Psyche's properties and provenance. Psyche's bulk density appears to be between 3,400 and 4,100 kg m−3. Psyche is thus predicted to have between ~30 and ~60 vol% metal, with the remainder likely low‐iron silicate rock and not more than ~20% porosity. Though their density is similar, mesosiderites are an unlikely analog to bulk Psyche because mesosiderites have far more iron‐rich silicates than Psyche appears to have. CB chondrites match both Psyche's density and spectral properties, as can some pallasites, although typical pallasitic olivine contains too much iron to be consistent with the reflectance spectra. Final answers, as well as resolution of contradictions in the data set of Psyche physical properties, for example, the thermal inertia measurements, may not be resolved until the NASA Psyche mission arrives in orbit at the asteroid. Despite the range of compositions and formation processes for Psyche allowed by the current data, the science payload of the Psyche mission (magnetometers, multispectral imagers, neutron spectrometer, and a gamma‐ray spectrometer) will produce data sets that distinguish among the models., Key Points New observations of asteroid (16) Psyche's physical properties indicate a different composition and properties than earlier data had shownThe best current analysis indicates that Psyche's density is 3,400‐4,100 kg m‐3, indicating a mixture of rock with 30‐60 vol% metalContradictions in data of Psyche physical properties may not be resolved until the NASA Psyche mission arrives at the asteroid

International audience

We present a new SIngle-scintillator Neutron and Gamma-Ray spectrometer (SINGR) instrument for use with both passive and active measurement techniques. Here we discuss, the application of SINGR for planetary exploration missions; however, hydrology, nuclear non-proliferation, and resource prospecting are all potential areas where the instrument could be applied. SINGR uses an elpasolite scintillator , Cs 2YLiCl6 :Ce (CLYC), that has been shown to have high neutron efficiency even at small volumes, with a gamma-ray energy resolution of approximately 4% full-width-at-half-maximum at 662 keV. Active gamma-ray and neutron (GRNS) measurements were performed with SINGR at the NASA Goddard Space Flight Center (GSFC) Goddard Geophysical and Astronomical Observatory (GGAO) outdoor test site using a pulsed neutron generator (PNG) to interrogate geologically relevant materials (basalt and granite monuments). These experimental results, combined with simulations, demonstrate that SINGR is capable of generating neutron die-away curves that can be used to reconstruct the bulk hydrogen abundance and the depth distribution of hydrogen within the monuments. We compare our experimental results with Monte Carlo N-Particle (MCNP) 6.1 transport simulations to constrain the uncertainties in depth and hydrogen abundance from the neutron die-away data generated by SINGR. For future planetary exploration missions, SINGR provides a single detector system for interrogating the shallow subsurface to characterize the presence and abundance of hydrated phases and to provide bulk elemental analysis.

Dawn at Ceres Guest Investigator Program award [NNX15AI29G]; NSF Graduate Research Fellowship award [DGE-1143653]

Prior to the arrival of the Dawn spacecraft at Ceres, the dwarf planet was anticipated to be ice‐rich. Searches for morphological features related to ice have been ongoing during Dawn's mission at Ceres. Here we report the identification of pitted terrains associated with fresh Cerean impact craters. The Cerean pitted terrains exhibit strong morphological similarities to pitted materials previously identified on Mars (where ice is implicated in pit development) and Vesta (where the presence of ice is debated). We employ numerical models to investigate the formation of pitted materials on Ceres and discuss the relative importance of water ice and other volatiles in pit development there. We conclude that water ice likely plays an important role in pit development on Ceres. Similar pitted terrains may be common in the asteroid belt and may be of interest to future missions motivated by both astrobiology and in situ resource utilization., Key Points Fresh complex craters on Ceres host distinctive pitted terrains that are morphologically similar to pitted materials on Mars and VestaPitted terrains on Ceres likely form via the rapid volatilization of molecular H2O entrained in impact materialsPitted terrains may be common morphological markers of volatile‐rich near‐surface material in the asteroid belt

We use data collected by the Dawn Gamma-Ray and Neutron Detector (GRaND) at Vesta to map compositions corresponding to nearly pure igneous lithologies in the howardite, eucrite, diogenite (HED) meteorite clan (samples likely from Vesta). At the ∼300-km spatial scale of GRaND measurements, basaltic eucrite occurs on only 3% of the surface, whereas cumulate eucrite and orthopyroxenitic diogenite are not detected. The basaltic eucrite region is generally coincident with an area of the surface with thick regolith, elevated H, and moderate crater density, and may represent the best compositional sample of primordial vestan crust. We observe an absence of pure orthopyroxenitic diogenite terrains in the Rheasilvia basin and its ejecta, an observation corroborated by VIR (0.1%), which suggests the south-polar crust was a polymict mixture of igneous lithologies (howardite) at the time of the Rheasilvia impact, or was a thick basaltic eucrite crust with heterogeneously distributed orthopyroxenitic diogenite plutons. The most dominant igneous composition detected (11% of the surface) corresponds to one of the least-abundant igneous lithologies in the HED meteorite collection, the Yamato Type B diogenites (plagioclase-bearing pyroxenites). The distribution of Type B diogenite is spatially correlated with post-Rheasilvia craters in the north-polar region that are in close proximity to the Rheasilvia basin antipode. This suggests that north-polar Type B plutonism may have been associated with the Rheasilvia impact event. We propose that this was either through 1) uplift of pre-existing plutons at the antipode through focusing of Rheasilvia impact stress, or 2) Rheasilvia impact antipodal crustal melting, creating magmas that underwent fractionation to produce Type B plutons.

Context. (1) Ceres is the largest body in the main asteroid belt and one of the most intriguing objects in the solar system, in part because of the discovery of water outgassing by the Herschel Space Observatory (HSO) and its still-debated origin. Ceres was the target of NASA’s Dawn spacecraft for 3.5 yr, which achieved a detailed characterization of the dwarf planet. The possible influence of the local flux of solar energetic particles (SEP) on the production of a Cerean exosphere and water vapor has been suggested, in addition to the sublimation of water ice that depends on the temperature, meaning the heliocentric distance. Aims. We used the opportunity of both the perihelion passage of (1) Ceres in April 2018, and the presence of Dawn in its vicinity (for measuring the SEP flux in real time) to check the influence of heliocentric distance and SEP flux on water outgassing. Methods. We searched for OH emission lines near the limb of Ceres in the near-UV with the UVES spectrograph mounted on the 8-m ESO Very Large Telescope. Two spectra were recorded when Ceres was close to its perihelion, in February 2018, and with Dawn spacecraft orbiting Ceres. It was possible to simultaneously measure energetic particles around Ceres at the time of our observations. Results. Our observations did not permit detection of OH emission lines to a very high sensitivity level. This level is estimated to correspond to a global water production rate of QH2O ∽ 2 × 1026 molecules s−1, similar to the water production rate derived from HSO observations. The solar energetic particles flux measured around Ceres was negligible at the time of these observations. Conclusions. Our observations support the idea that heliocentric distance (i.e., the sublimation of water ice) does not play a major role in the water emission from Ceres. This production rate could be either related to SEP events or to other mechanisms, possibly of endogenic origin.

We present a comprehensive global catalog of the geomorphological features with clear or potential relevance to subsurface ice identified during the Dawn spacecraft's primary and first extended missions at Ceres. We define eight broad feature classes and describe analyses supporting their genetic links to subsurface ice. These classes include relaxed craters; central pit craters; large domes; small mounds; lobate landslides and ejecta; pitted materials; depressions and scarps; and fractures, grooves, and channels. Features in all classes are widely distributed on the dwarf planet, consistent with multiple lines of observational evidence that ice is a key component of Ceres' crust. Independent analyses of multiple feature types suggest rheological and compositional layering may be common in the upper 10 km of the crust. Clustering of features indicates that ice concentration is heterogeneous on nearly all length scales, from 1 km to hundreds of kilometers. Impacts are likely the key driver of heterogeneity, causing progressive devolatilization of the low latitude and midlatitude crust on billion-year timescales but also producing localized enhancements in near surface ice content via excavation of deep ice-rich material and possible facilitation of cryomagmatic and cryovolcanic activity. Impacts and landslides may be the dominant mechanism for ice loss on modern Ceres. Our analysis suggests specific locations where future high-resolution imaging can be used to probe (1) current volatile loss rates and (2) the history of putative cryomagmatic and cryovolcanic features. The Cerean cryosphere and its unique morphology promise to be a rich subject of ongoing research for years to come.

The mineralogy and geochemistry of Ceres, as constrained by Dawn's instruments, are broadly consistent with a carbonaceous chondrite (CM/CI) bulk composition. Differences explainable by Ceres’s more advanced alteration include the formation of Mg-rich serpentine and ammoniated clay; a greater proportion of carbonate and lesser organic matter; amounts of magnetite, sulfide, and carbon that could act as spectral darkening agents; and partial fractionation of water ice and silicates in the interior and regolith. Ceres is not spectrally unique, but is similar to a few other C-class asteroids, which may also have suffered extensive alteration. All these bodies are among the largest carbonaceous chondrite asteroids, and they orbit in the same part of the Main Belt. Thus, the degree of alteration is apparently related to the size of the body. Although the ammonia now incorporated into clay likely condensed in the outer nebula, we cannot presently determine whether Ceres itself formed in the outer solar system and migrated inward or was assembled within the Main Belt, along with other carbonaceous chondrite bodies.

More than 200 years after its discovery, asteroid (4) Vesta is thought to be the parent body for the howardite, eucrite and diogenite (HED) meteorites. The Dawn spacecraft spent ∼14 months in orbit around this largest, intact differentiated asteroid to study its internal structure, geology, mineralogy and chemistry. Carrying a suite of instruments that included two framing cameras, a visible-near infrared spectrometer, and a gamma-ray and neutron detector, coupled with radio tracking for gravity, Dawn revealed a geologically and geochemically complex world. A constrained core size of ∼110–130 km radius is consistent with predictions based on differentiation models for the HED meteorite parent body. Hubble Space Telescope observations had already shown that Vesta is scarred by a south polar basin comparable in diameter to that of the asteroid itself. Dawn showed that the south polar Rheasilvia basin dominates the asteroid, with a central uplift that rivals the large shield volcanoes of the Solar System in height. An older basin, Veneneia, partially underlies Rheasilvia. A series of graben-like equatorial and northern troughs were created during these massive impact events 1–2 Ga ago. These events also resurfaced much of the southern hemisphere and exposed deeper-seated diogenitic lithologies. Although the mineralogy and geochemistry vary across the surface for rock-forming elements and minerals, the range is small, suggesting that impact processes have efficiently homogenized the surface of Vesta at scales observed by the instruments on the Dawn spacecraft. The distribution of hydrogen is correlated with surface age, which likely results from the admixture of exogenic carbonaceous chondrites with Vesta's basaltic surface. Clasts of such material are observed within the surficial howardite meteorites in our collections. Dawn significantly strengthened the link between (4) Vesta and the HED meteorites, but the pervasive mixing, lack of a convincing and widespread detection of olivine, and poorly-constrained lateral and vertical extents of units leaves unanswered the central question of whether Vesta once had a magma ocean. Dawn is continuing its mission to the presumed ice-rich asteroid (1) Ceres.

The globally-averaged concentrations of radioelements K and Th within Vesta’s regolith are determined from gamma ray spectra acquired by Dawn’s Gamma Ray and Neutron Detector (GRaND). Spectra measured by GRaND’s bismuth germanate (BGO) scintillator, while in close proximity to Vesta, are analyzed. Improvements in data reduction and analysis methods enable detection and quantification of K and Th. Ample precision is achieved using the entire data set acquired by Dawn during 5 months of low-altitude operations. A simple, analytic model, which can be applied to measurements of Vesta and Ceres, is used to determine radioelement concentrations from measured counting rates. Systematic errors in the analysis are evaluated using simulated gamma ray spectra for representative vestan meteorite compositions. Concentrations of K and Th within Vesta’s global regolith, measured by GRaND, are consistent with eucrite-rich howardite, and are distinct from most achondrites, all chondrites, and Mars meteorites. The K/Th ratio of Vesta (900 ± 400) is similar to the average ratio for howardite (approximately 1200). These radioelement data, along with major element ratios determined by nuclear spectroscopy, strongly support the hypothesis that Vesta is the parent body of the HEDs. The depletion of moderately-volatile elements implied by the measured K/Th ratio is consistent with early accretion of Vesta from a hot, incompletely condensed solar nebula and/or, less likely, subsequent removal of volatiles by energetic collisions or degassing of magmas.

Here, we construct a comprehensive howardite, eucrite, and diogenite (HED) bulk chemistry data set to compare with Dawn data. Using the bulk chemistry data set, we determine four gamma-ray/neutron parameters in the HEDs (1) relative fast neutron counts (fast counts), (2) macroscopic thermal neutron absorption cross section (absorption), (3) a high-energy gamma-ray compositional parameter (Cp), and (4) Fe abundance. These correspond to the four measurements of Vesta made by Dawn's Gamma Ray and Neutron Detector (GRaND) that can be used to discern HED lithologic variability on the Vestan surface. We investigate covariance between fast counts and average atomic mass ( ) in the meteorite data set, where a strong correlation (r2 = 0.99) is observed, and we demonstrate that systematic offsets from the fast count/ trend are linked to changes in Fe and Ni concentrations. To compare the meteorite and GRaND data, we investigate and report covariance among fast counts, absorption, Cp, and Fe abundance in the HED meteorite data set. We identify several GRaND measurement spaces where the Yamato type B diogenites are distinct from all other HED lithologies, including polymict mixtures. The type B's are diogenites that are enriched in Fe + pigeonite + diopside ± plagioclase, relative to typical, orthopyroxenitic diogenites. We then compare these results to GRaND data and demonstrate that regions north of ~70°N latitude on Vesta (including the north pole) are consistent with type B diogenites. We propose two models to explain type B diogenite compositions in the north (1) deposition as Rheasilvia ejecta, or (2) type B plutons that were emplaced at shallow depths in the north polar region and sampled by local impacts. Lastly, using principal component (PC) analysis, we identify unique PC spaces for all HED lithologies, indicating that the corresponding GRaND measurables may be used to produce comprehensive lithologic maps for Vesta.

The first map of bulk hydrogen concentrations in the lunar highlands region is reported. This map is derived using data from the Lunar Prospector Neutron Spectrometer (LP-NS). We resolve prior ambiguities in the interpretation of LP-NS data with respect to non-polar hydrogen concentrations by comparing the LP-NS data with maps of the 750 nm albedo reflectance, optical maturity, and the wavelength position of the thermal infrared Christiansen Feature. The best explanation for the variations of LP-NS epithermal neutron data in the lunar highlands is variable amounts of solar-wind-implanted hydrogen. The average hydrogen concentration across the lunar highlands and away from the lunar poles is 65 ppm. The highest hydrogen values range from 120 ppm to just over 150 ppm. These values are consistent with the range of hydrogen concentrations from soils and regolith breccias at the Apollo 16 highlands landing site. Based on a moderate-to-strong correlation of epithermal neutrons and orbit-based measures of surface maturity, the map of highlands hydrogen concentration represents a new global maturity index that can be used for studies of the lunar soil maturation process. We interpret these hydrogen concentrations to represent a bulk soil property related to the long-term impact of the space environment on the lunar surface. Consequently, the derived hydrogen concentrations are not likely related to the surficial enhancements (top tens to hundreds of microns) or local time variations of OH/H 2 O measured with spectral reflectance data.

On 6 March 2015, Dawn arrived at Ceres to find a dark, desiccated surface punctuated by small, bright areas. Parts of Ceres’ surface are heavily cratered, but the largest expected craters are absent. Ceres appears gravitationally relaxed at only the longest wavelengths, implying a mechanically strong lithosphere with a weaker deep interior. Ceres’ dry exterior displays hydroxylated silicates, including ammoniated clays of endogenous origin. The possibility of abundant volatiles at depth is supported by geomorphologic features such as flat crater floors with pits, lobate flows of materials, and a singular mountain that appears to be an extrusive cryovolcanic dome. On one occasion, Ceres temporarily interacted with the solar wind, producing a bow shock accelerating electrons to energies of tens of kilovolts.

We present improved Mars Odyssey Neutron Spectrometer (MONS) maps of near-surface Water-Equivalent Hydrogen (WEH) on Mars that have intriguing implications for the global distribution of "excess" ice, which occurs when the mass fraction of water ice exceeds the threshold amount needed to saturate the pore volume in normal soils. We have refined the crossover technique of Feldman et al. (2011) by using spatial deconvolution and Gaussian weighting to create the first globally self-consistent map of WEH. At low latitudes, our new maps indicate that WEH exceeds 15% in several near-equatorial regions, such as Arabia Terra, which has important implications for the types of hydrated minerals present at low latitudes. At high latitudes, we demonstrate that the disparate MONS and Phoenix Robotic Arm (RA) observations of near surface WEH can be reconciled by a three-layer model incorporating dry soil over fully saturated pore ice over pure excess ice: such a three-layer model can also potentially explain the strong anticorrelation of subsurface ice content and ice table depth observed at high latitudes. At moderate latitudes, we show that the distribution of recently formed impact craters is also consistent with our latest MONS results, as both the shallowest ice-exposing crater and deepest non-ice-exposing crater at each impact site are in good agreement with our predictions of near-surface WEH. Overall, we find that our new mapping is consistent with the widespread presence at mid-to-high Martian latitudes of recently deposited shallow excess ice reservoirs that are not yet in equilibrium with the atmosphere., 89 pages, 33 figures, revision submitted to Icarus

Water ice beneath the surface of Ceres The dwarf planet Ceres in the inner solar system is thought to have a crust made of a mixture of rock and ice. Prettyman et al. used neutron and gamma-ray spectroscopy from the Dawn spacecraft to peer below Ceres' surface and map the subsurface composition. They found evidence for water ice across the dwarf planet, with water making up a larger fraction of the material near the poles than around the equator. Together with their measurements of other elements, these results aid our understanding of Ceres' composition and constrain models of its formation. Science , this issue p. 55

The Dawn mission’s Framing Camera (FC) observed Asteroid (4) Vesta in 2011 and 2012 using seven color filters and one clear filter from different orbits. In the present paper we analyze recalibrated HAMO color cubes (spatial resolution ∼60 m/pixel) with a focus on dark material (DM). We present a definition of highly concentrated DM based on spectral parameters, subsequently map the DM across the Vestan surface, geologically classify DM, study its spectral properties on global and local scales, and finally, compare the FC in-flight color data with laboratory spectra. We have discovered an absorption band centered at 0.72 μm in localities of DM that show the lowest albedo values by using FC data as well as spectral information from Dawn’s imaging spectrometer VIR. Such localities are contained within impact-exposed outcrops on inner crater walls and ejecta material. Comparisons between spectral FC in-flight data, and laboratory spectra of meteorites and mineral mixtures in the wavelength range 0.4–1.0 μm, revealed that the absorption band can be attributed to the mineral serpentine, which is typically present in CM chondrites. Dark material in its purest form is rare on Vesta’s surface and is distributed globally in a non-uniform manner. Our findings confirm the hypothesis of an exogenic origin of the DM by the infall of carbonaceous chondritic material, likely of CM type. It further confirms the hypothesis that most of the DM was deposited by the Veneneia impact.

Most asteroids are collisional rubble from eons past, and few of them have survived intact. Vesta, the second most massive asteroid, is the only differentiated, rocky body in this category. This asteroid provides a unique view of the kinds of planetesimals that accreted to form the terrestrial planets. We know more about this asteroid than any other, thanks to its recently completed exploration by the orbiting Dawn spacecraft and studies of the ~1000 meteorites derived from it. The synergy provided by in situ analyses and samples has allowed an unparalleled understanding of Vesta's mineralogy, petrology, geochemistry, and geochronology.

The Dawn mission has provided new evidence strengthening the identification of asteroid Vesta as the parent body of the howardite, eucrite, and diogenite (HED) meteorites. The evidence includes Vesta's petrologic complexity, detailed spectroscopic characteristics, unique space weathering, diagnostic geochemical abundances and neutron absorption characteristics, chronology of surface units and impact history, occurrence of exogenous carbonaceous chondritic materials in the regolith, and dimensions of the core, all of which are consistent with HED observations and constraints. Global mapping of the distributions of HED lithologies by Dawn cameras and spectrometers provides the missing geologic context for these meteorites, thereby allowing tests of petrogenetic models and increasing their scientific value.

Global maps of the macroscopic thermal neutron absorption cross section of Vesta's regolith by the Gamma Ray and Neutron Detector (GRaND) on board the NASA Dawn spacecraft provide constraints on the abundance and distribution of Fe, Ca, Al, Mg, and other rock-forming elements. From a circular, polar low-altitude mapping orbit, GRaND sampled the regolith to decimeter depths with a spatial resolution of about 300 km. At this spatial scale, the variation in neutron absorption is about seven times lower than that of the Moon. The observed variation is consistent with the range of absorption for howardite whole-rock compositions, which further supports the connection between Vesta and the howardite, eucrite, and diogenite meteorites. We find a strong correlation between neutron absorption and the percentage of eucritic materials in howardites and polymict breccias, which enables petrologic mapping of Vesta's surface. The distribution of basaltic eucrite and diogenite determined from neutron absorption measurements is qualitatively similar to that indicated by visible and near infrared spectroscopy. The Rheasilvia basin and ejecta blanket has relatively low absorption, consistent with Mg-rich orthopyroxene. Based on a combination of Fe and neutron absorption measurements, olivine-rich lithologies are not detected on the spatial scales sampled by GRaND. The sensitivity of GRaND to the presence of mantle material is described and implications for the absence of an olivine signature are discussed. High absorption values found in Vesta's “dark” hemisphere, where exogenic hydrogen has accumulated, indicate that this region is richer in basaltic eucrite, representative of Vesta's ancient upper crust.

Surface composition information from Vesta is reported using fast neutron data collected by the gamma ray and neutron detector on the Dawn spacecraft. After correcting for variations due to hydrogen, fast neutrons show a compositional dynamic range and spatial variability that is consistent with variations in average atomic mass from howardite, eucrite, and diogenite (HED) meteorites. These data provide additional compositional evidence that Vesta is the parent body to HED meteorites. A subset of fast neutron data having lower statistical precision show spatial variations that are consistent with a 400 ppm variability in hydrogen concentrations across Vesta and supports the idea that Vesta's hydrogen is due to long-term delivery of carbonaceous chondrite material.

[1] The Framing Camera (FC) on the Dawn spacecraft provided the first view of 4 Vesta at sufficiently high spatial resolution to enable a detailed correlation of the asteroid's spectral properties with geologic features and with the vestoid (V-type) asteroids and the Howardite-Eucrite-Diogenite (HED) class of meteorites, both of which are believed to originate on Vesta. We combine a spectral analysis of the basin with visible and near-IR spectroscopy of vestoids and with archived data over the same spectral range for HED meteorites. The vestoids are only slightly more akin to the Rheasilvia basin than to Vesta as a whole, suggesting that the crustal material ejected is a well-mixed collection of eucritic and diogenitic materials. The basin itself is more diogenitic, implying Vesta is differentiated and the impact that created Rheasilvia uncovered a mineralogically distinct layer. The Rheasilvia basin exhibits a larger range in pyroxene band strengths than Vesta as a whole, further implying that the basin offers a view into a complex, differentiated protoplanet. The discrepancy between the spectral properties of the HED meteorites and Vesta, in particular the meteorites' deeper pyroxene absorption band and the redder color of the vestoids, can be explained by the abundance of smaller particles on Vesta and by the addition of low-albedo exogenous particles to its surface, which in turn are due to its larger gravity and longer exposure time to impact processing. Solar phase effects are slight and do not explain the spectral discrepancies between the HEDs, Vesta, and the vestoids.

Measurements of the high-energy gamma-ray flux emanating from asteroid 4 Vesta by the Dawn Gamma-Ray and Neutron Detector (GRaND) have revealed variability in the near-surface elemental composition of the Vestan surface. These observations are consistent with the presence of large (≥8 × 104 km2) regions with distinct, HED-like elemental compositions. The results agree broadly with other global measurements, such as the macroscopic neutron absorption cross section and spectral reflectance-derived mineralogic maps. Two distinct regions with eucrite-like elemental compositions have been identified, the first located primarily within the Lucaria and Marcia quadrangles and the second within Oppia quadrangle. The former region is collocated with some of the oldest, most heavily cratered terrain on Vesta. The interior of the 500 km diameter Rheasilvia impact basin is found to have a composition that is consistent with diogenite-like material. Taken together, these observations support the hypothesis that Vesta's original crust was composed of basaltic outflows in the form of eucritic-like material and that the Rheasilvia-basin-forming impact exposed lower-crustal, diogenite-like material. These measurements also constrain the maximum amount of mesosiderite-like material to