T. B. McCord

Character and Spatial Distribution of OH/H2O on the Surface of the Moon Seen by M3 on Chandrayaan-1

Lunar Water The Moon has been thought to be primarily anhydrous, although there has been some evidence for accumulated ice in permanently shadowed craters near its poles (see the Perspective by Lucey, published online 24 September). By analyzing recent infrared mapping by Chandrayaan-1 and Deep Impact, and reexamining Cassini data obtained during its early flyby of the Moon, Pieters et al. (p. 568, published online 24 September), Sunshine et al. (p. 565, published online 24 September), and Clark et al. (p. 562, published online 24 September) reveal a noticeable absorption signal for H2O and OH across much of the surface. Some variability in water abundance is seen over the course of the lunar day. The data imply that solar wind is depositing and/or somehow forming water and OH in minerals near the lunar surface, and that this trapped water is dynamic. Space-based spectroscopic measurements provide evidence for water or hydroxyl (OH) on the surface of the Moon The search for water on the surface of the anhydrous Moon had remained an unfulfilled quest for 40 years. However, the Moon Mineralogy Mapper (M3) on Chandrayaan-1 has recently detected absorption features near 2.8 to 3.0 micrometers on the surface of the Moon. For silicate bodies, such features are typically attributed to hydroxyl- and/or water-bearing materials. On the Moon, the feature is seen as a widely distributed absorption that appears strongest at cooler high latitudes and at several fresh feldspathic craters. The general lack of correlation of this feature in sunlit M3 data with neutron spectrometer hydrogen abundance data suggests that the formation and retention of hydroxyl and water are ongoing surficial processes. Hydroxyl/water production processes may feed polar cold traps and make the lunar regolith a candidate source of volatiles for human exploration.

Non‐water‐ice constituents in the surface material of the icy Galilean satellites from the Galileo near‐infrared mapping spectrometer investigation

We present evidence for several non-ice constituents in the surface material of the icy Galilean satellites, using the reflectance spectra returned by the Galileo near infrared mapping spectrometer (NIMS) experiment. Five new absorption features are described at 3.4, 3.88, 4.05, 4.25, and 4.57 μm for Callisto and Ganymede, and some seem to exist for Europa as well. The four absorption bands strong enough to be mapped on Callisto and Ganymede are each spatially distributed in different ways, indicating different materials are responsible for each absorption. The spatial distributions are correlated at the local level in complex ways with surface features and in some cases show global patterns. Suggested candidate spectrally active groups, perhaps within larger molecules, producing the five absorptions include C-H, S-H, SO2, CO2, and C≡N. Organic material like tholins are candidates for the 4.57- and 3.4-μm features. We suggest, based on spectroscopic evidence, that CO2 is present as a form which does not allow rotational modes and that SO2 is present neither as a frost nor a free gas. The CO2, SO2, and perhaps cyanogen (4.57 μm) may be present as very small collections of molecules within the crystal structure, perhaps following models for radiation damage and/or for comet and interstellar grain formation at low temperatures. Some of the dark material on these surfaces may be created by radiation damage of the CO2 and other carbon-bearing species and the formation of graphite. These spectra suggest a complex chemistry within the surface materials and an important role for non-ice materials in the evolution of the satellite surfaces.

Spectroscopic Characterization of Mineralogy and Its Diversity Across Vesta

A New Dawn Since 17 July 2011, NASAs spacecraft Dawn has been orbiting the asteroid Vesta—the second most massive and the third largest asteroid in the solar system (see the cover). Russell et al. (p. 684) use Dawns observations to confirm that Vesta is a small differentiated planetary body with an inner core, and represents a surviving proto-planet from the earliest epoch of solar system formation; Vesta is also confirmed as the source of the howardite-eucrite-diogenite (HED) meteorites. Jaumann et al. (p. 687) report on the asteroids overall geometry and topography, based on global surface mapping. Vestas surface is dominated by numerous impact craters and large troughs around the equatorial region. Marchi et al. (p. 690) report on Vestas complex cratering history and constrain the age of some of its major regions based on crater counts. Schenk et al. (p. 694) describe two giant impact basins located at the asteroids south pole. Both basins are young and excavated enough amounts of material to form the Vestoids—a group of asteroids with a composition similar to that of Vesta—and HED meteorites. De Sanctis et al. (p. 697) present the mineralogical characterization of Vesta, based on data obtained by Dawns visual and infrared spectrometer, revealing that this asteroid underwent a complex magmatic evolution that led to a differentiated crust and mantle. The global color variations detailed by Reddy et al. (p. 700) are unlike those of any other asteroid observed so far and are also indicative of a preserved, differentiated proto-planet. Spacecraft data provide a detailed characterization of the second most massive asteroid in the solar system. The mineralogy of Vesta, based on data obtained by the Dawn spacecraft’s visible and infrared spectrometer, is consistent with howardite-eucrite-diogenite meteorites. There are considerable regional and local variations across the asteroid: Spectrally distinct regions include the south-polar Rheasilvia basin, which displays a higher diogenitic component, and equatorial regions, which show a higher eucritic component. The lithologic distribution indicates a deeper diogenitic crust, exposed after excavation by the impact that formed Rheasilvia, and an upper eucritic crust. Evidence for mineralogical stratigraphic layering is observed on crater walls and in ejecta. This is broadly consistent with magma-ocean models, but spectral variability highlights local variations, which suggests that the crust can be a complex assemblage of eucritic basalts and pyroxene cumulates. Overall, Vesta mineralogy indicates a complex magmatic evolution that led to a differentiated crust and mantle.

The organic-rich surface of comet 67P/Churyumov-Gerasimenko as seen by VIRTIS/Rosetta

The VIRTIS (Visible, Infrared and Thermal Imaging Spectrometer) instrument on board the Rosetta spacecraft has provided evidence of carbon-bearing compounds on the nucleus of the comet 67P/Churyumov-Gerasimenko. The very low reflectance of the nucleus (normal albedo of 0.060 ± 0.003 at 0.55 micrometers), the spectral slopes in visible and infrared ranges (5 to 25 and 1.5 to 5% kÅ−1), and the broad absorption feature in the 2.9-to-3.6–micrometer range present across the entire illuminated surface are compatible with opaque minerals associated with nonvolatile organic macromolecular materials: a complex mixture of various types of carbon-hydrogen and/or oxygen-hydrogen chemical groups, with little contribution of nitrogen-hydrogen groups. In active areas, the changes in spectral slope and absorption feature width may suggest small amounts of water-ice. However, no ice-rich patches are observed, indicating a generally dehydrated nature for the surface currently illuminated by the Sun.

Vesta's shape and morphology

A New Dawn Since 17 July 2011, NASAs spacecraft Dawn has been orbiting the asteroid Vesta—the second most massive and the third largest asteroid in the solar system (see the cover). Russell et al. (p. 684) use Dawns observations to confirm that Vesta is a small differentiated planetary body with an inner core, and represents a surviving proto-planet from the earliest epoch of solar system formation; Vesta is also confirmed as the source of the howardite-eucrite-diogenite (HED) meteorites. Jaumann et al. (p. 687) report on the asteroids overall geometry and topography, based on global surface mapping. Vestas surface is dominated by numerous impact craters and large troughs around the equatorial region. Marchi et al. (p. 690) report on Vestas complex cratering history and constrain the age of some of its major regions based on crater counts. Schenk et al. (p. 694) describe two giant impact basins located at the asteroids south pole. Both basins are young and excavated enough amounts of material to form the Vestoids—a group of asteroids with a composition similar to that of Vesta—and HED meteorites. De Sanctis et al. (p. 697) present the mineralogical characterization of Vesta, based on data obtained by Dawns visual and infrared spectrometer, revealing that this asteroid underwent a complex magmatic evolution that led to a differentiated crust and mantle. The global color variations detailed by Reddy et al. (p. 700) are unlike those of any other asteroid observed so far and are also indicative of a preserved, differentiated proto-planet. Spacecraft data provide a detailed characterization of the second most massive asteroid in the solar system. Vesta’s surface is characterized by abundant impact craters, some with preserved ejecta blankets, large troughs extending around the equatorial region, enigmatic dark material, and widespread mass wasting, but as yet an absence of volcanic features. Abundant steep slopes indicate that impact-generated surface regolith is underlain by bedrock. Dawn observations confirm the large impact basin (Rheasilvia) at Vesta’s south pole and reveal evidence for an earlier, underlying large basin (Veneneia). Vesta’s geology displays morphological features characteristic of the Moon and terrestrial planets as well as those of other asteroids, underscoring Vesta’s unique role as a transitional solar system body.

Ammoniated phyllosilicates with a likely outer Solar System origin on (1) Ceres

Studies of the dwarf planet (1) Ceres using ground-based and orbiting telescopes have concluded that its closest meteoritic analogues are the volatile-rich CI and CM carbonaceous chondrites. Water in clay minerals, ammoniated phyllosilicates, or a mixture of Mg(OH)2 (brucite), Mg2CO3 and iron-rich serpentine have all been proposed to exist on the surface. In particular, brucite has been suggested from analysis of the mid-infrared spectrum of Ceres. But the lack of spectral data across telluric absorption bands in the wavelength region 2.5 to 2.9 micrometres—where the OH stretching vibration and the H2O bending overtone are found—has precluded definitive identifications. In addition, water vapour around Ceres has recently been reported, possibly originating from localized sources. Here we report spectra of Ceres from 0.4 to 5 micrometres acquired at distances from ~82,000 to 4,300 kilometres from the surface. Our measurements indicate widespread ammoniated phyllosilicates across the surface, but no detectable water ice. Ammonia, accreted either as organic matter or as ice, may have reacted with phyllosilicates on Ceres during differentiation. This suggests that material from the outer Solar System was incorporated into Ceres, either during its formation at great heliocentric distance or by incorporation of material transported into the main asteroid belt.

Dark material on Vesta from the infall of carbonaceous volatile-rich material

Localized dark and bright materials, often with extremely different albedos, were recently found on Vesta’s surface. The range of albedos is among the largest observed on Solar System rocky bodies. These dark materials, often associated with craters, appear in ejecta and crater walls, and their pyroxene absorption strengths are correlated with material brightness. It was tentatively suggested that the dark material on Vesta could be either exogenic, from carbon-rich, low-velocity impactors, or endogenic, from freshly exposed mafic material or impact melt, created or exposed by impacts. Here we report Vesta spectra and images and use them to derive and interpret the properties of the ‘pure’ dark and bright materials. We argue that the dark material is mainly from infall of hydrated carbonaceous material (like that found in a major class of meteorites and some comet surfaces), whereas the bright material is the uncontaminated indigenous Vesta basaltic soil. Dark material from low-albedo impactors is diffused over time through the Vestan regolith by impact mixing, creating broader, diffuse darker regions and finally Vesta’s background surface material. This is consistent with howardite–eucrite–diogenite meteorites coming from Vesta.

Dawn Arrives at Ceres: Exploration of a Small Volatile-Rich World

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.

Distinctive space weathering on Vesta from regolith mixing processes

The surface of the asteroid Vesta has prominent near-infrared absorption bands characteristic of a range of pyroxenes, confirming a direct link to the basaltic howardite–eucrite–diogenite class of meteorites. Processes active in the space environment produce ‘space weathering’ products that substantially weaken or mask such diagnostic absorption on airless bodies observed elsewhere, and it has long been a mystery why Vesta’s absorption bands are so strong. Analyses of soil samples from both the Moon and the asteroid Itokawa determined that nanophase metallic particles (commonly nanophase iron) accumulate on the rims of regolith grains with time, accounting for an observed optical degradation. These nanophase particles, believed to be related to solar wind and micrometeoroid bombardment processes, leave unique spectroscopic signatures that can be measured remotely but require sufficient spatial resolution to discern the geologic context and history of the surface, which has not been achieved for Vesta until now. Here we report that Vesta shows its own form of space weathering, which is quite different from that of other airless bodies visited. No evidence is detected on Vesta for accumulation of lunar-like nanophase iron on regolith particles, even though distinct material exposed at several fresh craters becomes gradually masked and fades into the background as the craters age. Instead, spectroscopic data reveal that on Vesta a locally homogenized upper regolith is generated with time through small-scale mixing of diverse surface components.

Distributions of CO2 and SO2 on the surface of Callisto

Absorption bands in the infrared reflectance spectra from the Galileo Near-Infrared Mapping Spectrometer (NIMS) which are attributed to the presence of CO2 and SO2 on the surface of Callisto have been analyzed and mapped in detail. CO2 of varying concentrations appears to exist everywhere on Callisto, except at higher latitudes, where it may be masked by frost. The CO2 concentration on the trailing hemisphere has a longitudinal distribution largely consistent with a sinusoid centered on the equator near 270° longitude. The approximately sinusoidal pattern suggests that exogenic effects related to Jupiters corotating magnetic field are involved. Closer inspection of both hemispheres reveals that in many cases, visibly bright and ice-rich impact craters have high CO2 concentrations within or near them. The CO2 sometimes appears to be associated more with dark material near the craters than with the water ice. These correlations suggest impact processes may also affect the distribution of CO2 on the surface of Callisto. The center of the absorption band has been refined to be at 4.258±0.004 μm. The presence of a single band shape and band minimum wavelength position in all data sets for the CO2 absorption implies the physical state of CO2 is similar over the surface of Callisto. The distribution of SO2 on the surface is less well defined owing to characteristically shallower band depths, but it appears generally mottled, with some areas of high concentrations correlated with ice-rich impact craters. Large-scale patterns include the depletion of SO2 in the polar regions and a depletion of SO2 on the trailing side relative to the leading side. There is no sinusoidal pattern to this depletion. The center of the SO2 band is determined to be between 4.01 and 4.02 μm.