Explore the vast range of titles available.

Organic compounds occur in some chondritic meteorites, and their signatures on solar system bodies have been sought for decades. Spectral signatures of organics have not been unambiguously identified on the surfaces of asteroids, whereas they have been detected on cometary nuclei. Data returned by the Visible and InfraRed Mapping Spectrometer on board the Dawn spacecraft show a clear detection of an organic absorption feature at 3.4 micrometers on dwarf planet Ceres. This signature is characteristic of aliphatic organic matter and is mainly localized on a broad region of ~1000 square kilometers close to the ~50-kilometer Ernutet crater. The combined presence on Ceres of ammonia-bearing hydrated minerals, water ice, carbonates, salts, and organic material indicates a very complex chemical environment, suggesting favorable environments to prebiotic chemistry.

Sodium and, in a lesser way, potassium atomic components of surface-bounded exospheres are among the brightest elements that can be observed from the Earth in our Solar System. Both species have been intensively observed around Mercury, the Moon and the Galilean Moons. During the last decade, new observations have been obtained thanks to space missions carrying remote and in situ instrumentation that provide a completely original view of these species in the exospheres of Mercury and the Moon. They challenged our understanding and modelling of these exospheres and opened new directions of research by suggesting the need to better take into account the relationship between the surface-exosphere and the magnetosphere. In this paper, we first review the large set of observations of Mercury and the Moon Sodium and Potassium exospheres. In the second part, we list what it tells us on the sources and sinks of these exospheres focusing in particular on the role of their magnetospheres of these objects and then discuss, in a third section, how these observations help us to understand and identify the key drivers of these exospheres.

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.

Knowledge of the surface temperature distribution on a comet’s nucleus and its temporal evolution at different timescales is key to constraining its thermophysical properties and understanding the physical processes that take place at and below the surface. Here we report on time-resolved maps of comet 67P/Churyumov–Gerasimenko retrieved on the basis of infrared data acquired by the Visible InfraRed and Thermal Imaging Spectrometer (VIRTIS) onboard the Rosetta orbiter in 2014, over a roughly two-month period in the pre-perihelion phase at heliocentric distances between 3.62 and 3.31 au from the Sun. We find that at a spatial resolution ≤15 m per pixel, the measured temperatures point out the major effect that self-heating, due to the complex shape of the nucleus, has on the diurnal temperature variation. The bilobate nucleus of comet 67P also induces daytime shadowing effects, which result in large thermal gradients. Over longer periods, VIRTIS-derived temperature values reveal seasonal changes driven by decreasing heliocentric distance combined with an increasing abundance of ice within the uppermost centimetre-thick layer, which implies the possibility of having a largely pristine nucleus interior already in the shallow subsurface.Diurnal and seasonal temperature variations of comet 67P’s nucleus are monitored by Rosetta’s imaging spectrometer VIRTIS during two months in 2014. The nucleus appears thermally homogeneous, with the temperature fluctuations mainly controlled by self-heating and heliocentric distance.

High-resolution near-infrared observations of the Occator bright areas on the dwarf planet Ceres suggest that the bright material is mostly made up of endogenous sodium carbonate. Ceres carbonates catch the eye NASA's Dawn orbiter probe has revealed localized bright areas on the surface of the dwarf asteroid-belt planet Ceres, most prominently in the Occator crater. These features were tentatively interpreted as containing a large amount of hydrated magnesium sulfates. Now Maria Cristina De Sanctis et al . present high-resolution near-infrared spectra of the Occator bright areas that suggest that the bright material consists mostly of endogenous sodium carbonate, mixed with a dark component and small amounts of phyllosilicates, as well as ammonium carbonate or ammonium chloride. The authors propose that these compounds are residues from the crystallization of brines, following upwelling through nearby fracture systems, together with entrained altered solids that reached the surface from below. Such a model requires a heat source, which may have been transient, triggered by impact heating for instance. Alternatively, internal temperatures may be above the eutectic temperature of subsurface brines, in which case fluids may exist at depth on Ceres today. The typically dark surface of the dwarf planet Ceres is punctuated by areas of much higher albedo, most prominently in the Occator crater 1 . These small bright areas have been tentatively interpreted as containing a large amount of hydrated magnesium sulfate 1 , in contrast to the average surface, which is a mixture of low-albedo materials and magnesium phyllosilicates, ammoniated phyllosilicates and carbonates 2 , 3 , 4 . Here we report high spatial and spectral resolution near-infrared observations of the bright areas in the Occator crater on Ceres. Spectra of these bright areas are consistent with a large amount of sodium carbonate, constituting the most concentrated known extraterrestrial occurrence of carbonate on kilometre-wide scales in the Solar System. The carbonates are mixed with a dark component and small amounts of phyllosilicates, as well as ammonium carbonate or ammonium chloride. Some of these compounds have also been detected in the plume of Saturn’s sixth-largest moon Enceladus 5 . The compounds are endogenous and we propose that they are the solid residue of crystallization of brines and entrained altered solids that reached the surface from below. The heat source may have been transient (triggered by impact heating). Alternatively, internal temperatures may be above the eutectic temperature of subsurface brines, in which case fluids may exist at depth on Ceres today.

Infrared spectra of (1) Ceres acquired at distances of 82,000 to 4,300 kilometres from the surface indicate widespread ammoniated phyllosilicates; the presence of ammonia suggests that material from the outer Solar System was incorporated into Ceres. Ammonia compounds on the surface of Ceres The VIR spectrometer onboard NASA's Dawn spacecraft has obtained infrared spectra of the dwarf planet Ceres at distances of 82,000 to 4,300 kilometres and at wavelengths of 0.4–5 μm, including the 2.6–2.9 μm spectral region not accessible to Earth-bound telescopes due to atmospheric absorption. The data indicate the widespread presence of ammoniated phyllosilicates across the asteroid's surface. No water ice could be detected, though small localized occurrences of water ice cannot be excluded. The discovery of ammonia implies 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. 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 1 , 2 . Water in clay minerals 3 , ammoniated phyllosilicates 4 , or a mixture of Mg(OH) 2 (brucite), Mg 2 CO 3 and iron-rich serpentine 5 , 6 have all been proposed to exist on the surface. In particular, brucite has been suggested from analysis of the mid-infrared spectrum of Ceres 6 . 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 H 2 O bending overtone are found—has precluded definitive identifications. In addition, water vapour around Ceres has recently been reported 7 , 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.

The dwarf planet Ceres is known to host phyllosilicate minerals at its surface, but their distribution and origin have not previously been determined. We used the spectrometer onboard the Dawn spacecraft to map their spatial distribution on the basis of diagnostic absorption features in the visible and near-infrared spectral range (0.25 to 5.0 micrometers). We found that magnesium- and ammonium-bearing minerals are ubiquitous across the surface. Variations in the strength of the absorption features are spatially correlated and indicate considerable variability in the relative abundance of the phyllosilicates, although their composition is fairly uniform. These data, along with the distinctive spectral properties of Ceres relative to other asteroids and carbonaceous meteorites, indicate that the phyllosilicates were formed endogenously by a globally widespread and extensive alteration process.

The Dawn spectrometer (VIR) is a hyperspectral spectrometer with imaging capability. The design fully accomplishes Dawn’s scientific and measurement objectives. Determination of the mineral composition of surface materials in their geologic context is a primary Dawn objective. The nature of the solid compounds of the asteroid (silicates, oxides, salts, organics and ices) can be identified by visual and infrared spectroscopy using high spatial resolution imaging to map the heterogeneity of asteroid surfaces and high spectral resolution spectroscopy to determine the composition unambiguously. The VIR Spectrometer—covering the range from the near UV (0.25 μm) to the near IR (5.0 μm) and having moderate to high spectral resolution and imaging capabilities—is the appropriate instrument for the determination of the asteroid global and local properties. VIR combines two data channels in one compact instrument. The visible channel covers 0.25–1.05 μm and the infrared channel covers 1–5.0 μm. VIR is inherited from the VIRTIS mapping spectrometer (Coradini et al. in Planet. Space Sci. 46:1291–1304, 1998 ; Reininger et al. in Proc. SPIE 2819:66–77, 1996 ) on board the ESA Rosetta mission. It will be operated for more than 2 years and spend more than 10 years in space.

Although olivine was expected to occur within the deep, south-pole basins of asteroid Vesta, which are thought to be excavated mantle rocks, spectral data from NASA’s Dawn spacecraft show that it instead occurs as near-surface materials in Vesta’s northern hemisphere. Surprises in store on Vesta Between July 2011 and September 2012, NASA's Dawn spacecraft was in orbit around the asteroid Vesta. In this paper, Dawn's Visible and Infrared Mapping Spectrometer (VIR) team presents a surprising finding — the signature of olivine on the asteroid's surface. Olivine is a major component of the mantle of differentiated bodies, including Earth. Vesta is a large asteroid, large enough to have differentiated into an Earth-like layered structure and the expectation was that olivine would be found within Vesta's deep, south-pole basins, thought to be excavated mantle rocks. Yet the spectroscopic data reveal olivine-rich material close to the surface in the northern hemisphere. An understanding of the differentiation processes that have occurred on Vesta will be invaluable as a window on the primordial Solar System, but these latest findings show that Vesta's evolutionary history is more complicated than was thought. Olivine is a major component of the mantle of differentiated bodies, including Earth. Howardite, eucrite and diogenite (HED) meteorites represent regolith, basaltic-crust, lower-crust and possibly ultramafic-mantle samples of asteroid Vesta, which is the lone surviving, large, differentiated, basaltic rocky protoplanet in the Solar System 1 . Only a few of these meteorites, the orthopyroxene-rich diogenites, contain olivine, typically with a concentration of less than 25 per cent by volume 2 . Olivine was tentatively identified on Vesta 3 , 4 , on the basis of spectral and colour data, but other observations did not confirm its presence 5 . Here we report that olivine is indeed present locally on Vesta’s surface but that, unexpectedly, it has not been found within the deep, south-pole basins, which are thought to be excavated mantle rocks 6 , 7 , 8 . Instead, it occurs as near-surface materials in the northern hemisphere. Unlike the meteorites, the olivine-rich (more than 50 per cent by volume) material is not associated with diogenite but seems to be mixed with howardite, the most common 7 , 9 surface material. Olivine is exposed in crater walls and in ejecta scattered diffusely over a broad area. The size of the olivine exposures and the absence of associated diogenite favour a mantle source, but the exposures are located far from the deep impact basins. The amount and distribution of observed olivine-rich material suggest a complex evolutionary history for Vesta.

From spectra and images of Vesta, it is suggested that the dark patches on Vesta are formed of infalling hydrated carbonaceous material and the bright patches are uncontaminated Vesta soil. A Dawn view of Vesta Between 16 July 2011 and 5 September 2012, NASA's space probe Dawn was orbiting Vesta, a protoplanet thought to have survived virtually intact since an early phase of Solar System formation. In this issue of Nature , two groups report on the encounter. Carle Pieters and co-workers find that space weathering on Vesta has followed a different course from that observed on the Moon and on Itokawa, the asteroid sampled in an Earth-return mission. On Vesta, weathering involved fine-scale regolith (soil) mixing that has removed clear traces of recent impact deposits. There are no signs of the nanophase metallic-particle deposits seen on the Moon and Itokawa. Thomas McCord and co-authors describe two main types of material on Vesta's surface: bright and dark. The bright material may be uncontaminated indigenous Vesta basaltic soil, with the darker material derived from low-albedo impactors. Dawn has now moved on and is due to rendezvous with the protoplanet Ceres in February 2015. Localized dark and bright materials, often with extremely different albedos, were recently found on Vesta’s surface 1 , 2 . 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 3 , 4 , 5 ), 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.