Search

The Visible-Infrared Mapping Spectrometer VIR (8) on the Dawn spacecraft (9) obtained multi-spectral images of Ceres' surface in the spectral range 0.4-5 µm with an instantaneous field of view of [...], 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).sub.2] (brucite), [Mg.sub.2]C[O.sub.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.sub.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.

We observed Ceres using the Framing Camera (FC) on board the Dawn spacecraft. The FC is equipped with a panchromatic (clear) filter and seven colour filters, covering the wavelength range [...], The dwarf planet (1) Ceres, the largest object in the main asteroid belt (1) 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 (2,3). 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 (4) of water vapour absorption near Ceres and previous detection of bound water and OH near and on Ceres (refs 5-7) 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 (8). 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 (9,10) may explain this haze. We conclude that Ceres must have accreted material from beyond the 'snow line' (11), which is the distance from the Sun at which water molecules condense.

We present observational evidence that carbonates and iron-rich clays are present on the surface of Ceres. These components are also present in CI chondrites and provide a means of explaining the unusual spectrum of this object as well as providing potential insight into its evolution. Keywords: Asteroid Ceres; Asteroids, composition; Infrared observations; Spectroscopy

The highest resolution (pixel scale 30 km) images of Ceres to date have been acquired by the Advanced Camera for Surveys onboard Hubble Space Telescope, through three wide band filters, centered at 535, 335, and 223 nm, covering more than one rotation of Ceres. The lightcurve at 535 nm agrees with earlier observations at V-band [Tedesco, E.F., Taylor, R.C., Drummond, J., Harwood, D., Nickoloff, I., Scaltriti, F., Schober, H. J., Zappala, V., 1983. Icarus 54, 23-29] in terms of magnitude, amplitude, and shape. The 0.04 magnitude lightcurve amplitude cannot be matched by Ceres' rotationally symmetric shape, and is modeled here by albedo patterns. The geometric albedos at the above three wavelengths are measured to be 0.087 [+ or -] 0.003, 0.056 [+ or -] 0.002, and 0.039 [+ or -] 0.003, respectively. V-band geometric albedo is calculated to be 0.090 [+ or -] 0.003, consistent with earlier observations [Tedesco, E.F., 1989. In: Binzel, R.P., Gehrels, T., Matthews, M.S. (Eds.), Asteroids n. Univ. of Arizona Press, Tucson, pp. 1090-1138]. A strong absorption band (30%) centered at about 280 nm is observed, but cannot be identified with either laboratory UV spectra or the spectra of Europa or Ganymede. The single-scattering albedo has been modeled to be 0.070 [+ or -] 0.002, 0.046 [+ or -] 0.002, and 0.032 [+ or -] 0.003, respectively. The photometric roughness of Ceres' surface is found to be about 44[degrees] [+ or -] 5[degrees] from photometric modeling using Hapke's theory, consistent with earlier radar observations [Mitchell, D.L., Ostro, S.J., Hudson, R.S., Rosema, K.D., Campbell, D.B., Velez, R., Chandler, J. E, Shapiro, I.I., Giorgini, J.D., Yeomans, D.K., 1996. Icarus 124, 113-133]. The first spatially resolved surface albedo maps of Ceres at three wavelengths have been constructed from HST observations, as well as the corresponding color maps. Eleven surface albedo features are identified, ranging in scale from 40-350 kM. Overall the range of these albedo and color variations is small compared to other asteroids and some icy satellites. Keywords: Asteroids; Photometry; Surfaces

Studies of the internal structure of asteroids, which are crucial for understanding their impact history and for hazard mitigation, appear to be in conflict for the S-type asteroids, Eros, Gaspra, and Ida. Spacecraft images and geophysical data show that they are fractured, coherent bodies, whereas models of catastrophic asteroidal impacts, family and satellite formation, and studies of asteroid spin rates, and other diverse properties of asteroids and planetary craters suggest that such asteroids are gravitationally bound aggregates of rubble. These conflicting views may be reconciled if 10-50 km S-type asteroids formed as rubble piles, but were later consolidated into coherent bodies. Many meteorites are breccias that testify to a long history of impact fragmentation and consolidation by alteration, metamorphism, igneous and impact processes. Ordinary chondrites, which are the best analogs for S asteroids, are commonly breccias. Some may have formed in cratering events, but many appear to have formed during disruption and reaccretion of their parent asteroids. Some breccias were lithified during metamorphism, and a few were lithified by injected impact melt, but most are regolith and fragmental breccias that were lithified by mild or moderate shock, like their lunar analogs. Shock experiments show that porous chondritic powders can be consolidated during mild shock by small amounts of silicate melt that glues grains together, and by friction and pressure welding of silicate and metallic Fe,Ni grains. We suggest that the same processes that converted impact debris into meteorite breccias also consolidated asteroidal rubble. Internal voids would be partly filled with regolith by impact-induced seismic shaking. Consolidation of this material beneath large craters would lithify asteroidal rubble to form a more coherent body. Fractures on Ida that were created by antipodal impacts and are concentrated in and near large craters, and small positive gravity anomalies associated with the Psyche and Himeros craters on Eros, are consistent with this concept. Spin data suggest that smaller asteroids 0.6-6 km in size are unconsolidated rubble piles. C-type asteroids, which are more porous than S-types, and their analogs, the volatile-rich carbonaceous chondrites, were probably not lithified by shock. Keywords: Asteroids; Meteorites; Impact processes; Surfaces, asteroids; Asteroids, Ceres, Eros, Gaspra, Ida, Mathilde, Vesta

Asteroids 1 Ceres, 2 Pallas, and 4 Vesta were observed with the 13-cm Arecibo radar and the 3.5-cm Goldstone radar during several apparitions between 1981 and 1995. These observations help to characterize the objects' surface properties. Echoes from Ceres and Pallas are [approximately]95% polarized ([[Mu].sub.C] = [[Sigma].sub.SC]/[[Sigma].sub.OC] [approximately equal to] 0.05) in the sense expected for specular (mirror) reflection yet broadly distributed in Doppler frequency, thus revealing surfaces that are smoother than the Moon at decimeter scales but much rougher (rms slopes > 20 [degrees]) on larger scales. Slopes on Ceres appear to be somewhat higher when viewed with the 3.5-cm wavelength, a trend that is observed for the terrestrial planets and the Moon. In contrast, echoes from Vesta are significantly depolarized, indicating substantial near-surface complexity at scales near 13 cm ([[Mu].sub.C] = 0.24 [+ or -] 0.04) and 3.5 cm ([[Mu].sub.C] = 0.32 [+ or -] 0.04), which is probably a consequence of Vesta's relatively strong basaltic surface material and may be a signature of large impact features inferred to be present on the surface. The low radar albedos of Ceres [Mathematical Expression Omitted] and Pallas [Mathematical Expression Omitted] are in the range expected for surfaces with a carbonaceous chondrite mineralogy. Pallas' distinctly higher albedo implies a [approximately]35% higher surface density, which could result from a lower regolith porosity and/or a higher specific gravity (zero-porosity density). Given a porosity of 45%, the specific gravities of the surface materials on Ceres and Pallas would be [approximately]2.3 and [approximately]3.0 g [cm.sup.-3], respectively, which would be consistent with (1) the presence of an additional silicate component on Pallas' surface (as inferred from spectroscopic observations) and (2) recent mass estimates, which suggest a higher mean (volume-averaged) density for Pallas than for Ceres.

Author(s): Alexandra Witze Author Affiliations: This article is part of our Nature Special: Pluto & Ceres. Nik Spencer/Nature Nik Spencer/Nature Nature Special: Pluto and Ceres Nik Spencer/Nature Related links Related [...], Graphical guide to the NASA missions that will provide the first close looks at Ceres and Pluto.

CALL IT THE YEAR of the dwarf planet. In 2015, scientists will get their first close-up look at two of the Solar System's biggest little rocks. The Dawn mission will [...]

The northern polar region of the dwarf planet Ceres contains lots of hydrogen and probably water, as revealed in an image taken by NASA's Dawn spacecraft and released on 22 [...]

A pair of bright spots glimmering inside an impact crater on the dwarf planet Ceres, mystifying scientists, could be coming from some kind of icy plume or other active geological [...]