A New Dawn Since 17 July 2011, NASA's 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 Dawn's 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 asteroid's overall geometry and topography, based on global surface mapping. Vesta's surface is dominated by numerous impact craters and large troughs around the equatorial region. Marchi et al. (p. 690 ) report on Vesta's 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 asteroid's 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 Dawn's 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.
Organic compounds detected on Ceres Water and organic molecules were delivered to the early Earth by the impacts of comets and asteroids. De Sanctis et al. examined infrared spectra taken by the Dawn spacecraft as it orbited Ceres, the largest object in the asteroid belt (see the Perspective by Küppers). In some small patches on the surface, they detected absorption bands characteristic of aliphatic organic compounds. The authors ruled out an external origin, such as an impact, suggesting that the material must have formed on Ceres. Together with other compounds detected previously, this supports the existence of a complex prebiotic chemistry at some point in Ceres' history. Science , this issue p. 719 ; see also p. 692
By October 7, 2011, the Dawn spacecraft mission will have completed its Survey orbit (3000 km from the body’s center of mass) and commenced high-altitude (950 km) mapping of 4Vesta, a large asteroid that experienced igneous differentiation and is thought to be the source of howardite-eucrite-diogenite (HED) meteorites. Dawn’s three payload instruments – Framing Camera, Visible and Near-Infrared Spectrometer, and Gamma Ray and Neutron Detector – are sensitive to the mineralogical and geochemical compositions of Vesta’s surface. Preliminary spectral mapping has revealed prominent color variations that imply compositional differences and/or different degrees of space weathering. Survey spectroscopic results will provide better information on the global distribution of pyroxenes and olivine at spatial scales <700 m/pixel. Information on the presence of surficial OH/H2O will be determined by spectroscopic data across 3 microns. Data on surface hydrogen may indicate the effects of space weathering, as on the Moon. Imaging and spectroscopic mapping at higher spatial resolution of 100-200 m, providing geologic context for the HEDs, will occur during October. Geochemical analysis results for Mg, Si, Fe, K, H, and Th to 300 km resolution by gamma-ray/neutron spectroscopy must await low-altitude (450 km) measurements beginning in December. Other elements not directly measured can be estimated from models of neutron absorption and constraints from HED mixing models. Together, the geochemical and mineralogical data should be sufficient to confirm Vesta as the source of HEDs, identify and map terranes dominated by the various volcanic and plutonic lithologies, search for materials not represented in the HED collection, and model the proportions of lithologies comprising the regolith and the crust. Spectroscopic mapping of a huge south polar structure and impact craters may reveal exposures of mantle or lower crustal materials. Dawn’s orbital data collected during a year in Vestan orbit, along with petrologic, geochemical, and isotopic data from HEDs, will also be used to constrain the bulk composition, thickness of the crust and mantle, mass of the core, chronology, and magmatic evolution of this lone surviving, large differentiated protoplanet.
In August 2011, Dawn went into orbit around asteroid 4 Vesta and during the different mission phases mapped most of its surface. The Dawn VIR-MS (Visible and Infrared Mapping Spectrometer) covers the spectral range of wavelengths 0.255 μm - 5.097 μm, giving information about the mineralogical composition of Vesta. For mapping purposes, Vesta's surface was divided in 15 quadrangles: here we analyze the quadrangle Av-14 (Urbinia) located in the southwest part of the asteroid (270°-360° E; 21°, 66° S) and the south pole quadrangle, Av-15 (Rheasilvia). Urbinia and Rheasilvia quadrangles appear less cratered than the north and the equatorial regions, and contains several different geologic units. The central and the southern parts of the Av-14 are characterized by a series of vertical scarps associated with the Rheasilvia ridge and groove terrain (RRGT); the upper part is flat and includes the equatorial cratered terrain (ECT) and two small areas of bright crater ray material (BCRM). The Rheasilvia quadrangle presents three types of terrains: Rheasilvia cratered mount terrain (RCMT) corresponding to the central pick, the RRGT, found also in the near quadrangle Urbinia and four areas of ejecta materials (EM) (Yingst et al., LPSC, 2012). The spectra of Vesta's surface are similar to those of HED (howardite, eucrite and diogenite) meteorites, characterized by two strong absorption features at 0.9 μm and 1.9 μm related to pyroxenes. The two features show different band depths and band centers, which can be associated with the grain size distribution, abundance of the absorbing minerals, and the presence of opaque materials. Pyroxenes are everywhere on Vesta at the VIR pixel scale of hundreds of meters. The distribution of the VIR band centers and the band depths shows an evident variability among the different regions of the asteroid. The parameters are often correlated with geological structures and are geographically located in different regions. Within the Urbina and Rheasilvia quadrangles, a particulary strong band depth is observed close to the RRGT and most of the terrains are mineralogically classified as ST-Southern Terrains (De Sanctis et al., LPSC, 2012). ST terrains are those characterized by with very deep 1 and 2 micron bands. HAMO and LAMO high-resolution data will provide more details and will allow for a finer analysis of the two quadrangles and all Vesta's surface. The authors acknowledge the support of the Dawn Science, Instrument and Operations Teams. This work was supported by the Italian Space Agency (ASI), ASI-INAF Contract.
The Asteroid Photometric Catalogue now contains photometric lightcurves for 584 asteroids. We discuss some of the guiding principles behind it. This concerns both observers who offer input to it and users of the product.
On Vesta, the thermal behavior of areas of unusual albedo seen at the local scale can be related to physical properties that can provide information about the origin of those materials. We used Dawn s Visible and Infrared Mapping Spectrometer (VIR) hyperspectral cubes to retrieve surface temperatures and emissivities, with high accuracy as long as temperatures are greater than ~180 K. Data acquired in the Survey phase (23 July through 29 August 2011) show several unusual surface features: 1) high-albedo (bright) and low-albedo (dark) material deposits, 2) spectrally distinct ejecta and pitted materials, 3) regions suggesting finer-grained materials. Some of the unusual dark and bright features were reobserved by VIR in the subsequent High-Altitude Mapping Orbit (HAMO) and Low- Altitude Mapping Orbit (LAMO) phases at increased pixel resolution. In this work we present temperature maps and emissivities of several local-scale features that were observed by Dawn under different illumination conditions and different local solar times. Data from VIR's IR channel show that bright regions generally correspond to regions with lower thermal emission, i.e. lower temperature, while dark regions correspond to areas with higher thermal emission, i.e. higher temperature. This behavior confirms that many of the dark appearances in the VIS mainly reflect albedo variations, and not, for example, shadowing. During maximum daily insolation, dark features in the equatorial region may rise to temperatures greater than 270 K, while brightest features stop at roughly 258 K, local solar time being similar. However, pitted materials, showing relatively low reflectance, have significantly lower temperatures, as a result of differences in composition and/or structure (e.g, average grain size of the surface regolith, porosity, etc.). To complement this work, we provide preliminary values of thermal inertia for some bright and dark features.
