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
Abstract We report new photometric lightcurve observations of the Lucy Mission target (11351) Leucus acquired during the 2017, 2018, and 2019 apparitions. We use these data in combination with stellar occultations captured during five epochs to determine the sidereal rotation period, the spin axis orientation, a convex shape model, the absolute scale of the object, its geometric albedo, and a model of the photometric properties of the target. We find that Leucus is a prograde rotator with a spin axis located within a sky-projected radius of 3° (1 σ ) from J2000 Ecliptic coordinates ( λ = 208°, β = +77°) or J2000 Equatorial Coordinates (R.A. = 248°, decl. = +58°). The sidereal period is refined to P sid = 445.683 ± 0.007 h. The convex shape model is irregular, with maximum dimensions of 60.8, 39.1, and 27.8 km. The convex model accounts for global features of the occultation silhouettes, although minor deviations suggest that local and global concavities are present. We determine a geometric albedo of p V = 0.043 ± 0.002. The derived phase curve supports a D-type classification for Leucus.
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.
We report new photometric lightcurve observations of the Lucy Mission target (11351) Leucus acquired during the 2017, 2018 and 2019 apparitions. We use these data in combination with stellar occultations captured during five epochs (Buie et al. 2020) to determine the sidereal rotation period, the spin axis orientation, a convex shape model, the absolute scale of the object, its geometric albedo, and a model of the photometric properties of the target. We find that Leucus is a prograde rotator with a spin axis located within a sky-projected radius of 3{\deg} (1$\sigma$) from J2000 Ecliptic coordinates ($\lambda=208\deg$, $\beta=+77\deg$) or J2000 Equatorial Coordinates (RA=248$\deg$, Dec=+58$\deg$). The sidereal period is refined to $P_{sid}=445.683\pm0.007$ h. The convex shape model is irregular, with maximum dimensions of (60.8, 39.1, 27.8) km. The convex model accounts for global features of the occultation silhouettes, although minor deviations suggest that local and global concavities are present. We determine a geometric albedo $p_V=0.043\pm0.002$. The derived phase curve supports a D-type classification for Leucus.
Dawn's global mapping of Vesta reveals that its observed south polar depression is composed of two overlapping giant impact features. These large basins provide exceptional windows into impact processes at planetary scales. The youngest, Rheasilvia, is 500 kilometers wide and 19 kilometers deep and finds its nearest morphologic analog among large basins on low-gravity icy satellites. Extensive ejecta deposits occur, but impact melt volume is low, exposing an unusual spiral fracture pattern that is likely related to faulting during uplift and convergence of the basin floor. Rheasilvia obliterated half of another 400-kilometer-wide impact basin, Veneneia. Both basins are unexpectedly young, roughly 1 to 2 billion years, and their formation substantially reset Vestan geology and excavated sufficient volumes of older compositionally heterogeneous crustal material to have created the Vestoids and howardite-eucrite-diogenite meteorites.
Abstract The objective of the NASA Psyche mission gravity science investigation is to map the mass distribution within asteroid (16) Psyche to elucidate interior structure and to resolve the question of whether this metal-rich asteroid represents a remnant metal core or whether it is a primordial body that never melted. Measurements of gravity will be obtained via the X-band telecommunication system on the Psyche spacecraft, collected from progressively lower mapping altitudes. Orbital gravity will allow an estimate of $GM$ GM to better than 0.001 km 3 s −2 . A spherical harmonic model of gravity to degree and order 10 will be achievable and, in concert with spherical harmonic data sets from topography and magnetometry, as well as surface composition data, will provide information regarding the spatial and radial distribution of mass that will be used to constrain the origin and evolution of (16) Psyche.
