ALBERT

Description: A partially differentiated interior for (1) Ceres deduced from its gravity field and shape Nature 537, 7621 (2016). doi:10.1038/nature18955 Authors: R. S. Park, A. S. Konopliv, B. G. Bills, N. Rambaux, J. C. Castillo-Rogez, C. A. Raymond, A. T. Vaughan, A. I. Ermakov, M. T. Zuber, R. R. Fu, M. J. Toplis, C. T. Russell, A. Nathues & F. Preusker Remote observations of the asteroid (1) Ceres from ground- and space-based telescopes have provided its approximate density and shape, leading to a range of models for the interior of Ceres, from homogeneous to fully differentiated. A previously missing parameter that can place a strong constraint on the interior of Ceres is its moment of inertia, which requires the measurement of its gravitational variation together with either precession rate or a validated assumption of hydrostatic equilibrium. However, Earth-based remote observations cannot measure gravity variations and the magnitude of the precession rate is too small to be detected. Here we report gravity and shape measurements of Ceres obtained from the Dawn spacecraft, showing that it is in hydrostatic equilibrium with its inferred normalized mean moment of inertia of 0.37. These data show that Ceres is a partially differentiated body, with a rocky core overlaid by a volatile-rich shell, as predicted in some studies. Furthermore, we show that the gravity signal is strongly suppressed compared to that predicted by the topographic variation. This indicates that Ceres is isostatically compensated, such that topographic highs are supported by displacement of a denser interior. In contrast to the asteroid (4) Vesta, this strong compensation points to the presence of a lower-viscosity layer at depth, probably reflecting a thermal rather than compositional gradient. To further investigate the interior structure, we assume a two-layer model for the interior of Ceres with a core density of 2,460–2,900 kilograms per cubic metre (that is, composed of CI and CM chondrites), which yields an outer-shell thickness of 70–190 kilometres. The density of this outer shell is 1,680–1,950 kilograms per cubic metre, indicating a mixture of volatiles and denser materials such as silicates and salts. Although the gravity and shape data confirm that the interior of Ceres evolved thermally, its partially differentiated interior indicates an evolution more complex than has been envisioned for mid-sized (less than 1,000 kilometres across) ice-rich rocky bodies.

Description: Images of Vesta taken by the Dawn spacecraft reveal large-scale linear structural features on the surface of the asteroid. We evaluate the morphology of the Vesta structures to determine what processes caused them to form and what implications this has for the history of Vesta as a planetary body. The dimensions and shape of these features suggest that they are graben similar to those observed on terrestrial planets, not fractures or grooves such as are found on smaller asteroids. As graben, their vertical displacement versus length relationship could be evaluated to describe and interpret the evolution of the component faults. Linear structures are commonly observed on smaller asteroids and their formation has been tied to impact events. While the orientation of the large-scale Vesta structures does imply that their formation is related to the impact events that formed the Rheasilvia and Veneneia basins, their size and morphology is greatly different from impact-formed fractures on the smaller bodies. This is consistent with new analyses that suggest that Vesta is fully differentiated, with a mantle and core. We suggest that impact into a differentiated asteroid such as Vesta could result in graben, while grooves and fractures would form on undifferentiated asteroids.

Description: Multispectral images (0.44 to 0.98 mum) of asteroid (4) Vesta obtained by the Dawn Framing Cameras reveal global color variations that uncover and help understand the north-south hemispherical dichotomy. The signature of deep lithologies excavated during the formation of the Rheasilvia basin on the south pole has been preserved on the surface. Color variations (band depth, spectral slope, and eucrite-diogenite abundance) clearly correlate with distinct compositional units. Vesta displays the greatest variation of geometric albedo (0.10 to 0.67) of any asteroid yet observed. Four distinct color units are recognized that chronicle processes--including impact excavation, mass wasting, and space weathering--that shaped the asteroid's surface. Vesta's color and photometric diversity are indicative of its status as a preserved, differentiated protoplanet.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Reddy, Vishnu -- Nathues, Andreas -- Le Corre, Lucille -- Sierks, Holger -- Li, Jian-Yang -- Gaskell, Robert -- McCoy, Timothy -- Beck, Andrew W -- Schroder, Stefan E -- Pieters, Carle M -- Becker, Kris J -- Buratti, Bonnie J -- Denevi, Brett -- Blewett, David T -- Christensen, Ulrich -- Gaffey, Michael J -- Gutierrez-Marques, Pablo -- Hicks, Michael -- Keller, Horst Uwe -- Maue, Thorsten -- Mottola, Stefano -- McFadden, Lucy A -- McSween, Harry Y -- Mittlefehldt, David -- O'Brien, David P -- Raymond, Carol -- Russell, Christopher -- New York, N.Y. -- Science. 2012 May 11;336(6082):700-4. doi: 10.1126/science.1219088.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Max Planck Institute for Solar System Research, Max-Planck-Strasse 2, 37191 Katlenburg-Lindau, Germany. reddy@mps.mpg.de〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/22582258" target="_blank"〉PubMed〈/a〉

Description: 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.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Jaumann, R -- Williams, D A -- Buczkowski, D L -- Yingst, R A -- Preusker, F -- Hiesinger, H -- Schmedemann, N -- Kneissl, T -- Vincent, J B -- Blewett, D T -- Buratti, B J -- Carsenty, U -- Denevi, B W -- De Sanctis, M C -- Garry, W B -- Keller, H U -- Kersten, E -- Krohn, K -- Li, J-Y -- Marchi, S -- Matz, K D -- McCord, T B -- McSween, H Y -- Mest, S C -- Mittlefehldt, D W -- Mottola, S -- Nathues, A -- Neukum, G -- O'Brien, D P -- Pieters, C M -- Prettyman, T H -- Raymond, C A -- Roatsch, T -- Russell, C T -- Schenk, P -- Schmidt, B E -- Scholten, F -- Stephan, K -- Sykes, M V -- Tricarico, P -- Wagner, R -- Zuber, M T -- Sierks, H -- New York, N.Y. -- Science. 2012 May 11;336(6082):687-90. doi: 10.1126/science.1219122.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉German Aerospace Center, Institute of Planetary Research, Berlin, Germany. ralf.jaumann@dlr.de〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/22582254" target="_blank"〉PubMed〈/a〉

Description: 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.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉De Sanctis, M C -- Ammannito, E -- Capria, M T -- Tosi, F -- Capaccioni, F -- Zambon, F -- Carraro, F -- Fonte, S -- Frigeri, A -- Jaumann, R -- Magni, G -- Marchi, S -- McCord, T B -- McFadden, L A -- McSween, H Y -- Mittlefehldt, D W -- Nathues, A -- Palomba, E -- Pieters, C M -- Raymond, C A -- Russell, C T -- Toplis, M J -- Turrini, D -- New York, N.Y. -- Science. 2012 May 11;336(6082):697-700. doi: 10.1126/science.1219270.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Istituto di Astrofisica e Planetologia Spaziali, Istituto Nazionale di Astrofisica, Rome, Italy. mariacristina.desanctis@iaps.inaf.it〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/22582257" target="_blank"〉PubMed〈/a〉

Description: Abstract We analyze landslides on Ceres using several quantitative approaches to constrain the composition and structure of the top few kilometers of Ceres’ crust. We focus on a subset of archetypal landslides classified morphologically as thick, steep‐snouted “type 1” (T1) flows and thin spatulate “type 2” (T2) flows (Schmidt et al., 2017) to explore the landslides’ mechanical properties. Our results confirm earlier observations showing that T1 landslides are typically found poleward of 70° latitude and T2 mostly equatorward of 70° latitude. Measurements of landslide drop height and runout length imply effective friction coefficients lower than common friction coefficients in any of Ceres’ identified or suggested non‐ice surface materials, including saturated clays. Our measurements of the volume and area of landslide scars suggest that T1 landslides can fail to greater depths than T2 for a given scar area, consistent with depth‐limited failure in T2 landslides. These results are consistent with a layer of lower shear strength material overlying a stronger layer in Ceres’ outer shell at low to mid latitudes, and a single layer without an overlying weak layer at polar latitudes. Combining these observations with known constraints on Ceres’ near‐surface composition, we propose that Ceres’ crust at low to mid latitudes consists of a topmost layer with an ice content in excess of the spectral and elemental detection depths, thins out at high latitudes, and overlies a stronger and more ice‐rich layer.

Description: 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.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉McCord, T B -- Li, J-Y -- Combe, J-P -- McSween, H Y -- Jaumann, R -- Reddy, V -- Tosi, F -- Williams, D A -- Blewett, D T -- Turrini, D -- Palomba, E -- Pieters, C M -- De Sanctis, M C -- Ammannito, E -- Capria, M T -- Le Corre, L -- Longobardo, A -- Nathues, A -- Mittlefehldt, D W -- Schroder, S E -- Hiesinger, H -- Beck, A W -- Capaccioni, F -- Carsenty, U -- Keller, H U -- Denevi, B W -- Sunshine, J M -- Raymond, C A -- Russell, C T -- England -- Nature. 2012 Nov 1;491(7422):83-6. doi: 10.1038/nature11561.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Bear Fight Institute, 22 Fiddler's Road, Box 667, Winthrop, Washington 98862, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/23128228" target="_blank"〉PubMed〈/a〉

Description: 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.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Pieters, C M -- Ammannito, E -- Blewett, D T -- Denevi, B W -- De Sanctis, M C -- Gaffey, M J -- Le Corre, L -- Li, J-Y -- Marchi, S -- McCord, T B -- McFadden, L A -- Mittlefehldt, D W -- Nathues, A -- Palmer, E -- Reddy, V -- Raymond, C A -- Russell, C T -- England -- Nature. 2012 Nov 1;491(7422):79-82. doi: 10.1038/nature11534.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA. carle_pieters@brown.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/23128227" target="_blank"〉PubMed〈/a〉

Description: The dwarf planet (1) Ceres, the largest object in the main asteroid belt 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. 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 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. 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 may explain this haze. We conclude that Ceres must have accreted material from beyond the 'snow line', which is the distance from the Sun at which water molecules condense.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Nathues, A -- Hoffmann, M -- Schaefer, M -- Le Corre, L -- Reddy, V -- Platz, T -- Cloutis, E A -- Christensen, U -- Kneissl, T -- Li, J-Y -- Mengel, K -- Schmedemann, N -- Schaefer, T -- Russell, C T -- Applin, D M -- Buczkowski, D L -- Izawa, M R M -- Keller, H U -- O'Brien, D P -- Pieters, C M -- Raymond, C A -- Ripken, J -- Schenk, P M -- Schmidt, B E -- Sierks, H -- Sykes, M V -- Thangjam, G S -- Vincent, J-B -- England -- Nature. 2015 Dec 10;528(7581):237-40. doi: 10.1038/nature15754.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Max Planck Institute for Solar System Research, Goettingen, Germany. ; Planetary Science Institute, Tucson, Arizona, USA. ; University of Winnipeg, Winnipeg, Canada. ; Freie Universitaet Berlin, Berlin, Germany. ; Technische Universitaet Clausthal, Clausthal-Zellerfeld, Germany. ; University of California, Los Angeles (UCLA), Los Angeles, California, USA. ; Johns Hopkins University, Laurel, Maryland, USA. ; Royal Ontario Museum, Toronto, Canada. ; TU Braunschweig, Braunschweig, Germany. ; Brown University, Providence, Rhode Island, USA. ; Jet Propulsion Laboratory, Pasadena, California, USA. ; Lunar and Planetary Institute, Houston, Texas, USA. ; Georgia Institute of Technology, Atlanta, Georgia, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/26659183" target="_blank"〉PubMed〈/a〉

Description: The Dawn spacecraft targeted 4 Vesta, believed to be a remnant intact protoplanet from the earliest epoch of solar system formation, based on analyses of howardite-eucrite-diogenite (HED) meteorites that indicate a differentiated parent body. Dawn observations reveal a giant basin at Vesta's south pole, whose excavation was sufficient to produce Vesta-family asteroids (Vestoids) and HED meteorites. The spatially resolved mineralogy of the surface reflects the composition of the HED meteorites, confirming the formation of Vesta's crust by melting of a chondritic parent body. Vesta's mass, volume, and gravitational field are consistent with a core having an average radius of 107 to 113 kilometers, indicating sufficient internal melting to segregate iron. Dawn's results confirm predictions that Vesta differentiated and support its identification as the parent body of the HEDs.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Russell, C T -- Raymond, C A -- Coradini, A -- McSween, H Y -- Zuber, M T -- Nathues, A -- De Sanctis, M C -- Jaumann, R -- Konopliv, A S -- Preusker, F -- Asmar, S W -- Park, R S -- Gaskell, R -- Keller, H U -- Mottola, S -- Roatsch, T -- Scully, J E C -- Smith, D E -- Tricarico, P -- Toplis, M J -- Christensen, U R -- Feldman, W C -- Lawrence, D J -- McCoy, T J -- Prettyman, T H -- Reedy, R C -- Sykes, M E -- Titus, T N -- New York, N.Y. -- Science. 2012 May 11;336(6082):684-6. doi: 10.1126/science.1219381.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA. ctrussell@igpp.ucla.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/22582253" target="_blank"〉PubMed〈/a〉