1. Vesta’s Shape and MorphologyABSTRACTVesta’s surface is characterized by abundant impact craters, some withpreserved ejecta blankets, large troughs extending around the equatorial region,enigmatic dark material, and widespread mass wasting, but as yet an absence ofvolcanic features. Abundant steep slopes indicate that impact-generated surfaceregolith is underlain by bedrock. Dawn observations confirm the large impactbasin (Rheasilvia) at Vesta’s south pole and reveal evidence for an earlier,underlying large basin (Veneneia). Vesta’s geology displays morphologicalfeatures characteristic of the Moon and terrestrial planets as well as those ofother asteroids, underscoring Vesta’s unique role as a transitional solar systembody.The Dawn spacecraft (1) arrived at Vesta on 17 July 2011 and began scienceoperations at the highest of three orbital altitudes. During the first orbital periodof 22 days (which define the survey phase), starting on 6 August 2011, the DawnFraming Camera (FC) (2) and the Visible and Infrared Spectrometer (VIR) (3)mapped the surface from an altitude of ~2700 km with image scales of ~260m/pixel (FC) and ~700 m/pixel (VIR), respectively. The survey mapping campaignwas designed to obtain at least three different viewing geometries of Vesta,under similar illumination conditions, in order to construct a model of surfacetopography and to generate a base map by using stereophotogrammetricanalyses. Because of Vesta’s oblique axis, the north-polar region above 50°Nremains dark until late August of 2012, when the spacecraft departs Vesta (4). Intotal, the Dawn FC acquired 1179 stereoscopic images during the survey orbitcovering ~80% of Vesta’s surface. The presence of an igneous crust on Vestaraises the prospect of surface morphology consistent with differentiatedterrestrial planets (5). Stereophotogrammetric analysis (6) of Vesta yields anoverall relief from –22.3 to 19.1 km (with respect to a biaxial reference ellipsoid
2. of 285 km by 229 km), which slightly exceeds previous estimates of relief basedon Hubble Space Telescope (HST) data [–25.4 to +13.7 km with respect to thesame ellipsoid (7)] by about 8%. The observed relief to size ratio of 15% isconsistent with previous discussions and predictions [e.g., (7, 8)] on the dynamicrange of topography of solar system bodies.Vesta’s surface exhibits complex topography at all spatial scales imaged so far(Fig. 1). Previous HST data revealed a large south polar basin with a centralmound (9). This feature has been named Rheasilvia and is centered at about301°E and 75°S (4) (Figs. 1 and 2). Stereo data reveal an approximately circularimpact basin with a diameter of 500 ± 20 km (the mound is ~180 km indiameter) displaying apparent lower crater density (10) and smoother texturedistinctive from the surrounding terrain. A steep scarp defines part of the outerperimeter of the basin. Besides the south polar region, other large depressionscan also be identified and may be remnants of large impact basins (Fig. 1) (10). Aregion with elevated topography, on average ~20 km higher than its surrounding,occurs between ~25°N to 35°S and ~200°E to 300°E and is called VestaliaTerra. The southernmost part of this region merges into the rim area of theRheasilvia basin. Such irregularly high plateaus like Vestalia Terra form parts ofthe Rheasilvia basin rim and are comparable to large arcuate ridges ofasymmetric ejecta blankets as predicted by impact models of rapidly rotatingbodies (11). However, their overall appearance does not completely fit the modelassumptions such as the impact direction. The western margin (at about 210°E)of Vestalia Terra, with an elevation change of ~2000 m, separates it from theyoung crater Marcia and adjacent craters to the north (Fig. 1). A scarp almostcompletely surrounds Vestalia Terra. Its northern edge is curvilinear, with a shapesuggestive of an impact basin margin at about 225°E to 270°E and 15°N.Additional elevated topography, on average ~15 km higher than thesurroundings, extends between 10°S to 55°S and 310°E to 110°E longitudewith its southern boundary again marked by the Rheasilvia basin rim. A second
3. large and older basin 400 ± 20 km wide, named Veneneia and centered at170°E and 52°S, is partly overlapped by the Rheasilvia structure.Fig. 1Global colorized hill-shaded digital terrain model in Mollweide projection (equal area).Large-scale structural features occur in the equatorial and northern regions ofVesta (Fig. 2), including sets of equatorial and northern troughs. By mapping thetroughs directly onto the Vesta shape model, they can be modeled as planes thatcut through the asteroid. A cluster of pole directions represents similarly orientedplanes (Fig. 2) and implies a common formation mechanism of correspondingstructures. The primary set of Vesta structures approximately encircle theasteroid, roughly oriented along the equator. In total, 86 linear structures withthis orientation were mapped at an image scale of about 260 m/pixel. Theequatorial troughs are wide, flat-floored, and bounded by steep scarps along~240° of longitude, circling about two-thirds of the asteroid. Troughs are mostlyco-planar, have lengths that vary from 19 to 380 km, and can be as wide as 15km. Muted troughs, grooves, and pit crater chains are evident in the remaininglongitude range of the asteroid, from ~150°E to ~270°E. The poles of theequatorial troughs cluster at about 120°E to 240°E and 78° (±10°)S. The other
4. set of large-scale linear structures extends to the northwest oriented ~30° fromthe equatorial troughs, starting at ~300°E longitude (Fig. 2). The most prominentstructure in this group is 390 km long and 38 km wide. Although larger than theequatorial troughs, the walls of the northern troughs display gentler slopes androunded edges. They show considerable infilling and heavy cratering, suggestingan older age. Analyses indicate that seven smaller troughs (31 to 212 km long)define planes that are parallel to the plane described by this large-scalestructure, also indicating that they all share a common formation mechanism.Their pole directions cluster at about 190°E and 60° (±10°)S. The poles of theglobal troughs agree within error bounds with the basin center of Rheasilvia at310°E, 75°S and Veneneia at 170°E, 52°S (Fig. 2). This strongly suggests thatthe formation of the troughs is related to the formation of the basins. Theorientations of both sets of large-scale structures are consistent with models oflarge impacts into an asteroid that predict fracturing concentric to the point ofimpact (12, 13). In addition, the older set of troughs is exposed only on thenorthern hemisphere, which may be due to obliteration of their southernextension by the Rheasilvia impact. This is consistent with the younger age ofRheasilivia and also indicates that all structures down to the equator weredestroyed or at least affected by the Rheasilvia impact.
5. Fig. 2Large-scale troughs are oriented along Vesta’s equator (white dashed lines) and also form theequator to the north (red dashed lines). The center positions of these circular troughs relative tothe equatorial plane of Vesta (top left) correspond to the center of the two southern basins (redand white × bottom left).Impact-related processes have played a dominating role in (re-)shaping Vesta’ssurface (Fig. 3). Impact craters on Vesta can be divided into four differentdegradation states: fresh craters, showing sharp, unmodified rims and ejectadeposits; craters with degraded, softened rims; craters with heavily degradedrims; and impact crater ruins showing almost no visible rims (fig. S1). Thesedifferent crater degradation states can be identified in a sample area covered bythe ejecta of a nearby 45-km-diameter crater at 307°E and 16°N named Lepida(fig. S1). The largest craters that are completely filled and covered in this areahave diameters of about 4 km. The crater depth to diameter ratio for simplecraters (6) implies a thickness of ejecta (regolith) in this area of about 0.8 ± 0.1km (14).
6. Topography plays an important role in crater formation and modificationprocesses on Vesta. There are numerous cases of craters formed on slopes,where pronounced collapse, slumping, and ejecta deposition occurred on theupslope side as well as the downslope side of the craters. This results in astrongly asymmetric morphology with a wider upslope crater wall topped by asharp crest and a narrower downslope wall with a smoothed crater rim (Fig. 3).Impact on steep slopes also causes asymmetric ejecta deposition with thedownslope ejecta covering and smoothing the corresponding crater rim.Compared with its radius, Vesta has significant relief, resulting in relatively steepslopes, locally exceeding 40° when referred to the ellipsoid. Gravitational slopesare slightly less steep but also reach up to ~40° on a lower-resolution shapemodel (~700 m). A considerable number of slopes on Vesta are most probablyabove the angle of repose and indicative of intact bedrock beneath (15). This isalso consistent with the appearance of large boulders in some parts of the ejecta.Impacts onto such steep surfaces, followed by slope failure, make resurfacingbecause of impacts, subsequent gravitational modifications, and seismic activityimportant geologic processes on Vesta that substantially alter the morphology ofgeologic features and add to the complexity of Vesta’s surface evolution.Smooth, flat regions occur in the interiors of some craters and in smalldepressions as pondlike accumulations (16) with well-defined geologicalcontacts, indicating that they are younger than their surroundings (Fig. 3). Theseponds are not randomly distributed across the surface but preferentially occur ina band between 10°S and 30°N with fewer ponds north of ~30° and even fewerponds in the Rheasilvia region. However, poor lighting conditions in the northernhemisphere may bias this observation. Similar, but smaller (<230 m diameter)smooth ponds were also reported from the surface of asteroid Eros (17). Pondson Vesta and Eros show several similarities, including preferred occurrence inequatorial regions, downslope asymmetry within craters on slopes, stratigraphicrelationships with adjacent terrain, and formation within craters and depressionswithout evidence for material flowing into these craters and depressions. On the
7. basis of current observations, several scenarios for the origin of these depositsseem plausible, including volcanism, impact sedimentation, impact melt, dustlevitation and transport, seismic shaking, or slumping of fine material.Fig. 3Detailed views of Vesta’s surface. (A1) Mountain with dark material exposed (image width = 48km, 61°E, 10°N); (B1) crater on a slope with a sharp crest uphill and slumping material coveringthe lower rim (image width = 44 km, 201°E, 58°S); (C1) scarp with mass wasting features [imagewidth = 180 km, 92°E, 55°S]; (A2, B2, and C2) color-coded image-textured three-dimensionalperspective views corresponding to the terrains in (A1),Dark material is common on Vesta; it is locally concentrated and mostlyassociated with impacts (fig. S2). Dark material is either exogenic in originbecause of carbon-rich material from low-velocity impactors (18) (i.e., from theimpact of a carbonaceous chondrites) or endogenic because of freshly exposedmafic material or impact melt locally mixed into the subsurface and excavated bylater impacts. Dark material on Vesta can be divided into four majorgeomorphologic classes (6): material emanating from the rims or walls of impactcraters or running downslope in fans into the crater and on the crater floorbecause of mass wasting processes; dark material associated with crater ejectapatches or continuous ejecta blankets; material associated with hill flanks andrelated to impacts on hills; and clusters of dark spots and extended linear darkfeatures. Dark material exposed by impact excavation often shows fine structures
8. indicating a spotty admixture within the regolith. Deposits of dark material areunevenly distributed across Vesta’s surface. The major regions with dark materialare at about 110°E to 160°E and 10°S to 10°N, 170°E to 225°E and 10°S to20°N, and 290°E to 330°E and 0° to 20°S (fig. S2 and Fig. 4).No unambiguous volcanic deposits have been identified, although they might beexpected given evidence from howardite-eucrite-diogenite meteorites (19). Allfeatures with lobate, lava flow–like morphologies occur in close proximity toimpact craters or topographic heights and are best interpreted as gravity-drivenmass flow deposits, impact ejecta deposits, or impact melt deposits, although theamount of impact melt is expected to be small (20). One feature, morphologysimilar to a terrestrial basaltic spatter vent feeding a dark flow-like deposit,occurs in the dark ejecta field of an impact crater. This feature could also beinterpreted as impact ejecta or melt deposits; thus, positive identificationremains ambiguous at image scales of 70 m/pixel. Two positive relief featurescontaining dark materials appear to be impact-sculpted hills (Fig.3). Thematerials associated with these dark or dark-rayed impact craters suggest eitheran exogenic origin of the dark material or excavation and exposure of subsurfacevolcanic dikes by the impacts (6). The present lack of volcanic relicts detected onVesta suggests that such features were only produced during the short period ofrapid cooling of Vesta’s interior within the first 100 million years after formationand have been eroded and gardened by impacts, in part evidenced by the darkmaterials seen in the walls of many impact craters. Volcanic materials should bedeeply buried by impact ejecta from the Rheasilvia and other large basins whoseejecta must cover the surface.
9. Fig. 4Geologic maps of Vesta based on survey orbit data.References and NotesC. T. Russell, C. A. Raymond, The DawnMission to Vesta and Ceres. Space Sci.Rev. 163, 3(2011).H. Sierks et al., The Dawn Framing Camera. Space Sci. Rev. 163, 263 (2011).C. M. De Sanctis et al., The VIRspectrometer. Space Sci. Rev. 163, 329 (2011).C. T. Russellet al., Science 336, 684 (2012).
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