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.

[1] 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.

21We report on a preliminary global geologic map of Vesta, based on data from the 22 Dawn spacecraft's High--Altitude Mapping Orbit (HAMO) and informed by Low--Altitude 23Mapping Orbit (LAMO) data. This map is part of an iterative mapping effort; the 24 geologic map has been refined with each improvement in resolution. Vesta has a 25 heavily--cratered surface, with large craters evident in numerous locations. The south 26 pole is dominated by an impact structure identified before Dawn's arrival. Two large 27 impact structures have been resolved: the younger, larger Rheasilvia structure, and the 28 older, more degraded Veneneia structure. The surface is also characterized by a system 29 of deep, globe--girdling equatorial troughs and ridges, as well as an older system of 30 troughs and ridges to the north. Troughs and ridges are also evident cutting across, and 31 spiraling arcuately from, the Rheasilvia central mound. However, no volcanic features 32 have been unequivocally identified. Vesta can be divided very broadly into three 33 terrains: heavily--cratered terrain; ridge--and--trough terrain (equatorial and northern); 34 and terrain associated with the Rheasilvia crater. Localized features include bright and 35 dark material and ejecta (some defined specifically by color); lobate deposits; and mass--36 wasting materials. No obvious volcanic features are evident. Stratigraphy of Vesta's 37 geologic units suggests a history in which formation of a primary crust was followed by 38 the formation of impact craters, including Veneneia and the associated Saturnalia 39Fossae unit. Formation of Rheasilvia followed, along with associated structural 40 deformation that shaped the Divalia Fossae ridge--and--trough unit at the equator. 41 Subsequent impacts and mass wasting events subdued impact craters, rims and 42 portions of ridge--and--trough sets, and formed slumps and landslides, especially within 43 crater floors and along crater rims and scarps. Subsequent to the formation of 44 Rheasilvia, discontinuous low--albedo deposits formed or were emplaced; these lie 45 stratigraphically above the equatorial ridges that likely were formed by Rheasilvia. The 46 last features to be formed were craters with bright rays and other surface mantling 47 deposits. Executed progressively throughout data acquisition, the iterative mapping 48 process provided the team with geologic proto--units in a timely manner. However, 49 interpretation of the resulting map was hampered by the necessity to provide the team 50 with a standard nomenclature and symbology early in the process. With regard to 51 mapping and interpreting units, the mapping process was hindered by the lack of 52 calibrated mineralogic information. Topography and shadow played an important role 53 in discriminating features and terrains, especially in the early stages of data acquisition. 54 55

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