Search

The Dawn spacecraft entered orbit around asteroid 4 Vesta in mid-July 2011. Since then, the Visible and InfraRed Imaging Spectrometer (VIR) acquired hyperspectral images of Vesta's surface in the overall wavelength range from 0.25 to 5.1 μm. During the Approach and Survey mission phases (23 July through 29 August 2011), VIR obtained resolved images of Vesta with spatial resolution between 1.31 km and 0.68 km/pix. More than 65% of the surface, from the South Pole up to ˜40°N, was observed under different illumination conditions and local solar elevations. Further data were acquired in the subsequent HAMO and LAMO phases. Based on this datasets, the Dawn Team is conducting mineralogical mapping of Vesta's surface in the form of 15 quadrangle maps. In this work we present the results of the spectroscopic analysis carried out for the quadrangle Av-10 ("Oppia"), spanning Vesta's latitude 22S° - 22N° and longitude 288° - 360°. Vesta is known to have a basaltic surface from visible/near-infrared reflectance spectroscopy. Although pyroxenes' spectral signatures at about 0.9 and 1.9 µm are ubiquitous on Vesta and largely dominate its surface composition, significant variability in slope, strength and wavelength position of the center of these features can be measured, which indicate multiple physical surface processes. Absorption band depth is sensitive to abundance, texture and multiple scattering effects. On the other hand, the position of the band centers is sensitive to Calcium and Magnesium abundance in pyroxenes, which is crucial to establish a link with Howardite, Eucrite and Diogenite meteorites' clan (HEDs). On the basis of several spectral indices, the surface of Vesta can be divided into different terrain types, which are mostly related to specific geological units or morphological surface features. By combining VIR channels and ratios into colour images, the presence and abundance of these materials can be revealed. Among the terrain types identified on Vesta, Copious Ejecta Terrains (CET) are the most abundant in the Av-10 quadrangle. These terrains are characterized by low reflectance in the VIS and shallow pyroxene bands, compared to Mid-Latitude Terrains (MLT). Combined with the visible albedo of Vesta's surface, the RGB composite made by using "Clementine" colours shows that: 1) bright material are yellow/green, 2) dark material blue/violet, and 3) spectrally distinct material (ejecta) are red/purple. In the Oppia quadrangle, some unnamed craters have ejecta showing deeper absorptions at 1.9 µm. Conversely, other small craters partly show dark ejecta, which appear blue in the Clementine ratio color composite and exhibit VIR spectra with a strongly reduced pyroxene signature. However, the most notable feature of quadrangle Av-10 is the Oppia crater (6.5°S/307.8°E), from which the quadrangle itself is named. The Oppia crater is surrounded by extended ejecta material, classified as CET. These ejecta show significant differences in pyroxenes' signatures (reduced band depths) and they can be distinguished in the above ratio color composite by their prominent red colour compared to the surrounding terrains.

Since the arrival of the Dawn spacecraft at Vesta the Visible and InfraRed Imaging Spectrometer (VIR) has acquired hyperspectral images of Vesta's surface in the wavelength range from 0.25 to 5.1µm. As part of the analysis of Vesta's surface composition a series of 15 quadrangle maps have been produced showing the results derived from the spectroscopic analysis of the VIR data. We present the results of the spectroscopic analysis achieved for the quadrangle Av-12 (Sextilia), which covers Vesta's surface between 21°S - 66° S and 90° - 180° E. The region is dominated by the impact crater Sextila (~39°S/146°E) with its very well preserved continuous ejecta blanket, as well as two huge tectonic scarps (Argonium and Matronalia Rupes), which define the margin of Vesta's southern impact basin Rheasilvia (Krohn et al., 2012, this session). VIR spectra show clear evidence of Vesta's basaltic surface composition. However, significant variability in slope, strength and wavelength position of the prominent pyroxene absorptions near 0.9 and 1.9µm indicate multiple physical surface processes (De Sanctis et al., this session). Based on the combination of the visible albedo, spectral parameters like the depths of the pyroxene absorptions as well as color ratio composites using the VIR channels centering at 749nm/438nm (Red), 749nm/917nm (Green) and 438/749nm (Blue) Vesta's surface can be divided into different terrain types, which are mostly related to specific geological units or morphological surface features (De Sanctis et al., this session). Among these terrain types the quadrangle Av-12 can be divided very broadly into: 1) The Southern Terrain (ST), which dominates the southern part of the quadrangle V-12 and extends into the Rheasilvia impact basin, exhibits very deep pyroxene absorptions. 2) The pyroxene signature in the Mid-Latitude Terrains (MLT) located mainly between 20 and 50°S is slightly subdued in comparison to the ST but still distinct. On a local scale, fresh bright material associated to a strong pyroxene signature appears in the vicinity of several small impact craters and their ejecta, whereas others partly show low albedo ("dark") ejecta with a strongly subdued pyroxene signature. Several large impact craters like the prominent impact crater Sextilia (~20km in diameter), from which the quadrangle itself is named, shows bright material on the crater rim, probably representing fresh material excavated due to mass wasting processes. Other peculiar impact craters like Helena (~41°S/122°E) partially resemble impact craters like Sextilia but also show portions of the crater buried by low albedo material with a subdued pyroxene signature. Only one of the two prominent scarps (i.e. Matronalia Rupes) shows a pronounced spectral signature of fresh material with very deep pyroxene absorptions characteristic for the ST terrain. The authors gratefully acknowledge the support of the Dawn Instrument, Operations, and Science Teams. This work is supported by NASA through the Dawn project and the German Space Agency (DLR).

Asteroid 4 Vesta is currently observed by NASA's Dawn orbiter. The Dawn Team is conducting mineralogical mapping of the surface in the form of 15 quadrangle maps, and here we report results from the mapping of the two adjacent quadrangles Marcia (Av-8) and Numisia (Av-9). The mapping is based on a Framing Camera (FC) color band ratio and Visible and InfraRed (VIR) hyperspectral images. Av-8 Marcia Quadrangle covers 144˚-216˚E longitude while Av-9 Numisia extends from 216˚-288˚E longitude; both quadrangles are located in the equatorial region from ±21˚ latitude. Av-8 Marcia is dominated by a cratered regions, which is a low-albedo ejecta field containing craters Marcia, (68 km by 58 km), Calpurnia (54 km by 52 km), and Minucia (26 km by 23 km). A hill with a dark-rayed crater, named Aricia Tholus, is also nearby (size about 42.5 km and 28 km). Av-9 Numisia is dominated by Vestalia Terra, a distinct region of Vesta. Many of the impact craters in Av-9 have both bright and dark layers in their walls and also distinct ejecta lobes. The largest feature in Av-9 is the 33 km diameter Numisia crater which is centrally located at 7°S, 247°E. The other significant feature is the 15 km diameter Cornelia crater. Cornelia, smaller than Numisia is apparently more interesting has it shows a greater color diversity and an extensive ray system. In particular FC color data show an inner dark albedo region surrounded by an extended light colored ray system, this is also shown in the VIR images. Both FC color ratio images and VIR images show compositional variations within the Marcia quadrangle (De Sanctis et al., LPSC2012). The band depth distribution is uneven in this region and associated with the geological structures present in the quadrangles. On average the dominant terrains are the so called ET ( Equatorial Terrains) characterized by intermediate band depths, but associated with Marcia we see also terrains with very deep band depth similar to the ST (Southern Terrains) and deep band depth as the MLT (Mid Latitude Terrains). The low albedo ejecta field, derived from the Marcia and Calpurnia craters (located in Av-9), mantles underlying older terrains and is associated with the CET (Copious ejecta Terrains): the band depth are very shallow. VIR data show strong 1 µm and 2 µm absorptions exposed within the walls and floor of impact crater Marcia, suggesting excavation of pyroxene-rich material and/or variations in particle sizes. FC color ratio images, as well as VIR color ratio (Red:750/430 nm; Green :750/920 nm; Blue:430/750 nm) show color differences in Marcia and Calpurnia. The color and spectral data show the difference between the eastern side of the Marcia and Calpurnia ejecta field from its western side. Moreover it appears an unusual diffuse deposit surrounding an unnamed crater on the western side of the quadrangle. Further study is underway to investigate the significance of these color variations. The authors gratefully acknowledge the support of the Dawn Instrument, Operations, and Science Teams. This work is supported by an Italian Space Agency (ASI) grant and by NASA through the Dawn project and a Dawn at Vesta Participating Scientist grant.

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 Dawn spacecraft is currently orbiting the main belt asteroid 4 Vesta to evaluate the character of this small protoplanet and its role in early solar system evolution. Compositional data from the Gamma Ray and Neutron Detector (GRaND) are being acquired and will be available soon. Spectral data acquired with the Visible and Infrared Mapping Spectrometer (VIR) and the Framing Camera (FC) instruments provide compositional information in a spatial context. Available VIR data provide the high spectral resolution necessary for mineral identification and characterization while FC data provide selected spectral channels, but at a higher spatial resolution that enable geologic relations to be better discerned. As these different data are integrated and analyzed, broad compositional variations are observed in the mineralogy across Vesta. The diversity seen on a global scale and direct associations with HED mineralogy is discussed in DeSanctis et al. and Ammannito et al. [this meeting]. Geologic mapping of Vesta has been initiated for 15 well-defined quadrangles and coordinated mineralogical mapping has commenced at that higher scale. We discuss the mineralogical diversity observed in quadrangle Av-13 Tuccia, which spans a region between 21° -66°S latitude and 180° -270° E longitude on Vesta. The Av-13 Tuccia quadrangle includes three major geologic terrains [see Kneissl et al., this meeting]: Vestalia Terra located in the north of the quadrangle, the Equatorial Cratered Terrain, and the Ridge-and-Groove Terrain which is part of the Rheasilvia Formation. As such, the Tuccia quadrangle includes regions which transition from the equatorial terrain to the giant impact basin of Rheasilvia. Prominent spectral variations are mapped across the surface by FC and VIR, largely associated with relatively fresh impact craters. Diagnostic absorption bands of pyroxene are readily detected. The relative strength of these absorptions is seen to vary in a regular spatial context, largely correlated with overall albedo of surface material and often at high spatial resolution associated with individual features. Diagnostic variations in absorption band center and shape are also observed. Deposits of an unusual crater located on a steep slope at 59° S, 200° E exhibits some of the shortest, most symmetric absorption bands (e.g., centered near 0.920-0.925 μm), but with highly variable band strength in a well-defined spatial context. In contrast, the small fresh crater to the north on the rim of Tuccia crater has exposed material of a different composition that exhibits broader diagnostic absorption bands centered at a longer wavelength, suggestive of pyroxenes that are slightly more Fe-rich. As VIR and FC data continue to be acquired, processed and integrated, additional details of the mineral character and diversity of Vesta will be revealed in order to discern local and regional geologic evolution. The authors are grateful for the dedication of the Dawn Instrument and Operations Teams and for support from NASA's Discovery Program and Dawn's international partners.

The Dawn spacecraft is currently orbiting 4 Vesta, returning data about the geology, composition, shape, and internal structure of this differentiated asteroid. Here we present compositional interpretations for the Av-4 Domitia quadrangle, one of the 15 quadrangles currently being mapped in terms of both geology and mineralogy by the Dawn Team. The maps are based on data acquired by Dawn's Framing Camera (FC) and Visible and Infrared Mapping Spectrometer (VIR-MS). The FC images the surface through one broadband clear filter and seven narrow-band filters from 0.44 to 0.98 µm at pixel scales of ~250 meters from Dawn's survey orbit and less than 20 meters from its low-altitude mapping orbit (LAMO). VIR-MS maps the surface in more than 850 spectral channels from 0.25 to 5.1 µm with pixel scales from 800 meters in the survey orbit to ~50 meters from LAMO. The Av-4 Domitia quadrangle covers the region from 21-66° N and 180-270° E. It is currently winter in the northern hemisphere of Vesta, and only the southern ~half of this quadrangle has been in sunlight since Dawn's arrival. The portion viewed thus far is part of Vesta's Northern Heavily Cratered Terrain that includes a NW-SE trending trough system, and exposures of dark material. The current illumination conditions (high incidence angles) are not ideal for interpreting spectral reflectance data, and much of the variation in band depth and center is thus far related to variations in incidence angle. However, some regional trends are evident. Absorption bands are generally similar to the global average; band centers are intermediate and appear to increase toward the east, consistent with a mixed-Eucritic composition. The authors acknowledge the support of the Dawn Science, Instrument and Operations Teams. This work was supported by the NASA Dawn Project under contract from UCLA and by the NASA Dawn at Vesta Participating Scientist program.

We present the mineralogical map of a quadrant of the southern hemisphere of Vesta spanning 0-90 degrees longitude, and -21 to -66 degrees latitude; a region named Pinaria. The region, named after the Roman vestal virgin (c. 600 B.C.), includes an approximately 37km diameter crater, also named Pinaria. Several additional large craters are in this region as is the western most region of the rim of Rhea Silvia, named Matronalia Rupes. Mineralogical maps are based on data acquired by the Visible and Infrared Mapping Spectrometer (VIR-MS) and the Framing Camera (FC) on the Dawn spacecraft that has been orbiting Vesta since July 2011. VIR-MS is sensitive to wavelengths from 0.25um to 5.1um with a spatial resolution that depends upon the mission phase: nominally from 2.5 up to 0.8 km/pixel during the approach, 0.8 km/pixel during survey, 0.2 km/pixel during the high altitude orbit (HAMO) and about 0.05 km/pixel during the low altitude orbit (LAMO). This spatial resolution does not include the effects of the spacecraft's nor Vesta's motion. FC data from Survey orbit with a spatial resolution of about 250 m/pixel have been mapped using filter band parameters selected to enhance the anticipated mineralogy of Vesta. Global color maps of Vesta's surface using these color differences and ratios are generated. VIR data show that Vesta's surface is dominated by pyroxenes, with no evidence for the presence of other minerals observed at the scale of the survey measurements. The spectral parameters of the two major pyroxene absorption bands including band centers, depths and band areas and their variation within the Pinaria region, suggest mineralogical variation representing different compositional and/or textural terrains. Matronalia Rupes has band parameters suggesting different composition or grain size possibly resulting from down slope motion of regolith revealing different material beneath. The authors gratefully acknowledge the support of the Dawn Instrument, Operations, and Science Teams. This work is supported by an Italian Space Agency (ASI) grant, the DLR, MPI and by NASA through the Dawn project and the Dawn at Vesta Participating Scientist grant.

In July 2011 the Dawn spacecraft entered orbit around the Main Belt asteroid 4 Vesta utilizing three different instruments to map the asteroid [1]. The Main Belt is the source region of most impactors in the inner solar system [2]. We compare the obtained crater size-frequency distribution (CSFD) of Vesta with that of the Moon and other Main Belt asteroids such as 951 Gaspra, 243 Ida, and 21 Lutetia. We also compare our results of crater counting on Vesta with K/Ar-Ar reset ages of HED meteorites, which most likely originated from Vesta [3]. To properly compare the lunar CSFD with that of the asteroids we applied scaling laws [4] to account for various impact velocities, surface gravities as well as material properties between the investigated bodies. We found well defined lunar-like CSFDs of impact craters on all four asteroids. The CSFD of Vesta and Lutetia had to be constructed from several individual measurements following [5]. We were able to derive lunar-like chronologies for each asteroid utilizing intrinsic collision probabilities [6], lunar-like CSFDs and the ground truth-derived lunar chronology. Since the Moon and the Main Belt asteroids share the same main impactor source, it is straightforward to also assume a very similar time-dependent impact rate over the solar system history. Alternative chronologies, which are based on computer models of the LHB [7], result in surface ages >4.5 Ga, which are highly unlikely for Vesta. Our lunar-like chronology for Vesta is able to match three out of four peaks in age probabilities of HED meteorites [3]. We measured the age of the Rheasilvia basin with 3.70 +/-0.02 Ga, which coincides with a wide-spread resurfacing age on Vesta. An underlying older basin, partially obliterated by the Rheasilvia impact was formed 3.81 +/-0.05 Ga ago. Finally the large basins and craters >150 km in diameter and the densest cratered areas on Vesta indicate a global resurfacing event 4.00 +/-0.02 Ga ago. This represents the oldest age we can infer from Vesta by this technique. Acknowledgement: This work has been supported by the German Space Agency (DLR) on behalf of the Federal Ministry of Economics and Technology, grant 50 OW 1101.

Asteroid 4 Vesta is currently under investigation by NASA's Dawn orbiter. The Dawn Science Team is conducting mineralogical mapping of Vesta's surface in the form of 15 quadrangle maps, and here we report results from the mapping of Floronia quadrangle Av-5. The maps are based on the data acquired by the Visible and Infrared Mapping Spectrometer (VIR-MS) and the Framing Camera (FC) (De Sanctis et al., this meeting). This abstract is focused on the analysis of band ratios, as well as the depth and position of the 2-µm absorption band of pyroxenes, but additional information will be presented. Absorption band depth is sensitive to abundance, texture and multiple scattering effects. Absorption band position is controlled by composition, shorter wavelength positions indicate less Calcium (and more Magnesium) in pyroxenes. The inferred composition is compared with that of Howardite, Eucite and Diogenite meteorites (HEDs). Diogenites are Mg-rich with large orthopyroxene crystals suggesting formation in depth; Eucrites are Ca-poor pyroxene, with smaller crystals. Av-5 Floronia Quadrangle is located between ~20-66˚N and 270˚-360˚E. It covers a portion of the heavily-cratered northern hemisphere of Vesta, and part of it is in permanent night, until August 2012. Long shadows make the visualization of albedo variations difficult, because of limited effectiveness of photometric corrections. Most of the variations of the band depth at 2 µm are partly affected by illumination geometry in this area. Only regional tendencies are meaningful at this time of the analysis. The 2-µm absorption band depth seems to be deeper towards the south of the quadrangle, in particular to the south of Floronia crater. It is not possible to interpret the value of the band depth in the floor the craters because of the absence of direct sunlight. However, the illuminated rims seem to have a deeper 2-µm absorption band, as does the ejecta from an unnamed crater located further south, within quadrangle Av-10 (Tosi et al., 2010, this meeting). The absorption band seems slightly shifted towards shorter wavelengths in the neighborhood of the same crater, which may indicate a more diogenitic composition, consistent with materials of the deeper crust. Relationships between craters, ejecta and composition will be investigated further. The authors acknowledge the support of the Dawn Science, Instrument and Operations Teams. This work was supported by the NASA Dawn Project under contract from UCLA and by the NASA Dawn at Vesta Participating Scientist program.

NASA's Dawn spacecraft arrived at the asteroid 4 Vesta in July 2011 and is now collecting imaging and spectroscopic data during its one-year orbital mission. The maps we present are based on information obtained by the Visible and Infrared Mapping Spectrometer VIR-MS and the multi-color Framing Camera FC. VIR covers the wavelength range between 0.25 to 5.1 µm while FC covers the range 0.4 to 1.0 µm. The VIR instrument has a significant higher spectral resolution than FC but the latter achieves higher spatial resolution data. As part of the geological and mineralogical analysis of the surface, a series of 15 quadrangles have been defined covering the entire surface of Vesta. We report about the mapping results of quadrangle Av-6 (Gegania) and Av-7 (Lucaria). The Gegania quadrangle is dominated by old craters showing no ejecta blankets and rays while several small fresh craters do. The most obvious geologic features are a set of equatorial troughs, a group of three ghost craters of similar diameter (~57 km), an ejecta mantling of the Gegania crater and three smaller craters showing bright and dark ejecta rays. The quadrangle contains two main geologic units: 1) the northern cratered trough terrain and 2) the equatorial ridge and trough terrain. The quadrangle shows moderate variation in Band II center wavelength and Band II depth. FC color ratio variations of some recent craters and their ejecta are linked to the bright and dark material. The bright material is possibly excavated eucritic material while the dark material could be remnants of a CM2 impator(s) or an excavated subsurface layer of endogenic origin. The most prominent geologic features in the Lucaria quadrangle are the 40 km long hill Lucaria Tholus, a set of equatorial troughs, some relatively fresh craters with bright and dark material and mass wasting. The quadrangle contains three main geologic units: 1) the northern cratered trough terrain, 2) the equatorial ridge and trough terrain, and 3) the equatorial cratered terrain. The quadrangle exhibits moderate variation in Band II center wavelength and only small variation in Band II depth. FC ratio maps show a distinct variation of the pseudo 1-µm band depth. Ejecta material showing steeper VIS spectral slope is located in the western part of the quadrangle.

Since the arrival of the Dawn spacecraft at 4 Vesta on July 16, 2011 the Visible and InfraRed Imaging Spectrometer (VIR) has acquired hyperspectral images of Vesta's surface, which enable to characterize Vesta's mineralogical composition in the wavelength range from 0.25 to 5.1µm. As part of the analysis of Vesta's surface composition the science team is preparing a series of 15 quadrangle maps showing the results derived from the spectroscopic analysis of the VIR and FC color data. We present preliminary results of the spectroscopic analysis achieved for the quadrangles Av-2 (Belicia) and Av-3 (Caparronia), which show Vesta's surface between 21°N - 66° N°, 0° - 90°E and 90° - 180° E, respectively. These results are based on the analysis of the combination of the visible albedo, spectral parameters including the position, depth of the pyroxene absorptions, as well as color ratio composites using the VIR channels centering at 749nm/438nm (Red), 749nm/917nm (Green) and 438nm/749nm (Blue). Vesta's rotation axis, however, is tilted ~29° with respect to its orbital plane. Since Dawn arrived during northern winter, portions of Vesta north of ~45° N are dominated by extended shadows or have not yet been imaged due to permanent night. Thus, limited FC color or VIR hyperspectral data have been available for the quadrangles Av-2 and Av-3. The illuminated parts are dominated by a heavily-cratered northern terrain with ancient troughs and grooves and named after the prominent relatively large impact craters Belicia (~37°N/48°E) and Caparronia (~36°N/167°E). Numerous impact craters of different size, morphology, and state of surface degradation are apparent. Most spectral variations are strongly affected by the extreme illumination conditions, making the analysis of albedo variations and spectral signatures rather difficult. Their interpretation thus remains. Nevertheless, VIR spectra show clear evidence of Vesta's surface composition similar to those of HED (howardite, eucrite and diogenite) meteorites. The prominent pyroxene absorptions near 0.9 and 1.9µm show different band depths and band centers, which are associated with the presence and abundance of the mafic minerals as well as grain size. Within the quadrangles Av-2 and Av3, band centers appear to shift slightly to shorter wavelength from W to E following the trend of the equatorial region. A similar trend can be observed with respect to the depth of the pyroxene absorptions with the absorption deepening eastward. Locally, bright material associated with strong pyroxene absorptions is observed on crater walls of a few relatively large impact craters with pronounced topography. Either these impact craters are relatively young or fresh material became exposed due mass wasting processes. The effects of photometry for under these illumination conditions are being assessed. The authors gratefully acknowledge the support of the Dawn Instrument, Operations, and Science Teams. This work is supported by NASA through the Dawn project and the German Space Agency (DLR).

NASA's Dawn spacecraft is spending one year in orbit around asteroid 4 Vesta to characterize its geology, chemical and mineralogical composition, topography, shape, and internal structure. The Dawn Team is conducting geological mapping of the surface in the form of one global and 15 quadrangle maps. Here we report results from the mapping of Rheasilvia quadrangle Av-15. Mapping is based on a Framing Camera (FC) mosaic produced from High Altitude Mapping Orbit (HAMO) data with a spatial resolution of ~70 m/pixel, supplemented by a Digital Terrain Model (DTM: lateral spacing of 450 m/pixel and vertical accuracy of ~30 meters), FC color images, and Visible and InfraRed (VIR) hyperspectral images. Av-15 Rheasilvia Quadrangle covers the southern pole of Vesta and stretches north to -21°S. Vesta has three dominant terrains: A heavily-cratered northern terrain with ancient troughs and grooves, an intermediately-cratered equatorial terrain bearing prominent flat-floored, E-W-trending troughs, and the relatively lightly-cratered south polar region, containing the Rheasilvia impact basin and related terrains. This quadrangle is dominated by the central mound complex of the Rheasilvia impact basin. Primary geologic features of this region include: (1) the Rheasilvia complex, including the central mound terrain, ridge-and-groove terrain, and smoother terrain; (2) slump material; and (3) impact craters and associated material. The Rheasilvia formation encompasses the central mound complex, two trends of ridges and grooves (only 5% of the quad area is covered by this terrain), and patches of smoother, less-cratered terrain on the mound itself. The mound, which covers nearly 60% of the quadrangle area, is ~22 km high and ~180 km wide, with a discontinuous bounding scarp and low crater density. The ridge-and groove terrain consists of ridges and grooves radiating approximately 90°-270°, and ridges and troughs or ridge and groove complexes that arch or curve as they extend out from from the central mound unit. VIR data indicates that the basin's central mound has a uniform mineralogy, while the basin floor shows more heterogeneity, though both indicate the presence of howardite-diogenite rich materials. Along the base of the bounding scarp occur patches of material characterized by a low crater density and smoother, somewhat granular-textured morphology. Irregularly-bounded patches of very smooth material 25-35 km across are also present, often located on slopes or topographically lower regions. We interpret most of these deposits as slump material emplaced as a result of the effects of basin formation and settling. Most terrain in the quadrangle is heavily cratered. The authors gratefully acknowledge the support of the Dawn Instrument, Operations, and Science Teams. This work is supported by NASA through Dawn at Vesta Participating Scientist grant #NNX10AR19G.

NASA’s Dawn spacecraft began orbiting the inner main belt asteroid 4 Vesta on July 16, 2011, and will characterize the geology, elemental and mineralogical composition, topography, and internal structure of Vesta before departing for asteroid 1 Ceres in late 2012. As part of the Dawn data analysis, the science team is constructing geological mapping of the surface, in the form of 15 quadrangle maps. This abstract reports preliminary results from mapping of quadrangle Av-3, named for Caparronia crater.

By 7-October-2011, the Dawn mission will have completed Survey orbit and commenced high altitude mapping of 4Vesta. We present a preliminary analysis of data acquired by Dawn’s Framing Camera (FC) and the Visual and InfraRed Spectrometer (VIR) to map mineralogy and surface temperature, and to detect and quantify surficial OH. The radiometric calibration of VIR and FC is described. Background counting data acquired by GRaND are used to determine elemental detection limits from measurements at low altitude, which will commence in November. Geochemical models used in the interpretation of the data are described. Thermal properties, mineral-, and geochemical-data are combined to provide constraints on Vesta’s formation and thermal evolution, the delivery of exogenic materials, space weathering processes, and the origin of the howardite, eucrite, and diogenite (HED) meteorites.

The FC cameras onboard the Dawn spacecraft are expected to map the asteroid 4 Vesta in seven different colors from Survey Orbit in August 2011. We will present the first immediate results of the spectral mapping of the visible surface from FC images along with their association with surface compositional units. The first medium resolution observations of Vesta have been performed in July 2011