Mapping Vesta: First Results from Dawn’s Survey Orbit

The geologic objectives of the Dawn Mission [1] are to derive Vesta’s shape, map the surface geology, understand the geological context and contribute to the determination of the asteroids’ origin and evolution.Geomorphology and distribution of surface features will provide evidence for impact cratering, tectonic activity, volcanism, and regolith processes. Spectral measurements of the surface will provide evidence of the compositional characteristics of geological units. Age information, as derived from crater sizefrequency distributions, provides the stratigraphic context for the structural and compositional mapping results, thus revealing the geologic history of Vesta. We present here the first results of the Dawn mission from data collected during the approach to Vesta, and its first discrete orbit phase – the Survey Orbit, which lasts 21 days after the spacecraft had established a circular polar orbit at a radius of ~3000 km with a beta angle of 10°-15°

The Geology of the Marcia Quadrangle of Asteroid Vesta: Assessing the Effects of Large, Young Craters

We used Dawn spacecraft data to identify and delineate geological units and landforms in the Marcia quadrangle of Vesta as a means to assess the role of the large, relatively young impact craters Marcia (approximately 63 kilometers diameter) and Calpurnia (approximately 53 kilometers diameter) and their surrounding ejecta field on the local geology. We also investigated a local topographic high with a dark-rayed crater named Aricia Tholus, and the impact crater Octavia that is surrounded by a distinctive diffuse mantle. Crater counts and stratigraphic relations suggest that Marcia is the youngest large crater on Vesta, in which a putative impact melt on the crater floor ranges in age between approximately 40 and 60 million years (depending upon choice of chronology system), and Marcia's ejecta blanket ranges in age between approximately 120 and 390 million years (depending upon choice of chronology system). We interpret the geologic units in and around Marcia crater to mark a major Vestan time-stratigraphic event, and that the Marcia Formation is one of the geologically youngest formations on Vesta. Marcia crater reveals pristine bright and dark material in its walls and smooth and pitted terrains on its floor. The smooth unit we interpret as evidence of flow of impact melts and (for the pitted terrain) release of volatiles during or after the impact process. The distinctive dark ejecta surrounding craters Marcia and Calpurnia is enriched in OH- or H-bearing phases and has a variable morphology, suggestive of a complex mixture of impact ejecta and impact melts including dark materials possibly derived from carbonaceous chondrite-rich material. Aricia Tholus, which was originally interpreted as a putative Vestan volcanic edifice based on lower resolution observations, appears to be a fragment of an ancient impact basin rim topped by a dark-rayed impact crater. Octavia crater has a cratering model formation age of approximately 280-990 million years based on counts of its ejecta field (depending upon choice of chronology system), and its ejecta field is the second oldest unit in this quadrangle. The relatively young craters and their related ejecta materials in this quadrangle are in stark contrast to the surrounding heavily cratered units that are related to the billion years old or older Rheasilvia and Veneneia impact basins and Vesta's ancient crust preserved on Vestalia Terra

New Insights Into Subsurface Stratigraphy Northwest of Ascraeus Mons, Mars, Using the SHARAD and MARSIS Radar Sounders

The Tharsis Montes volcanoes on Mars are the source of laterally extensive lava flows and other volcanic deposits generating a complex stratigraphy throughout the Tharsis Volcanic Province. We use SHAllow RADar (SHARAD) and Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) observations in a region northwest of Ascraeus Mons to determine the composition, density, thickness, and spatial distribution of these emplaced volcanic materials. We identified subsurface reflectors along 43 SHARAD and five MARSIS observations. Reflectors in the volcanic plains are interpreted to be sequences of basaltic lava flows with interspersed pyroclastic material, dust, or regolith during a hiatus in activity. Others correspond to the base of three major flow fields. Several plain reflectors were detected by both MARSIS and SHARAD. Other notable reflectors were identified near Ascraeus' flank where lava buried glacially derived sediment. We derived thickness and other material properties for flows using their distinct topographic boundary visible in the radar images. Permittivity ranged from 7.0 to 11.2 corresponding to lava flow densities of 3.20–3.52 g/cm3. Flow thicknesses ranged from 19.8 to 60.2 m. Loss tangents were low for the flow fields ranging from 0.024 to 0.043. Loss tangents in the plains ranged from 0.010 to 0.076. Higher loss tangents correspond to lossier regions that may have higher concentrations of radar absorbing minerals like hematite. Surface roughness controls where reflectors are detected. SHARAD detects the base of three out of the four flow fields in this region with muted surface roughness from dust mantling and erosion. © 2022. American Geophysical Union. All Rights Reserved.6 month embargo; first published: 31 May 2022This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Geologic Mapping Methods for a Mission-Driven Mapping Scenario: The Dawn at Vesta Example

We report on methods used to create a global geologic map of Vesta based on data from the Dawn spacecraft’s High-Altitude Mapping Orbit (HAMO) data. Each of Dawn’s several orbital phases at Vesta provided increasingly higher spatial resolution data that fed into three main iterations of the global map. The first iteration was created during the approach phase and was based on clear filter data from the Framing Camera (FC), which at this stage covered the surface at 3-9 km/pixel resolution. The second iteration used FC clear filter data at ~200 m/pixel resolution and a Digital Terrain Model derived from image data acquired during Survey orbit. The third iteration was based on data from the High-Altitude Mapping Orbit (HAMO; spatial resolution ~61 m/pixel). We mapped three broad terrain types: heavily-cratered, ridge-and-trough (equatorial and northern fossae), and terrain associated with the Rheasilvia and Veneneia impact structures. Local features include bright and dark material and ejecta, lobate deposits, and mass-wasting materials. Stratigraphy of Vesta’s geologic units suggests a history in which primary crust formation was followed by first the Veneneia and then the Rheasilvia impact events, along with associated structural deformation that shaped the Saturnalia and Divalia Fossae Formations respectively. Subsequent impacts and mass wasting events subdued impact craters and parts of ridge-and-trough sets, and formed slumps and landslides. Discontinuous low-albedo deposits also formed or were emplaced; these lie stratigraphically above the equatorial fossae. The youngest features are bright-rayed craters and other surface mantling deposits. Lessons learned in this mapping effort include: (1) iterative mapping provides teams with a robust way to organize knowns and unknowns, feeding forward into subsequent science decisions; (2) the process must include enough people working concurrently as well as collaboratively and individually, to be efficient enough to serve tactical needs; and (3) generic descriptors of features should be retained for as long as possible — during the iterative mapping process, ideally until features can be observed with the highest resolution. This lessens bias and potentially reduces confusion (among the team and the community) as feature interpretations evolve

Geologic mapping of ejecta deposits in Oppia Quadrangle, Asteroid (4) Vesta

Abstract Oppia Quadrangle Av-10 (288–360°E, ±22°) is a junction of key geologic features that preserve a rough history of Asteroid (4) Vesta and serves as a case study of using geologic mapping to define a relative geologic timescale. Clear filter images, stereo-derived topography, slope maps, and multispectral color-ratio images from the Framing Camera on NASA’s Dawn spacecraft served as basemaps to create a geologic map and investigate the spatial and temporal relationships of the local stratigraphy. Geologic mapping reveals the oldest map unit within Av-10 is the cratered highlands terrain which possibly represents original crustal material on Vesta that was then excavated by one or more impacts to form the basin Feralia Planitia. Saturnalia Fossae and Divalia Fossae ridge and trough terrains intersect the wall of Feralia Planitia indicating that this impact basin is older than both the Veneneia and Rheasilvia impact structures, representing Pre-Veneneian crustal material. Two of the youngest geologic features in Av-10 are Lepida (∼45 km diameter) and Oppia (∼40 km diameter) impact craters that formed on the northern and southern wall of Feralia Planitia and each cross-cuts a trough terrain. The ejecta blanket of Oppia is mapped as ‘dark mantle’ material because it appears dark orange in the Framing Camera ‘Clementine-type’ color-ratio image and has a diffuse, gradational contact distributed to the south across the rim of Rheasilvia. Mapping of surface material that appears light orange in color in the Framing Camera ‘Clementine-type’ color-ratio image as ‘light mantle material’ supports previous interpretations of an impact ejecta origin. Some light mantle deposits are easily traced to nearby source craters, but other deposits may represent distal ejecta deposits (emplaced >5 crater radii away) in a microgravity environment

Geologic Mapping of Av-10 Oppia Quadrangle of Asteroid 4 Vesta

NASA's Dawn spacecraft is collecting a variety of imaging, spectral, and elemental abundance data to characterize the geology, geochemistry, shape and internal structure of Vesta. Geologic mapping of Vesta's surface is being conducted at the global scale and as a series of 15 regional quadrangles. We report results from the mapping of quadrangle Av-10 (Oppia). A mosaic of monochrome (clear filter) Framing Camera (FC) images from the High Altitude Mapping Orbit (HAMO) (70 m/pixel) serves as the base image with additional information about the surface gathered from Visible and Infra-Red (VIR) hyperspectral images, FC color ratio images, and a Digital Terrain Model (DTM: lateral spacing of 450 m/pixel and vertical accuracy of ~30 meters) derived from FC images. Quadrangle Av-10 Oppia is located within the equatorial region of Vesta, and covers 22°S to 22°N latitude and 288° to 0°/360°E longitude. The four global units in Av-10 are 1) cratered terrain in the north, 2) a broad, topographic low (Feralia Planitia) that dominates the central portion of the quadrangle and 3) a topographically higher area towards the south, and 4) equatorial troughs and ridges that cut across the quadrangle between 10°S and 10°N latitude. The northern cratered terrain is one of the older geologic units in Av-10. A portion of this terrain was excavated by the impact that formed Feralia Planitia, one of the larger basin-like features on Vesta. Feralia Planitia is 270 km across and ~15 km deep in relation to the surrounding topographically higher terrain. This topographic low, which is bound by Vestalia Terra to the west (see quadrangle Av-9 Numisia), has been reshaped by Oppia crater on the southern basin boundary, an unnamed crater (50 km) on the northern basin boundary, and by the ridge and trough terrain. A portion of a large northern trough (Saturnalia Fossa) is present in the northwest corner of the quadrangle (see quadrangles Av-4 Domitia and Av-9 Numisia) and E-W-trending equatorial troughs (Divalia Fossa) are located in the eastern half of the (see quadrangle Av-6 Gegania). Oppia crater (D=34 km, 8°S, 309°E) has a sharp rim and a smooth ejecta blanket with a low abundance of impact craters that indicates the crater is relatively young. In FC color-ratio images using approximations of Clementine ratios (Red: 750/430 nm; Green: 750/920 nm; Blue: 430/750 nm), the ejecta blanket has a distinct coloration in comparison to the surrounding terrain. Further study is underway to better understand the nature of this ejecta blanket. 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 #NNH09ZDA001N

A compositional view of Av-10 Oppia quadrangle

In an ongoing effort to map various quadrangles of Vesta from a compositional perspective, here we use data from the Visible and InfraRed (VIR) mapping spectrometer [1] to assess mineralogical evidences and trends across the Av-10 "Oppia" quadrangle. We focus on features observed at the local scale, to highlight diagnostic compositional signatures and relationships with similar findings observed in other quads. Combining information from different datasets (mineralogy, geology, and topography) may ultimately help decipher the origin of these structures and properly fit them in the context of the evolution of the entire asteroid