An in-depth study of Marcia Crater, Vesta

After visiting the second most massive asteroid Vesta from July 2011 to September 2012, the Dawn spacecraft is now on its way to asteroid Ceres. Dawn observed Vesta with three instruments: the German Framing Camera (FC), the Italian Visible and InfraRed mapping spectrometer (VIR), and the American Gamma Ray and Neutron Detector (GRaND) [1]. Marcia crater (190°E, 10°N; 68 x 58 km) is the largest of three adjacent impact structures: Marcia (youngest), Calpurnia, and Minucia (oldest). It is the largest well-preserved post-Rheasilvia impact crater, shows a complex geology [2], is young [2], exhibits evidence for gully-like mass wasting [3], contains the largest location of pitted terrain [4], has smooth impact melt ponds [5], shows enhanced spectral pyroxene signatures on its inner walls [2], and has low abundances of OH and H in comparison to the surrounding low-albedo terrain [6, 7]. Geophysically, the broad region of Marcia and Calpurnia craters is characterized by a higher Bouguer gravity, indicating denser material [9]. Williams et al. [2] have produced a detailed geologic map of Marcia crater and the surrounding terrain. They identified several units within Marcia crater, including bright crater material, pitted terrain, and smooth material. Units outside Marcia, include undivided crater ejecta material, bright lobate material, dark lobate material, and dark crater ray material [2]. Because of its extensive ejecta and fresh appearance, the Marcia impact defines a major stratigraphic event, postdating the Rheasilvia impact [2]. However, the exact age of Marcia crater is still under debate. Compositionally, Marcia crater is characterized by higher iron abundances, which were interpreted as more basaltic-eucrite-rich materials suggesting that this region has not been blanketed by diogenitic materials from large impact events [10, 11]. Using FC data, [13] identified "gray material" associated with the ejecta blanket of Marcia crater. This material is characterized by a 0.75-mm reflectance of ~15%, a shallow visible slope, and a weak R(0.75 µm)/R(0.92 µm) ratio [12], which is still high compared to immediately adjacent terrains. The most prominent thermal feature in Marcia is the pitted terrain on its floor [8]. Temperatures of the pitted floor of Marcia are significantly lower than in the surrounding terrains, when observed under similar solar illumination. Denevi et al. [4] argued that the morphology and geologic setting are consistent with rapid degassing of volatile-bearing materials following an impact, which would lead to an increased local density and/or a higher thermal conductivity [8]. References: [1] Russell et al. (2007), Earth Moon Planets 101; [2] Williams et al. (2014), submitted to Icarus; [3] Scully et al. (2013), LPSC 45; [4] Denevi et al. (2012), Science 338; [5] Williams, D.A., et al. (2013) PSS, in press, j.pss.2013.06.017 [6] De Sanctis et al. (2012b) Astrophys. J. Lett. 758; [7] Prettyman et al. (2012), Science 338; [8] Tosi et al. (2014), submitted to Icarus; [9] Konopliv et al. (2013) Icarus, in press; [10] Yamashita et al. (2013), Met. Planet. Sci. 48; [11] Prettyman et al. (2013), Met. Planet. Sci. 48; [12] Reddy et al. (2012), Science 33

Constraining Solar System Bombardment Using In Situ Radiometric Dating

The leading, but contentious, model for lunar impact history includes a pronounced increase in impact events at around 3.9 Ga. This late heavy bombardment would have scarred Mars and the terrestrial planets, influenced the course of biologic evolution on the early Earth, and rearranged the very architecture of our Solar System. But what if it's not true? In the last decade, new observations and sample analyses have reinterpreted basin ages and "pulled the pin" on the cataclysm - we may only have the age of one large basin (Imbrium). The Curie mission would constrain the onset of the cataclysm by determining the age of a major pre-Imbrium lunar basin (Nectaris or Crisium), characterize new lunar lithologies far from the Apollo and Luna landing sites, including the basalts in the basin-filling maria and olivine-rich lithologies in the basin margins, and provide a unique vantage point to assess volatiles in the lunar regolith from dawn to dusk

The Crater Chains of Vesta - a Morphological Study

Abstract The Dawn spacecraft spent 141 days in the Low Mapping Orbit (LAMO) around Vesta obtaining 10251 images with an average resolution of 20m per pixel. The surface of Vesta displays a diverse geological structure - numerous impact craters, extensive ejecta blankets, giant troughs around the equator, impact basins, and regions with enigmatic dark as well as bright material (1). The LAMO imaging data set has been used to search, over the entire illuminated surface, for crater chains, which are groups of depressions (round or elongated) in mostly linear alignments - resembling a chain of pearls. More than 300 features were identified as crater chains most of which have an East - West orientation. Based on morphology the observed crater chains were divided into 5 subgroups. 1) Single crater chain (A) - a linear alignment of craters associated with other types of linear features (e.g. grooves, faults, troughs, ridges, or trenches). 2) Single crater chain (NA) - a linear alignment of craters not associated with other types of linear features. 3) Crater chain families - segments of short crater chains aligned parallel to each other. Some are associated with lineaments and some are not. 4) Swarm - a wide “band” of craters stretching in the East - West direction. 5) Special cases which do not fit in the other groups or are mixtures of the other types. The global distribution of crater chains and crater counts will be used to analyze the various morphological groups. Hypotheses for the formation mechanisms of the different groups will be explored. (1) Jaumann, et al, 2012 - Science 336,687-69

The Color of 4 Vesta and Lithology Diversity: First Results from Dawn Survey Orbit

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 201

Surveying Vesta's styles of space weathering and surface mixing

The Dawn spacecraft's mission at Vesta [1] has revealed a world that, although much smaller than the Moon or Mercury, has experienced planet-like processes and undergone a complicated geological evolution [2]. One fascinating characteristic of Vesta is the manner in which the regolith evolves in response to exposure to the space environment. In general, vestan space weathering is dominated by admixture of low-reflectance material delivered by carbonaceous chondrite (CC) impactors [3-7]. As a result, freshly exposed vestan basaltic material tends to become darker with time, and the strong absorption bands (near 1000 and 2000 nm) caused by ferrous iron in pyroxene become shallower. Darkening and decreased band contrast are hallmarks of lunar space weathering, however on the Moon these are accompanied by a strong increase in the continuum slope (reddening) [e.g., 8, 9]. The cause of the spectral changes on the Moon is the accumulation of micro- and nanophase metallic iron as a result of melting and vaporization by micrometeoroid bombardment and/or solar-wind sputtering [reviewed by 10]. We are conducting a survey of impact mixing and regolith maturation trends in different regions of Vesta. The goals are to document the range of space weathering styles on Vesta, and to examine how the observed trends can give clues to the composition of the material that is undergoing space weathering. The findings should help to further understanding of space weathering in the asteroid belt, and hence as a general phenomenon across the Solar System. Here we present results from two locations that illustrate Vesta's spectral diversity: Vibidia crater and near Oppia crater

Thermal measurements of dark and bright surface features on Vesta as derived from Dawn/VIR

Remote sensing data acquired during Dawn’s orbital mission at Vesta showed several local concentrations of high-albedo (bright) and low-albedo (dark) material units, in addition to spectrally distinct meteorite impact ejecta. The thermal behavior of such areas seen at local scale (1–10 km) is related to physical properties that can provide information about the origin of those materials. We use Dawn’s Visible and InfraRed (VIR) mapping spectrometer hyperspectral data to retrieve surface temperatures and emissivities, with high accuracy as long as temperatures are greater than 220 K. Some of the dark and bright features were observed multiple times by VIR in the various mission phases at variable spatial resolution, illumination and observation angles, local solar time, and heliocentric distance. This work presents the first temperature maps and spectral emissivities of several kilometer-scale dark and bright material units on Vesta. Results retrieved from the infrared data acquired by VIR show that bright regions generally correspond to regions with lower temperature, while dark regions correspond to areas with higher temperature. During maximum daily insolation and in the range of heliocentric distances explored by Dawn, i.e. 2.23–2.54 AU, the warmest dark unit found on Vesta rises to a temperature of 273 K, while bright units observed under comparable conditions do not exceed 266 K. Similarly, dark units appear to have higher emissivity on average compared to bright units. Dark-material units show a weak anticorrelation between temperature and albedo, whereas the relation is stronger for bright material units observed under the same conditions. Individual features may show either evanescent or distinct margins in the thermal images, as a consequence of the cohesion of the surface material. Finally, for the two categories of dark and bright materials, we were able to highlight the influence of heliocentric distance on surface temperatures, and estimate an average temperature rate change of 1% following a variation of 0.04 AU in the solar distance

Global inventory and characterization of pyroclastic deposits on Mercury: New insights into pyroclastic activity from MESSENGER orbital data

We present new observations of pyroclastic deposits on the surface of Mercury from data acquired during the orbital phase of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. The global analysis of pyroclastic deposits brings the total number of such identified features from 40 to 51. Some 90% of pyroclastic deposits are found within impact craters. The locations of most pyroclastic deposits appear to be unrelated to regional smooth plains deposits, except some deposits cluster around the margins of smooth plains, similar to the relation between many lunar pyroclastic deposits and lunar maria. A survey of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over a prolonged interval. Measurements of surface reflectance by MESSENGER indicate that the pyroclastic deposits are spectrally distinct from their surrounding terrain, with higher reflectance values, redder (i.e., steeper) spectral slopes, and a downturn at wavelengths shorter than similar to 400nm (i.e., in the near-ultraviolet region of the spectrum). Three possible causes for these distinctive characteristics include differences in transition metal content, physical properties (e.g., grain size), or degree of space weathering from average surface material on Mercury. The strength of the near-ultraviolet downturn varies among spectra of pyroclastic deposits and is correlated with reflectance at visible wavelengths. We suggest that this interdeposit variability in reflectance spectra is the result of either variable amounts of mixing of the pyroclastic deposits with underlying material or inherent differences in chemical and physical properties among pyroclastic deposits