Delivery of Dark Material to Vesta via Carbonaceous Chondritic Impacts

NASA's Dawn spacecraft observations of asteroid (4) Vesta reveal a surface with the highest albedo and color variation of any asteroid we have observed so far. Terrains rich in low albedo dark material (DM) have been identified using Dawn Framing Camera (FC) 0.75 {\mu}m filter images in several geologic settings: associated with impact craters (in the ejecta blanket material and/or on the crater walls and rims); as flow-like deposits or rays commonly associated with topographic highs; and as dark spots (likely secondary impacts) nearby impact craters. This DM could be a relic of ancient volcanic activity or exogenic in origin. We report that the majority of the spectra of DM are similar to carbonaceous chondrite meteorites mixed with materials indigenous to Vesta. Using high-resolution seven color images we compared DM color properties (albedo, band depth) with laboratory measurements of possible analog materials. Band depth and albedo of DM are identical to those of carbonaceous chondrite xenolith-rich howardite Mt. Pratt (PRA) 04401. Laboratory mixtures of Murchison CM2 carbonaceous chondrite and basaltic eucrite Millbillillie also show band depth and albedo affinity to DM. Modeling of carbonaceous chondrite abundance in DM (1-6 vol%) is consistent with howardite meteorites. We find no evidence for large-scale volcanism (exposed dikes/pyroclastic falls) as the source of DM. Our modeling efforts using impact crater scaling laws and numerical models of ejecta reaccretion suggest the delivery and emplacement of this DM on Vesta during the formation of the ~400 km Veneneia basin by a low-velocity (<2 km/sec) carbonaceous impactor. This discovery is important because it strengthens the long-held idea that primitive bodies are the source of carbon and probably volatiles in the early Solar System.Comment: Icarus (Accepted) Pages: 58 Figures: 15 Tables:

Pyroxene reflectance spectra: Minor absorption bands and effects of elemental substitutions

Reflectance spectra of a suite of compositionally diverse pyroxenes exhibit variable spectral properties which are associated with various elemental substitutions. Those associated with transition series elements, such as Cr, Ti, V, and Mn, give rise to a number of minor absorption bands in the visible spectral region and, in some cases (e.g., Mn), at longer wavelengths. Substitutions by other cations, such as Li and Zr, do not result in distinct absorption bands. The spectra of these pyroxenes are dominated by the transition series elements that may be present in the samples. The visible wavelength region of low-calcium pyroxene reflectance spectra exhibit a number of absorption bands which are attributable to Fe2+ spin-forbidden crystal field transitions. Fe2+-Fe3+ intervalence charge transfer absorption bands near 0.77 μm are common in terrestrial pyroxenes which contain both ferrous and ferric iron. Collectively, these results indicate that the presence of various transition series elements can be detected in reflectance spectra and that their oxidation states and site occupancies can also be determined on the basis of unique spectral characteristics present in their reflectance spectra, thereby greatly increasing the range of compositional information which can be derived from analysis of their reflectance spectra.This study was supported by grants-in-aid of research from the Natural Sciences and Engineering Research Council of Canada (NSERC), Sigma Xi, The Scientific Research Society, the Geological Society of America, a research contract from the Canadian Space Agency, and a start-up grant from the University of Winnipeg.This study was supported by grants-in-aid of research from the Natural Sciences and Engineering Research Council of Canada (NSERC), Sigma Xi, The Scientific Research Society, the Geological Society of America, a research contract from the Canadian Space Agency, and a start-up grant from the University of Winnipeg.https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2001JE00159