Delivery of dark material to Vesta via carbonaceous chondritic impacts

Abstract

a b s t r a c t 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 lm 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/s) 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.

The geological nature of dark material on Vesta and implications for the subsurface structure

“…This might suggest that the projectile was composed of dark material and the dark striations are the result of projectile material mixed into the ejecta. Survival of the impactor is plausible due to the low impact velocities at Vesta's position in the main asteroid belt (O'Brien et al, 2011;Reddy et al, 2012). However, cropping out of dark material in the crater wall, $200-300 m below the crater rim at exactly such positions where the striations originate make an excavation from the subsurface more likely.…”

“…The local characteristics of dark material, which clearly separates it from areas without dark material, as well as the uneven distribution across the surface of Vesta, suggest a distribution process that focuses on the deposition of small amounts of material rather than global coverage. On a global scale the densest concentrations of dark material, outcrops as well as subsurface patches, appear to be correlated with the northern and northwestern rim of the Veneneia south polar basin (Jaumann et al, 2012b;Otto et al, 2012;Reddy et al, 2012), extending about 130 km inside as well as outside the rim structure (Fig. 1).…”

“…Alternatively, it may occur as blocky or layered outcrops in crater walls and as mass-wasting deposits on crater flanks and floors. One apparent conclusion from these observations (Jaumann et al, 2012b;McCord et al, 2012;Reddy et al, 2012) is that the nature of the dark material deposits, whatever their sources, is strongly influenced by impact mixing, gardening and mass-wasting. In addition to the small, well-defined dark material deposits, there are also large regions of low albedo surface material, often with indistinct boundaries, that appear to include dark material.…”

“…However, regions of very low albedo on Vesta's surface were first discovered by the Dawn mission (Jaumann et al, 2012a;Russell et al, 2012;McCord et al, 2012;Reddy et al, 2012). These dark material deposits are non-randomly distributed over the surface and often associated with geological/morphological features (Jaumann et al, 2012b;McCord et al, 2012).…”

Survival of the impactor during hypervelocity collisions – I. An analogue for low porosity targets

“…The investigation of the projectile, and projectile debris, during hypervelocity impacts is crucial to explain the observations of mixed mineralogies on the surface of asteroids. Such phenomena, which have been observed only relatively recently, are the source of the olivine and dark material deposits observed on Vesta (McCord et al 2012;Reddy et al 2012) and probably of the "Black Boulder" on (25143) Itokawa (Hirata & Ishiguro 2011). Mixing of asteroid material with different lithology through impacts is also necessary to explain the nature of the Near-Earth asteroid 2008 TC3, a multi-lithology body whose formation mechanism is still not completely understood (Jenniskens et al 2009;Bischoff et al 2010).…”