The unique spectral and geomorphological characteristics of pitted impact deposits associated with Marcia crater on Vesta

Abstract

The occurrence of pitted impact deposits (PIDs) and widespread OH-bearing minerals on the differentiated asteroid Vesta shows that volatiles delivered by exogenic sources play an important role for surface appearance and processes on airless bodies. The young, large crater Marcia features widely spread PIDs in its crater-fill and ejecta deposits. In order to derive more details on their formation, we studied the spectral and geomorphological properties of PIDs associated with Marcia. Our observations imply that the formation of PIDs is linked to topography-controlled accumulation of ejecta and the accompanying increase of deposit thickness as well as internal heat. This is best illustrated where PIDs are located within pre-existing craters (i.e., pre-Marcia), enabling the estimation of pre-impact topography and deposit thickness. Previous studies reported a lower than average reflectance at visible wavelengths for the crater-fill PIDs, which we confirm. However, several PIDs outside the crater exhibit higher reflectance and pyroxene band strength than the average Vesta and the majority of PIDs exhibit higher reflectance and pyroxene band strength with respect to their immediate surroundings (which belong to the same impact deposit). Furthermore, at least six PIDs (many are ambiguous) show resolvable, diminished absorption bands at 2.8 μm, corresponding to a depletion in OH. These spectral observations are most consistent with less contamination by exogenic carbonaceous chondrite materials, i.e., a loss of hydroxyl groups as well as dark components. Our observations are inconsistent with differences in grain size, pyroxene composition, glass content or age-related differences. Regarding reflectance at 750 nm and pyroxene band depth (750/917 [nm]), we present a unique spectral trend for PIDs with respect to ‘typical Vestan’ materials, suggesting a likewise unique formation process occurring on this asteroid. The crater-fill PIDs display ambiguous spectral characteristics that we attribute to post-impact inflow of dark material from the crater rim. PIDs are ubiquitous in the Marcia region (>100 individual clusters). Yet volatiles lost at these sites might only represent a minor fraction (<2%) of volatiles still present in the host ejecta. An average PID only lost ~1 wt% of volatiles upon devolatilization, implying that exceptionally high volatile contents are not critical for PID formation on Vesta.