The Pitted Impact Deposits on Asteroid 4 Vesta: An In-Depth Analysis

Abstract

This work analyzes geomorphological devolatilization features on the atmosphereless asteroid Vesta. In particular, it predominantly analyzes the features associated with the crater Marcia, as those add up to more than 96% on the whole body and are the only ones occurring within the ejecta of a crater. These features are called Pitted Impact Deposits, or short PIDs, and they have been previously identified on Mars, Vesta and the dwarf planet Ceres. PIDs are characterized by closely spaced pits that occur as locally confined clusters of several kilometers length. The individual pits can overlap and often share boundaries. They lack raised rims as would be typical of an impact crater and are polygonal to circular in shape. Furthermore, they show distinct spectral characteristics like higher reflectance at 750 nm and more intense pyroxene absorptions near 0.9 and 1.9 µm. It is evident through previous studies that PIDs formed via degassing or devolatilization of parts of the ejecta, yet the detailed mechanism behind remained debated. Additionally to the previously known dominance of pyroxene-rich HED-like material on Vesta (HED: howardite, eucrite, diogenite meteorites), many studies have reported on the existence of OH-bearing material and the so-called ‘dark material’ which has been proposed to originate from influx of carbonaceous chondrite material. The identified OH is likely bound within the crystal structure of phyllosilicates (that are commonly found in carbonaceous chondrites) and is able to leave the crystal structure when heated. All remote observations shown here were obtained by NASA’s Dawn mission. The presented analysis show that the formation of PIDs is controlled by the proportion of an ejecta deposit’s volume to surface area, or depth. Many PIDs are located in small craters or at steep topographic slopes that existed prior to the Marcia impact or to the time of PID formation. This enables the estimation of the pre-existing topography and therefore of the shape and extent of the post-impact ejecta deposit. The accumulation of ejecta at these sites leads to a slower cooling of the ejecta deposit, a larger volume not exposed to the space environment and an extended possibility for volatiles to escape from its host mineral. The depth of the ejecta deposit appears to control the shape and extent of the developing devolatilization vents and therefore, of the individual pits themselves. The proportion of volatiles within the whole ejecta is estimated here to be less than 2 wt% and the proportion of lost volatiles at the PIDs’ sites to ~1 wt%. This shows that particularly elevated volatile contents are not needed to form PIDs. PIDs are always part of a larger impact deposit not featuring a pitted surface. This suggest that the original material of both deposit parts were identical or very similar. The spectral changes of the PIDs with respect to their immediate surroundings are not consistent with variations in grain size, roughness or glass content. One possibility of creating similar spectral characteristics is the removal of darkening agents. At high temperatures, organic material (which are commonly dark) can decompose, as do other components of carbonaceous chondrites. Tochilinite for example is a major constituent in the carbonaceous chondrite Murchison and decomposes already at 400 °C. However, the mere removal of dark components cannot explain all aspects of spectral characteristics shown for PIDs, i.e. the ratio of pyroxene band strength to reflectance at 750 nm with respect to other more typical regions on Vesta. This thesis furthermore presents laboratory experiments in order to explain the PIDs’ spectral characteristics. Both terrestrial and meteoritic materials were used. These were assembled to adequately represent the Vestan regolith. The experiments involving the heating of these analog materials show that hematite formed due to oxidation and therefore, a strong reddening of the visible spectral slope of the samples was observed. This is not observed on Vesta, yet might be explained. First, the ejecta deposit on Vesta is likely very heterogeneous, as has been shown by previous studies. This can lead to a smaller and slower extent of these oxidation processes. Second, the estimated devolatilization duration is in the range of hours to days, possibly inhibiting the onset of hematite formation or the incapability of remote spectrometers to record the small amounts that might have already formed. Third, the processes of space weathering of hematite over geological timescales are not well-known, which could additionally play a role. In combination with existing literature, the laboratory experiments presented here show that an oxidizing environment together with higher temperatures (≥400 °C, well below the minerals’ melting point) can result in similar spectral characteristics, i.e. higher reflectance and pyroxene band strength, as shown for PIDs. The underlying process could involve the migration of Fe2+ to the grain surfaces, where it increases the relative iron abundance that electromagnetic energy (i.e., light) encounters first which in turn intensifies the pyroxene absorption. Many studies have observed similar processes where Fe2+ is converted to Fe3+ and forms hematite (Fe2O3) or other iron (hydr-)oxides. An alternative explanation also includes the migration of Fe2+, yet to its preferred crystallographic M2 site, where it likewise would intensify the pyroxene absorption. Original pyroxene crystals might have been disordered regarding their cation distribution, whereas the temperature increase would enable the cations to migrate to their preferred position. This work shows that oxidation processes can occur on planetary bodies thought to be dry. The surficial contamination with carbonaceous chondrite material enables this process, which might be relevant to future space missions and could influence the search for organic matter.