Effects of Thermal Metamorphism on Spectral Properties of a Carbonaceous Asteroid Regolith Simulant

English Abstract:

Thermal processing has been invoked as a possible cause of some of the spectral features of the B-type near Earth asteroid Bennu, the target of the upcoming OSIRIS-REx asteroid sample return mission. In particular, thermal metamorphism may explain the anomalous ‘blue’ spectral slope (i.e., increasing reflectance towards short wavelength). It is expected that, to first order at least, the regolith of Bennu will resemble known carbonaceous chondrite meteorites (though it will very likely differ in detail). We have developed a regolith simulant consisting of nanophase magnetite (5%), natural serpentine (85%), shungite (5%; shungite is a refractory organic mineraloid), and troilite (5%). This mixture was designed to simulate the composition of CM2 chondrites largely in terms of phases expected to have the greatest influence on CM spectral properties; CMs are one of the most common carbonaceous chondrite types. The simulant was heated under partial vacuum (10-70 mbar) with a dry N₂ purge. The samples were heated at 100ºC increments from 100ºC to 1200ºC. X-ray diffraction data were collected at each heating stage, before and after heating, along with reflectance spectra covering 350-2500 nm. Overall reflectance increased with heating. Spectral changes include loss of OH-related spectral features above 700ºC, consistent with the dehydroxylation of serpentine. Enstatite and forsteritic olivine were observed by XRD at 900ºC. Hematite formed at the expense of magnetite beginning at 600ºC, but was less apparent with increasing temperatures and nearly undetectable above 1100ºC. It appears that heating alone cannot account for the spectral properties of Bennu, perhaps indicating that alternative mechanisms (e.g., enhanced aqueous or hydrothermal alteration, space weathering) or physical properties (grain size, compaction state) must be explored.