Ceres: UV-Visible Spectra as a Window to Carbon on Low-Albedo Asteroids

Abstract:

Carbon and carbonaceous species, though ubiquitous throughout the solar system, are not well understood on planetary surfaces due to carbon’s spectral blandness at visible-near infrared wavelengths. In the ultraviolet (UV), however, carbon is spectrally active and spectrally variable, depending on the amount of processing (thermal, radiolytic) experienced by the surface. Thus, the UV is a key region in which we can not only identify carbonaceous species but also track the level of processing of different surfaces throughout the solar system.

We find that intimate mixtures of phyllosilicates and carbons can reproduce the overall characteristics of a range of UV-visible primitive asteroid spectra. Amorphous carbon is expected to be a weathering product of native carbonaceous compounds and carbon materials delivered by micrometeoroid impacts. Graphitized carbons are consistent with carbon-rich species on the surface of an airless body, losing H to weathering processes such as charged particle bombardment.

Observations of dwarf planet Ceres using the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telecope in the spectral range 1160-5700 Å were used to identify the spectral signatures of graphitized carbon. In this study, we revisit HST/STIS data of Ceres. Here, we derive absolute spectral albedos across the surface to compare with spectral models and thus derive abundances of compositional components. In additional to carbon compounds and phyllosilicates, we consider salts and irradiated salts, and how they can contribute to the UV-visible spectrum of Ceres.