Abstract
Measurements from NASA’s Dawn Mission to Vesta and Ceres have revealed the presence of numerous bright spots (albedo: ~0.45) generally in association with impact craters on Ceres’ dark surface (albedo: 0.09 - 0.10), raising questions about material mineralogy and the geochemical evolution of these deposits. Cerealia facula and the Vinalia faculae, bright deposits that lie within Occator Crater (20 °N, 239 °E), have significantly reddened spectral slopes in the ultraviolet-visible region, as measured by Dawn’s Framing Camera, which suggest the presence of magnesium sulfate hexahydrate (MgSO4-6H2O) – surprising in Ceres’ low-pressure surface environment. In addition, the measured reflectance across the visible spectrum gradually decreases with increasing distance from the center of Cerealia Facula, potentially a function of material dehydration or space weathering. More recently, Dawn’s onboard Visible Near-Infrared Mapping Spectrometer (VIR) has confirmed the presence of carbonate salts in most of the bright regions, including calcite, dolomite, and magnesite, but also sodium carbonates (natrite / thermonatrite / natron) particularly within Occator Crater. The presence of salts, along with the definitive identification of phyllosilicates in the planetary global mineralogy, implies the likelihood of aqueous alteration on Ceres sometime within its geologic history.
We used model experiments to study the spectral effects due to Ceres’ harsh low-pressure, cryogenic space environment, as well as the impact of solar radiation, on salts identified on Ceres’ surface. We utilized standard laboratory analytical techniques such as UV-Vis-IR reflectance spectroscopies, X-ray photoelectron-spectroscopy, and X-ray powder diffraction, in conjunction with soft X-Ray and ion irradiation, high / ultra-high vacuum, and liquid nitrogen cooling, to simulate and study the chemistry and dehydration kinetics occurring on Ceres’ surface. Chemical and physical change are then related to optical signatures for use in astronomical observation.
In particular, we analyzed the presence and stability of hydrated and anhydrous magnesium sulfates and sodium carbonates already identified on Ceres’ surface, under experimental Ceres-like cryogenic, low pressure conditions. Our results show that dehydration alone cannot account for the systematic reflectance variation observed across Cerealia facula. Instead, we identified progressively increasing UV absorption with vacuum exposure from the formation of defects within both the hydrated and anhydrous materials which likely contributes to Ceres’s unusual, near-UV spectra. From an isothermal Avrami-model of dehydration, we also determined the lifetime and dehydration pathway for the hydrated materials across different temperature regimes. Additionally, we investigated potential darkening or brightening of carbonate salts by interaction with solar soft X-rays and ions, since space weathering by solar ions can darken silicate materials as seen on the Moon and other mafic-mineral-containing asteroids. We observed visible darkening and the development of near-UV and visible spectral absorption features by Al K-alpha X rays, likely due to the formation of color-centers, and overall darkening across the same spectral range by 4 keV He ions. The magnitude of the darkening can potentially be utilized as a geologic chronometer for Ceres’ bright regions, and our results indicate that Ceres is (or has recently been) geologically active.
