Visible and near-infrared reflectance spectra of thermally processed synthetic sulfides as a potential analog for the hollow forming materials on Mercury

We have obtained reflectance spectra in the visible spectral range of magnesium, calcium and manganese sulfides before and after thermal processing at Mercury dayside temperatures in vacuum conditions. All measurements were made at the Planetary Emissivity Laboratory (PEL) at DLR. These experiments are motivated by the recent findings of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission of hollows on Mercury and the notion that sulfides might be a candidate material for their formation. From these new measurements, we infer that all sulfides studied here display a diagnostic feature at or near 0.6 μm. The spectral contrast of this diagnostic feature can be strongly affected by heating of the samples to Mercury daytime temperatures of approximately 500 °C in a vacuum environment. Both the spectral slope and the color observed before and after thermal processing showed significant changes. The laboratory work also indicates that sulfides can be thermally decomposed at temperature below 500 °C, well below their melting point. Our results are consistent with models explaining hollows on Mercury's surface by the thermal decomposition of sulfides.