SULFUR AT MERCURY, ELEMENTAL AT THE POLES AND SULFIDES IN THE REGOLITH

We propose that the bright radar spots seen at Mercury's north and south poles are a result of volume scattering from elemental sulfur. Further we suggest that the high average index of refraction for surface materials and notable microwave transparency of the Mercurian regolith can be explained by the presence of iron sulfides (troilite, pyrrhotite) and low-iron sulfides (daubreelite and sphalerite). Sulfides deposited throughout the regolith are a result of fumarolic and magmatic events following initial accretion of meteoroids rich in troilite and pyrrhotite to the planet. Calculations are made illustrating how solar abundances of sulfur volatilized from such a surface can provide an adequate supply over the history of the planet to be cold-trapped at high latitudes, be covered partially with dust, and be seen as bright regions in radar images. To test our hypothesis, we suggest a series of observations in the ultraviolet to search for the SI multiplet centered at 1814 Angstrom, in the midinfrared to search for emissivity features at 7.7 and 11.8 mu m, and at millimeter wavelengths to search for sulfureous molecules like H2S and SO2. (C) 1995 Academic Press, Inc