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 sou th poles are a result of volume scattering from elemental sulfur. Furt her we suggest that the high average index of refraction for surface m aterials and notable microwave transparency of the Mercurian regolith can be explained by the presence of iron sulfides (troilite, pyrrhotit e) and low-iron sulfides (daubreelite and sphalerite). Sulfides deposi ted throughout the regolith are a result of fumarolic and magmatic eve nts following initial accretion of meteoroids rich in troilite and pyr rhotite to the planet. Calculations are made illustrating how solar ab undances of sulfur volatilized from such a surface can provide an adeq uate supply over the history of the planet to be cold-trapped at high latitudes, be covered partially with dust, and be seen as bright regio ns in radar images. To test our hypothesis, we suggest a series of obs ervations in the ultraviolet to search for the SI multiplet centered a t 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 sulf ureous molecules like H2S and SO2. (C) 1995 Academic Press, Inc