An observational comparison of mercury and the moon

Abstract

Observations of the neutral sodium (Na) and neutral potassium (K) atoms in the tenuous atmospheres of the Moon and Mercury have been made, analyzed and interpreted. An atmospheric number density of ∼300 times more Na and ∼100 times more K are observed at Mercury than at the Moon. This is not consistent with a meteoritic source for these constituents unless losses and recycling are very different. Potassium observations in the lunar atmosphere indicate fewer accommodated atoms than expected from thermalization of the extended component through gas-surface interactions. A mechanism for redistributing the thermal and non-thermal populations is presented. Thermal infrared observations between 7.5 and 12.5 μm have been made at both bodies in an attempt to gain understanding of the relationship of the surfaces to the alkali atmospheres. Mercury has been found to have a surface composition at three locations more felsic than that of the lunar south-polar highlands. Spectral indications of high alkali feldspar have been observed. One mafic location on Mercury's surface has been determined. Calculations using a grain-boundary and regolith diffusion model have shown that the differences in abundances of Na and K and the ratios of Na/K at both bodies can be explained by a supply of atoms diffusing upward through the sub-surface materials over geologic time. Enhancements in K seen at the longitude of Caloris and the antipodal point of up to five (5) and four (4) respectively, point to a sub-surface source for at least part of the tenuous alkali atmosphere at Mercury.