Impact‐induced thermal effects in the lunar and Mercurian regoliths

Abstract

While the Moon and Mercury are similar objects in a geomorphologic sense, they also possess important differences, particularly in the context of small‐scale impact phenomena. Not only is the surface of Mercury notably hotter than that of the Moon, but the impact flux is also more intense at Mercury due to higher impact velocities and a greater spatial density of micrometeoroids. By extrapolating the terrestrial micrometeoroid flux to the Moon and Mercury, it is found that the impact rate at Mercury is 5.5 times greater and the mean impact velocity more than 60 percent higher than at the Moon. A model of impact melting and vaporization applied to the lunar and mercurian environments indicates that almost 14 times more impact melt and over 20 times more vapor are generated per unit time in the mercurian regolith. Although the surface temperature plays a role in determining the melt and vapor volumes, the difference in impact velocity appears to be markedly more important. “Average” craters formed by identical projectiles under the two velocity distributions on the two planets should be very similar in size, because differences in the kinetic energy of the “average” impactors will be offset by the differences in gravitational acceleration. As a result, variations in regolith mixing should depend more on the actual impact rates than on disparities in excavated volumes. Each “average” impact on Mercury, however, will produce more than twice as much impact melt as its lunar counterpart, introducing more glass into the mercurian regolith. Although substantial quantities of impact vapor are produced on both planets, mixing of the regolith should occur quickly enough in each case to spread the deposited vapor over large surface areas; the resulting vapor coatings should be negligibly small for most purposes. A much larger agglutinate population should exist on Mercury, and individual agglutinates should be similar to those of the lunar highlands. Reflectance spectra and visual albedos indicate that the mercurian crust possesses relatively small quantities of Fe, perhaps similar to those of Apollo 16 light‐ and dark‐matrix breccias. Laboratory studies have demonstrated that, as the magnetic fractions (which are correlated with agglutinate abundances) of soils derived from these breccias increase, the differences in 0.565‐mm (visible light) reflectivity between the soils diminish. Should the regional variations in Fe‐content of the mercurian crust be similar to those of these Apollo 16 samples, then the intense agglutination environment could account for the lack of optical contrast across the mercurian surface. At the same time, the very large fraction of impact glass in the regolith would imply that most pyroxenes have been fused and incorporated into these glasses, seriously attenuating the Fe 2+crystal‐field absorption near 0.9‐mm. Supporting this picture are the most recently published spectral observations of Mercury, which provide little or no evidence of such an absorption feature.