Age constraints of Mercury's polar deposits suggest recent delivery of ice

Abstract

Surface ice at the poles of Mercury appears as several-m-thick deposits that are composed of nearly pure water. We provide new age estimates of the surfaces of Mercury's polar deposits from combined analyses of Poisson statistics and direct observations of crater densities within permanently shadowed, radar-bright regions imaged by the MESSENGER spacecraft. These age estimates conservatively suggest that ice was delivered to Mercury within the last ∼330 Myr. The geologically young ages suggest that the surfaces have been recently refreshed, and this may be accomplished by the delivery of ice in a young impactor or impactors. A single, recent impactor is more consistent with the relative purity of the ice, as suggested by the Earth-based radar observations. In contrast to ice on Mercury, observations of the lunar poles are suggestive of a highly patchy distribution of surface frost. The patchiness of lunar polar deposits is consistent with long exposure times to the space weathering environment. Given enough time, the polar deposits on Mercury may age into a more heterogeneous spatial distribution, similar to that on the Moon.