Craters hosting radar‐bright deposits in Mercury's north polar region: Areas of persistent shadow determined from MESSENGER images

Abstract

Radar‐bright features near Mercury's poles were discovered in Earth‐based radar images and proposed to be water ice present in permanently shadowed areas. Images from MESSENGER's one‐year primary orbital mission provide the first nearly complete view of Mercury's north polar region, as well as multiple images of the surface under a range of illumination conditions. We find that radar‐bright features near Mercury's north pole are associated with locations persistently shadowed in MESSENGER images. Within 10° of the pole, almost all craters larger than 10 km in diameter host radar‐bright deposits. There are several craters located near Mercury's north pole with sufficiently large diameters to enable long‐lived water ice to be thermally stable at the surface within regions of permanent shadow. Craters located farther south also host radar‐bright deposits and show a preference for cold‐pole longitudes; thermal models suggest that a thin insulating layer is required to cover these deposits if the radar‐bright material consists predominantly of long‐lived water ice. Many small (<10 km diameter) and low‐latitude (extending southward to 66°N) craters host radar‐bright material, and water ice may not be thermally stable in these craters for ~1 Gy, even beneath an insulating layer. The correlation of radar‐bright features with persistently shadowed areas is consistent with the deposits being composed of water ice, and future thermal modeling of small and low‐latitude craters has the potential to further constrain the nature, source, and timing of emplacement of the radar‐bright material.