Images of surface volatiles in Mercury's polar craters acquired by the MESSENGER spacecraft

Images acquired by NASA's MESSENGER spacecraft haverevealed the morphology of frozen volatiles in Mercury's permanentlyshadowed polar craters and provide insight into the mode of emplacementand evolution of the polar deposits. The images show extensive,spatially continuous regions with distinctive reflectance properties. Asite within Prokofiev crater identified as containing widespread surfacewater ice exhibits a cratered texture that resembles the neighboringsunlit surface except for its uniformly higher reflectance, indicatingthat the surficial ice was emplaced after formation of the underlyingcraters. In areas where water ice is inferred to be present but coveredby a thin layer of dark, organic-rich volatile material, regions withuniformly lower reflectance extend to the edges of the shadowed areasand terminate with sharp boundaries. The sharp boundaries indicatethat the volatile deposits at Mercury's poles are geologically young,relative to the time scale for lateral mixing by impacts, and either arerestored at the surface through an ongoing process or were deliveredto the planet recently.