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

Abstract

Images acquired by NASA's MESSENGER spacecraft have revealed the morphology of frozen volatiles in Mercury's permanently shadowed polar craters and provide insight into the mode of emplacement and evolution of the polar deposits. The images show extensive, spatially continuous regions with distinctive reflectance properties. A site within Prokofiev crater identified as containing widespread surface water ice exhibits a cratered texture that resembles the neighboring sunlit surface except for its uniformly higher reflectance, indicating that the surficial ice was emplaced after formation of the underlying craters. In areas where water ice is inferred to be present but covered by a thin layer of dark, organic-rich volatile material, regions with uniformly lower reflectance extend to the edges of the shadowed areas and terminate with sharp boundaries. The sharp boundaries indicate that the volatile deposits at Mercury's poles are geologically young, relative to the time scale for lateral mixing by impacts, and either are restored at the surface through an ongoing process or were delivered to the planet recently.

How thick are Mercury’s polar water ice deposits?

“…More recently, the sensitivity of MDIS images has allowed the imaging of permanently shaded regions using scattered light (Chabot et al 2014). The results are similar to the MLA albedo measurements, albeit at visible wavelengths, with anomalous reflectance regions largely coincident with radar-bright ones.…”

Impact Gardening as a Constraint on the Age, Source, and Evolution of Ice on Mercury and the Moon

“…Many of Mercury's radar‐bright ice deposits are covered by a layer of low‐reflectance material that has been interpreted to be the carbonaceous leftovers of ice that has sublimated or “thermal lag” (Crites et al, ; Delitsky et al, ; Neumann et al, ; Paige et al, ; Syal et al, ). MESSENGER neutron data and thermal models show that these low albedo lag deposits are up to 10–30 cm thick (Lawrence et al, ; Paige et al, ), and images indicate that the low‐reflectance deposits directly overlie ice deposits, terminating sharply at the boundary of radar‐bright regions (Chabot et al, ).…”

“…Some impacts will dig deeper; for example, we predict that there is a 50% chance that ice buried under cm of thermal lag will have been excavated once over 10 Myr. Much as Apollo core 7002‐7009 showed an anomalously young deeply penetrating impact (Nishiizumi et al, ; Figure ), rare, larger impacts on Mercury may penetrate the lag and exhume small amounts of ice onto the surface, causing observed variations in surface brightness among thermal lag regions (Chabot et al, , ; Deutsch et al, ). Quantitatively linking the efficiency of lag‐penetrating impacts and up‐sampling of ice at low model probability and the surface albedo variations awaits a more complete dataset.…”

The permanently shadowed regions of dwarf planet Ceres

“…On Mercury, the cold traps are filled with water ice [Slade et al, 1992;Lawrence et al, 2013;Neumann et al, 2013;Paige et al, 2013;Chabot et al, 2014]. On the Moon, the circumstances are more complex [e.g., Feldman et al, 1998;Zuber et al, 2012;.…”