Comparison of areas in shadow from imaging and altimetry in the north polar region of Mercury and implications for polar ice deposits

Abstract

Earth-based radar observations and results from the MESSENGER mission have provided strong evidence that permanently shadowed regions near Mercury's poles host deposits of water ice. MESSENGER's complete orbital image and topographic datasets enable Mercury's surface to be observed and modeled under an extensive range of illumination conditions. The shadowed regions of Mercury's north polar region from 65°N to 90°N were mapped by analyzing Mercury Dual Imaging System (MDIS) images and by modeling illumination with Mercury Laser Altimeter (MLA) topographic data. The two independent methods produced strong agreement in identifying shadowed areas. All large radar-bright deposits, those hosted within impact craters ≥6 km in diameter, collocate with regions of shadow identified by both methods. However, only ∼46% of the persistently shadowed areas determined from images and ∼43% of the permanently shadowed areas derived from altimetry host radar-bright materials. Some sizable regions of shadow that do not host radar-bright deposits experience thermal conditions similar to those that do. The shadowed craters that lack radar-bright materials show a relation with longitude that is not related to the thermal environment, suggesting that the Earth-based radar observations of these locations may have been limited by viewing geometry, but it is also possible that water ice in these locations is insulated by anomalously thick lag deposits or that these shadowed regions do not host water ice.

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

“…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.…”

A whole new Mercury: MESSENGER reveals a dynamic planet at the last frontier of the inner solar system

“…Topography of part of Mercury's northern hemisphere (30°N to 90°N) from the Mercury Laser Altimeter, underlain by a map of volcanic plains (darker hues for any given color), showing locations of hollows (red [ Thomas et al , ]), and craters greater than 10 km in diameter for which at least part of the interior is both in permanent shadow and is radar bright (white open circles [ Chabot et al , ; Deutsch et al ., ]). Lambert azimuthal equal area projection centered on the North pole, with grid lines 10° in latitude and 15° in longitude.…”

How thick are Mercury’s polar water ice deposits?

“…• (Deutsch et al 2016). Results from automated crater detection algorithms using only DEMs and not imagery have been reported in the past for crater finding on both the Moon (Luo et al 2013;Xie et al 2013;Li et al 2015) and Mars (Bue & Stepinski 2007;Stepinski et al 2009;Salamuniccar & Loncaric 2010;Di et al 2014), but not yet Mercury.…”

“…• boundary, all of the craters in Table 1 are also present in the catalogue of Deutsch et al (2016).…”

“…These pixels slightly oversample the instrumental resolution of ∼ 1.5 km, and there may be systematic location mismatches on the order of ∼ 2 km between this radar grid and the MLA DEM (Harmon et al 2011;Chabot et al 2013;Deutsch et al 2016). Root mean square measurement uncertainties on the individual pixel σSC values are ∼ 0.014 and a threshold of 0.1 was adopted by default to separate radar-bright (σSC > 0.1) pixels from radar-dark ones.…”