New Illumination and Temperature Constraints of Mercury’s Volatile Polar Deposits

Abstract

Images from the Mercury Dual Imaging System (MDIS) aboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging mission reveal low-reflectance polar deposits that are interpreted to be lag deposits of organic-rich, volatile material. Interpretation of these highest-resolution images of Mercury’s polar deposits has been limited by the available topography models, so local high-resolution (125 m pixel −1 ) digital elevation models (DEMs) were made using a combination of data from the Mercury Laser Altimeter (MLA) and from shape-from-shading techniques using MDIS images. Local DEMs were made for eight of Mercury’s north polar craters; these DEMs were then used to create high-resolution simulated image, illumination, and thermal models. The simulated images reveal that the pixel brightness variations imaged within Mercury’s low-reflectance deposits are consistent with scattered light reflecting off of topography and do not need to be explained by volatile compositional differences as previously suggested. The illumination and thermal models show that these low-reflectance polar deposits extend beyond the permanently shadowed region, more than 1.0 km in some locations, and correspond to a maximum surface temperature of greater than 250 K but less than 350 K. The low-reflectance boundaries of all eight polar deposits studied here show a close correspondence with the surface stability boundary of coronene (C 24 H 12 ). While coronene should only be viewed as a proxy for the myriad volatile compounds that may exist in Mercury’s polar deposits, coronene’s surface stability boundary supports the idea that Mercury’s low-reflectance polar deposits are composed of macromolecular organic compounds, consistent with the hypotheses of exogenous transport and in situ production.

Aspects of thermal modeling using digital terrain models

“…In fact, this is the first time that model temperatures on the complete polar MLA DTM are reported; this is especially of importance for temperatures at latitudes above ≈ 85 • N. In a previous study conducted during the ongoing MESSENGER mission, MLA tracks were still too sparse or they did not exist at these latitudes (Paige et al 2013). Other studies have concentrated on single craters or nonpolar regions (Hamill et al 2020;Bauch et al 2021). Due to the 3 : 2 spin-orbit resonance of Mercury, so-called hot (at 0 • E and 180 • E) and warm (at 90 • E and 270 • E) longitudes emerge (Siegler et al 2013).…”

Surface Roughness Variation Across Polar Ice Deposit Boundaries on Mercury

“…In the construction of these DEMs, Hamill et al (2020) used track bundle-adjustment methods to correct for spacecraft position knowledge and pointing parameters and to mitigate orbital geolocation errors. Specifically, Hamill et al (2020) created two types of topography products for the craters in our study. The first was created using tracks from the MESSENGER MLA.…”

“…T max is a primary factor controlling the long-term stability of surface volatiles, and thus the distribution of polar ice deposits on Mercury (e.g., Paige et al, 2013;Schorghofer & Taylor, 2007;Vasavada et al, 1999). Finally, we analyze surface roughness as a function of water-ice and coronene stability depths, representing the depths these volatiles would be lost at a sublimation rate <1 mm/Gyr, as modeled by Hamill et al (2020) down to 2.5 m below the surface.…”

“…Additionally, Hamill et al. (2020) created high‐resolution digital elevation models (DEMs) for each of these craters that enable this analysis. In the construction of these DEMs, Hamill et al.…”

Water Group Exospheres and Surface Interactions on the Moon, Mercury, and Ceres

“…According to MESSENGER near-infrared reflectance measurements of the surface, the coldest N. Schörghofer et al areas are bright, whereas areas with higher predicted temperatures, where water ice is expected to be short-lived on the very surface but long-lived at shallow depth, are dark . The dark surface deposits may be devolatilized sublimation lag deposits (Neumann et al 2013;Hamill et al 2020) or due to radiation processing of various cold-trapped species (Crites et al 2013;Delitsky et al 2017). Mercury's polar deposits have sharp boundaries, which suggests that the deposits are geologically young or that their boundaries are refreshed by an ongoing process (Chabot et al 2018a).…”