Elsevier

Icarus

Volume 149, Issue 1, January 2001, Pages 1-15
Icarus

Regular Article
High-Resolution Radar Imaging of Mercury's North Pole

https://doi.org/10.1006/icar.2000.6544Get rights and content

Abstract

The recently upgraded Arecibo S-band (λ12.6-cm) radar was used to make delay-Doppler images of Mercury's north polar region, where earlier observations had shown strong echoes from putative ice deposits in craters. The image resolution of 1.5–3 km is a substantial improvement over the 15-km resolution of the older Arecibo images (J. K. Harmon et al. 1994, Nature369, 213–215). The new observations confirm all the original polar features and reveal many additional features, including several at latitudes as low as 72–75°N and several from craters less than 10 km in diameter. All of the new features located on the Mariner-imaged side of the planet can be matched with known craters or other shaded areas. We find the north pole to be located 65 km from the original Mariner-based pole and 15 km from the new Mariner-based pole of M. S. Robinson et al. (1999, J. Geophys. Res.104, 30,847–30,852). The improved resolution reveals fine structure in the radar features and their respective host craters, including radar shadowing/highlighting by central peaks and rim walls, rim terracing, and preferential concentration of radar-bright deposits in shaded southern floor areas. The radar features' high brightness, circular polarization inversion (μc=1.25), and confinement to regions permanently shaded from direct sunlight are all consistent with volume scattering from a cold-trapped volatile such as clean water ice. The sizes and locations of most of the features show good agreement with the thermal model of A. R. Vasavada, D. A. Paige, and S. E. Wood (1999, Icarus141, 179–193) for insulated (buried) water ice, although the problems of explaining radar features in small craters and the rapid burial required at lower latitudes suggest that other factors may be suppressing ice loss after emplacement.

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      Furthermore, in some craters, surface volatiles exhibit a cratered texture similar to the sunlit portion of the crater floor, suggesting that the surficial ice was emplaced after the formation of the underlying small craters (Chabot et al., 2014). This is also more consistent with Case I. Finally, regolith gardening models (Crider and Killen, 2005) suggest that the ice deposits were emplaced <50 Myr ago in order to explain the enhanced radar backscatter (Slade et al., 1992; Harmon and Slade, 1992; Butler et al., 1993; Harmon et al., 1994, 2001, 2011; Harmon, 2007). It is possible, however, that there are examples of both Case I and Case II craters within the PSRs.

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