High-resolution radar imaging of Mercury's north pole

The recently upgraded Arecibo S-band (lambda 12.6-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, Nature 369, 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 degreesN 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 hi. 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 (mu (c) = 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, Icarus 141, 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. (C) 2001 Academic Press.