Mercury: Radar images of the equatorial and midlatitude zones

Radar imaging results for Mercury's non-polar regions are presented. The dual-polarization, delay-Doppler images were obtained from several years of observations with the upgraded Arecibo S-band (lambda 12.6-cm) radar telescope. The images are dominated by radar-bright features associated with fresh impact craters. As was found from earlier Goldstone-VLA and pre-upgrade Arecibo imaging, three of the most prominent crater features are located in the Mariner-unimaged hemisphere. These are: "A;'' an 85-km-diameter crater (348 degrees W, 34 degrees S) whose radar ray system may be the most spectacular in the Solar System; "B," a 95-km-diameter crater (343 degrees W, 58 degrees N) with a very bright halo but less distinct ray system; and "C," an irregular feature with bright ejecta and rays distributed asymmetrically about a 125-km source crater (246 degrees W, 11 degrees N). Due south of "C" lies a "ghost" feature (242 degrees W, 27 degrees S) that resembles "A'' but is much fainter. An even fainter such feature is associated with Bartok Crater. These may be two of the best mercurian examples of large ejecta/ray systems observed in an intermediate state of degradation. Virtually all of the bright rayed craters in the Mariner 10 images show radar rays and/or bright rim rings, with radar rays being less common than optical rays. Radar-bright craters are particularly common in the H-7 quadrangle. Some diffuse radar albedo variations are seen that have no obvious association with impact ejecta. In particular, some smooth plains regions such as the circum-Caloris plains in Tir, Budh, and Sobkou Planitiae and the interiors of Tolstoj and "Skinakas" basins show high depolarized brightness relative to their surroundings, which is the reverse of the mare/highlands contrast seen in lunar radar images. Caloris Basin, on the other hand, appears dark and featureless in the images. (c) 2006 Elsevier Inc. All rights reserved.