THE WESTERN-HEMISPHERE OF VENUS - 3.5 CM DUAL CIRCULAR-POLARIZATION RADAR IMAGES

We present new dual circular-polarization radar maps of the western he misphere of Venus. The results are from a 1993 experiment imaging Venu s with 3.5 cm radar. Continuous-wave right circularly polarized flux w as transmitted toward Venus from the 70 m Deep Space Network antenna i n Goldstone, California, The echo was received in both the same sense (SS) and the opposite sense (OS) of circular polarization at the Very Large Array in New Mexico, By spatially reconstructing the echo with t he interferometer er, maps of Venusian radar albedo were made for each of two days of observation in both OS (echo principally due to specul ar reflection) and SS (diffuse echo) channels, On both days, the sub-e arth longitude was near 300 E. The SS maps are dominated by a signific ant component of diffuse backscatter from the 285 E longitude highland s: Beta, Phoebe, and Themis Regiones. Beta Regio includes radar-anomal ous regions with high reflectivity and low emissivity, The nature of t hese altitude-related electrical properties on Venus is one of the out standing surface process questions that remain after Magellan. Our exp eriment adds the first full-disk polarization ratio (mu(c)) maps to th e discussion, The data show that different geology determines differen t radar scattering properties within Beta. Diffuse scattering is very important in Beta, and may be due to either surface or volume scatteri ng. We find a strong correlation of the SS albedo sigma(ss) with altit ude R-p (km) in Beta, sigma(ss) proportional to 0.3R(p). Also, sigma(o s) proportional to 0.7R(p). The onset of this relationship is at the R -p similar to 6054 km planetary radius contour, The nature and morphol ogy of the highland radar anomalies in Beta is consistent with a diffu se scattering mechanism. In Beta Regio we find mu(c) > 0.5 in general, with mu(c) as high as 0.8 between Rhea and Theia Montes, to the west of Devana Chasma, These values are compatible with measurements of blo cky terrestrial lava flows if surface scattering dominates, If volume scattering Is important, the high RCP cross-sections may indicate an i mportant decrease in embedded scatterer size with altitude, which coul d be related to enhanced weathering. (C) 1997 Academic Press.