MERCURY - FULL-DISK RADAR IMAGES AND THE DETECTION AND STABILITY OF ICE AT THE NORTH-POLE

The first full-disk radar images of Mercury were obtained on August 8 and 23, 1991. These images were constructed using the Very Large Array (VLA) in Socorro, New Mexico, to receive and map radar flux at 3.5 cm (X band) which was continuously transmitted from the 70-m Jet Propuls ion Laboratory/Deep Space Network antenna at Goldstone, California. Ap proximately 77% of the surface was imaged, at resolutions as good as 1 50 km. About half of the hemisphere photographed by Mariner 10 was ima ged, as well as most of the hemisphere which has not previously been p hotographed. At the time of the observations, the north pole was visib le, and the feature with the highest same sense (SS) circular reflecti vity in the images is near the nominal polar position. The peak SS ref lectivity of this feature was 7.9%, and the circular polarization rati o throughout much of the feature is > 1. Our best estimate of the size of the feature is that its diameter is less than or similar to 350 km . We interpret the feature to be indicative of the presence of ices be cause of the signal strength and polarization characteristics. The ice s must be very clean and were thus probably deposited in a relatively short time period. The most likely place to find these ices is in any permanently shadowed areas in the polar regions, which would be very, cold. In comparison to absolute reflectivities of other icy bodies and regions in the solar system, the reflectivity of the north polar feat ure is slightly depressed. This is probably because the ices do not fi ll an observational resolution cell (i.e., there is incomplete areal c overage), or are covered by a layer of dust or soil, which absorbs som e of the incoming and outgoing radar energy, or both. A covering layer would also protect any ices from erosion by energetic sources. Other prominent features on the unphotographed hemisphere correspond to posi tions where atmospheric sodium enhancements have been measured from Ea rth. This may indicate that these locations are large basins similar t o the Caloris basin, where an atmospheric potassium enhancement has be en measured, which may be the result of increased degassing in the dis rupted surface and near surface. Direct comparison of the radar echoes from our features with those from the Caloris basin are hard to make, as Caloris was not imaged by us in a favorable geometry. Features on the photographed hemisphere are smaller and mostly associated with cra ters and crater complexes, most notably the Kuiper crater and its envi rons.