PHYSICAL PROPERTIES OF LUNAR IMPACT CRATER EJECTA USING 70-CM RADAR OBSERVATIONS

Earth-based radar observations of the Moon provide a means of probing the physical properties of the lunar regolith. Radar observations at 70 cm can be used in conjunction with shorter-wavelength radar, thermal infrared, and orbital multispectral observations to develop a picture of the physical and chemical structure of the regolith to several metersÕ depth. In this work, we use 70-cm wavelength radar data to examine the physical properties and spatial distribution of ejecta around nearside impact craters with radar-dark haloes. Numerous craters with haloes characterized by low 70-cm radar return occur in the maria and highlands. These haloes extend 1-2 crater diameters from the rim, outboard of a zone of radar-bright rough ejecta that varies in width between craters; in some instances the radar-dark halo reaches the rim of the crater. We have investigated ~25 such craters using both previous and newly acquired radar observations at 70 cm wavelength, and have compared data with Clementine, Lunar Orbiter, and Apollo Panoramic Camera observations in order to distinguish between the following possible explanations for the presence of the dark haloes. 1) The low-return areas may indicate the presence of a regolith component with high loss tangent relative to the surroundings. For the mare craters, this would imply a layer of higher-TiO2 basalt within the underlying lava flow units. For the highland craters, this would require excavation of mare basalt and its incorporation into the terra regolith; the mixed deposit would have a lower total radar echo than pure highlands material. 2) The low-return haloes may result from the presence of a relatively block-free mantling deposit produced by the impact process. The second model has an advantage in that it does not require fortuitous sub-surface deposits of high-loss materials in the target area. Previously documented thermal infrared observations of craters support the mantling deposit hypothesis. We present results indicating that physical (i.e., block size) variations are likely responsible for the low radar return at 70 cm. Further quantification of these block size variations has application to thermal infrared observations of craters on Mars, and potentially for interpretation of Messenger data for Mercury.