Earth-based observations of radar-dark crater haloes on the Moon: Implications for regolith properties

Earth-based radar can be used in conjunction with other data sets collected remotely or in situ to characterize the physical and chemical properties of the lunar regolith. We use previously existing and newly acquired Earth-based radar measurements at 70-cm wavelength to investigate the properties of distinctive low-return haloes surrounding 39 nearside impact craters. These haloes are distinct from the well-known optically dark haloes associated with volcanic craters or with impacts into cryptomare deposits. We examine two possible mechanisms for formation of radar-dark haloes: (1) excavation of high-TiO₂, high-loss tangent basalt and (2) deposition of a mantling layer produced in the impact process that is depleted in meter-sized blocks. Comparison of the radar data with Apollo and Lunar Orbiter images indicates a general spatial correspondence between radar-dark crater haloes and radial ridged ejecta facies distal to the radar-bright crater rims. Comparison with Clementine UV/VIS multispectral data indicates no apparent spatial correlation between regions of enhanced TiO₂ content and radar-dark haloes; though variations in TiO₂ content may play a role for some mare craters, they are insufficient to account for the observed offsets in radar echo power. We conclude that the radar-dark haloes result from a block-poor ejecta deposit that disappears over time with meteoroid bombardment. A more detailed and comprehensive analysis of ejecta degradation processes and rates following acquisition of additional high-resolution radar data could lead to a refinement of the lunar geologic timescale.