Mini-RF and Arecibo Observatory Bistatic Observations of the Moon

Abstract:

The Mini-RF instrument aboard NASA’s Lunar Reconnaissance Orbiter (LRO) is a hybrid dual-polarized synthetic aperture radar (SAR) and has been operating in concert with the Arecibo Observatory since 2012 to collect bistatic radar data of the Moon. Approximately 28 bistatic observations, covering a variety of terrains on the lunar nearside, have been made using this mode of operation. Laboratory data and analog experiments, at optical wavelengths, have shown that the scattering properties of lunar materials (e.g., the roughness) can be sensitive to variations in bistatic angle. This sensitivity manifests as an opposition effect and likely involves contributions from shadow hiding at low angles and coherent backscatter near 0°. Analog experiments and theoretical work have shown that water ice is also sensitive to variations in bistatic angle. Differences in the character of the opposition response of these materials offer an opportunity to differentiate between them, an issue that has been problematic for previous radar studies of the Moon.

Observations that include mare materials, highland materials, and pyroclastic deposits have not shown an opposition response over for bistatic angles of ~0.1° to 30°. Observations of the ejecta blankets of young, fresh craters have shown an opposition response for the same range of bistatic angles. The character of the response varies for each crater and is likely related to differences in target properties of the surface and exposure age of the blanket. Observations of portions of the floor of the south polar crater Cabeus have also been acquired for the same range of bistatic angles. The character of the radar response from the crater, as a function of bistatic angle, appears unique with respect to all other lunar terrains observed.