Radar remote sensing of pyroclastic deposits in the southern Mare Serenitatis and Mare Vaporum regions of the Moon

We use polarimetric radar observations to study the distribution, depth, and embedded rock abundance of nearside lunar pyroclastic deposits. Radar images were obtained for Mare Vaporum and the southern half of Mare Serenitatis; the imaged areas contain the large Rima Bode, Mare Vaporum, Sulpicius Gallus, and Taurus-Littrow pyroclastic deposits. Potential pyroclastic deposits at Rima Hyginus crater, the Tacquet Formation, and a dome in Mare Vaporum are also included. Data were acquired at S band (12.6 cm wavelength) using Arecibo Observatory and the Green Bank Telescope in a bistatic configuration. The S band images have resolutions between 20 and 100 m/pixel. The pyroclastic deposits appear dark to the radar and have low circular polarization ratios at S band wavelengths because they are smooth, easily penetrable by radar waves, and generally contain few embedded blocks. Changes in circular polarization ratio (CPR) across some of the pyroclastic deposits show areas with increased rock abundance as well as deposits that are shallower. Radar backscatter and CPR maps are used to identify fine-grained mantling deposits in cases where optical and near-infrared data are ambiguous about the presence of pyroclastics. The Tacquet Formation in southern Serenitatis, areas near Hyginus crater, and a dome in Mare Vaporum have lower-backscatter cross sections than would be expected for mare basalts of similar estimated titanium content. Combined with very low CPR values, this is strong evidence that these areas are covered in fine-grained pyroclastic mantling material.