Evidence for Fine-Grained Material at Lunar Red Spots: Insights from Thermal Infrared and Radar Data Sets

Lunar red spots, mostly found in the nearside maria, are small volcanic features that are proposed to be the result of non-mare volcanism. Studies have shown that a number of red spots are highly silicic, and are spectrally distinct from both highlands and mare compositions. In this work, we use data from LRO Diviner, Mini-RF, and Arecibo to investigate the material properties of a number of red spots. We create updated albedo maps using Diviner daytime solar reflectance data to use as an input to our thermophysical model, and calculate the rock abundance (RA) and H-parameter values that best fit Diviner nighttime thermal infrared radiance measurements. H-parameter can be considered analogous to the thermal inertia of the regolith, with high H-parameter corresponding to low thermal inertia. We find that the red spots generally have low RA, and do not have uniform H-parameter but contain localized regions of high H-parameter. We additionally compare our Diviner results with radar datasets from LRO Mini-RF and Arecibo, finding that the red spots have low circular polarization ratio (CPR) in many of the same locations that show low RA and high H-parameter. Low RA, high H-parameter, and low CPR indicates a relative lack of rocks larger than ~10 cm, which is consistent with previous findings of a mantling of fine-grained pyroclastic material at a number of the red spots. Areas with high H-parameter but that do not show clear signs of pyroclastics in other datasets may be evidence of previously undiscovered pyroclastics, or could be due to the unique physical properties (e.g., porosity, rock strength/breakdown resistance) of the silicic rocks that make up the red spots.