Near-surface Vertical Structure of Lunar Volcanic Terrains from Radar and Infrared Data

Introduction: Lunar volcanism creates a wide variety of features such as domes, rilles, and large pyroclastic deposits. Mapping the subsurface column structure of the upper part of these deposits can provide key information about how they were formed and how the surface regolith developed through subsequent weathering. In addition, many possible lunar landing sites (e.g., the Constellation sites) are in volcanic areas, and it is critical to determine the surface and subsurface rock populations prior to landing, roving, or remote sampling. Data collected by Lunar Reconnaissance Orbiter (LRO) enable a new comparison of radar and thermal infrared data at a spatial scale (tens of meters) relevant both to understanding local geology and analyses of human and robotic landing sites. Prior radar data have revealed buried flows and rocks within some pyroclastics deposits (e.g., Aristarchus) [1], while other deposits have radar polarimetry values that suggest very thick mounds of fine (centimeter-or-less sized) material [2]. Thermal infrared data also reveal changes in surface properties across large pyroclastic deposits [3]. Our goal is to use data from multiple wavelength regions to derive vertical structure maps that provide an improved estimate of the thickness and degree of regolith mixing in different types of volcanic settings. In cases where the thermal data reveal differences in surface structure, we can also use thermal models to investigate the burial depth of the rocks sensed by the LRO Diviner radiometer [4,5]. Measuring the subsurface at multiple depths: A multi-wavelength approach provides information about the vertical structure of the upper surface. Radar can penetrate up to ~10 times the wavelength (depending on the dielectric properties) and is sensitive to buried rocks, while thermal infrared data probes the upper few centimeters of the surface where embedded rocks influence the thermal signature. Radar is sensitive to rocks with sizes at least 10-20% of the radar wavelength, while thermal infrared data is sensitive to rocks with a size and burial depth that can influence the surface diurnal heating. We use radar data at 70 and 12.6 cm wavelengths from LRO and ground-based sources, thermal infrared cooling curves and derived products (e.g., rock abundance and regolith temperature) from LRO Diviner, and optical imaging, to map changes in the size, depth, and abundance of rocks across volcanic terrains. Aristarchus (large pyroclastic deposit): Wellstudied Aristarchus is the largest, and possibly thickest, pyroclastic deposit, and is often considered a leading landing site for future missions. Ground-based radar images acquired at 70-cm wavelength show an area of increased brightness that likely corresponds to lava flows that have been buried by subsequent pyroclastic deposits [1]. Shorter wavelength S-band (12.6 cm) data only shows a small portion of these flows. If the mantling pyroclastics above the flows are thin, small impacts are more likely to excavate buried rocks and alter the thermal signature compared to surrounding thick pyroclastics. However, the buried flows are not visible in Diviner rock abundance or regolith temperature data [3], and cooling curves do not reveal temperature differences between the units. These buried flows are therefore likely buried several centimeters to several tens of centimeters at their shallowest depth of burial. Tranquillitatis Domes (small pyroclastic deposits; hollow terrain): Small domes are particularly interesting due to their wide range of surface types. The Cauchy 5 dome in Mare Tranquillitatis (Fig. 1) has been shown to have low S-band radar circular polarization ratio (CPR) values similar to those of large pyroclastics like Aristarchus. Optical images reveal that the dome has an unusual “lunar hollows” texture with mul-