Surface Morphology of Domes in the Marius Hills and Mons Rümker Regions of the Moon from Earth-Based Radar Data

Clusters of domes are a relatively rare landform within the lunar maria. While they are spectroscopically similar to mare basalts emplaced by high-volume, low-viscosity eruptions, the large-scale morphology of the domes is more consistent with slower emplacement and/or more viscous magma. We use new Earth-based radar data, at 12.6 and 70 cm wavelengths, to study the decimeter- to meter-scale “roughness”, interpreted as rock abundance at the surface and buried within the probing range of the radar signal, of domes in the Marius Hills and Mons Rümker regions. The circular polarization ratio (CPR) is well calibrated for both lunar datasets and can be compared with data for terrestrial volcanic and impact features. Domes in the Marius Hills have high 12.6-cm and 70-cm CPR values, similar to those of near-rim ejecta from young lunar craters like Tycho, large ejecta fragments at Meteor Crater, and to blocky lavas such as SP flow in Arizona. The Marius domes have high 70-cm backscatter relative to the surrounding maria, while at 12.6 cm there is little echo contrast. We infer that a rugged flow surface morphology, associated with the original dome-forming eruption rather than post-emplacement modification, exists beneath a thin regolith. The blocky structure of the dome-forming lava must arise from some combination of compositional change, effusion rate, and/or cooling effects not typically observed in basaltic landforms on Earth. The 80-km dome complex Mons Rümker has very different radar properties. Most of this complex has low 12.6-cm backscatter and CPR relative to nearby mare terrain. At 70-cm wavelength, most of the complex is radar-dark, but there is a radar-bright lobate feature that extends eastward from the center. There is no evidence for high-titanium basalts that could explain such low radar returns. Much of Mons Rümker is likely mantled by fine-grained pyroclastic material.