Surface morphology of domes in the Marius Hills and Mons Rümker regions of the Moon from Earth‐based radar data

Clusters and coalesced complexes of domes represent a relatively rare landform within the extensive, low‐relief, lunar maria. While they are typically 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 to study the decimeter‐ to meter‐scale 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. Domes of the Marius Hills are characterized by high circular polarization ratio (CPR) values at 12.6‐cm and 70‐cm wavelength, similar to the near‐rim ejecta deposits of young lunar craters like Tycho, areas of large ejecta fragments near the rim of Meteor Crater, and blocky lavas such as SP flow in Arizona. We infer that a very rugged flow surface morphology, associated with the original dome‐forming eruption rather than postemplacement modification, exists beneath at most a few meters of regolith. Magmas with greater silica content than sampled lunar basalts are unlikely, so 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 the Earth. Much of the 80‐km diameter Mons Rümker has low backscatter and CPR at 12.6‐cm wavelength relative to nearby mare terrain. At 70‐cm wavelength, the northern and most of the southern parts of the complex are radar dark, but there is a radar bright feature that extends from the central region eastward into Oceanus Procellarum. We conclude that much of Mons Rümker is mantled by rock‐poor, fine‐grained pyroclastic material; the radar bright feature in the 70‐cm map is detectable owing to some combination of greater roughness (like the Marius domes) and thinner mantling cover.