An exceptional grouping of lunar highland smooth plains: Geography, morphology, and possible origins

An exceptional deposit covering an area of ∼7700 km2, displaying morphology indicative of an originally fluid material, occurs near 42.2°N, 167.4°E on the lunar farside. The material occurs as smooth, flat deposits (here termed “ponds”) on the bottoms of many craters and in other topographic depressions, as well as a veneer across the majority of the region. Within this area, the ponded deposits and widespread-veneer have an estimated volume of ∼8 km3; the veneer constitutes the great majority of this volume. This material appears to have flowed downslope across the surface, collecting in flat-surface accumulations. The surfaces of the ponds and veneer are only lightly cratered, indicating a young (i.e., late Copernican) age. Four possible modes of origin are investigated: basin ejecta, pyroclastic volcanism, effusive volcanism, and ballistically emplaced impact-melt. Volcanism and basin ejecta appear to be inconsistent with the observed morphology: an implausible number of vents are required for volcanism and the morphological properties do not resemble basin ejecta. We suggest that ballistically emplaced impact melt is most consistent with the observations. Possible source craters for impact melt, based on minimum required size (>20 km diameter) and age, are at least 250 km distant and cannot be definitively tied to the pond deposits. This discovery places important new constraints on our knowledge of the distribution of impact melt relative to the parent crater.