Emplacement mechanism of ponded light plains on the Moon: Insight from topography roughness

Abstract

Light plains on the Moon are distinctive smooth deposits formed by large impact basins. Featuring diverging morphology and frequent spatial association with large secondary craters, light plains were interpreted to be formed either by ground-hugging ejecta flows and/or debris flows triggered by landing of impact ejecta. Light plains located in closed depressions, term ponded light plains (PLPs) here, cover ∼3% of the lunar global surface. They lack diagnostic geomorphology that could be ascribed to secondary impacts or primary ejecta, suggesting a possibly different emplacement mechanism, which has not been investigated before. PLPs feature comparable morphology with both lunar mare that were mainly emplaced by effusive volcanism and large impact melt pools that were deposited by melt-rich ejecta in rims and terraces of large impact basins. For such smooth plain materials, topography roughness at small scales might be an indicative information for their different emplacement styles. This study analyses topography roughness for PLPs and similar-sized melt pools that were formed by the Orientale basin and also coeval mare units on the Moon. We find that mare units have smaller roughness than same-aged melt pools, because large entrained blocks are abundant in melt pools that likely yielded larger viscosity. We further notice that PLPs have a medium topography roughness between coeval mare units and large melt pools, and the topography roughness of PLPs is closer to that of mare units. Correlated observations using high-resolution images suggest that the smoother topography of PLPs than cogenetic melt pools are mainly due to the minor existence of entrained blocks in PLPs. This comparison suggests that while PLPs and melt pools are both basin-related smooth ejecta deposits, PLPs may feature a lower viscosity during the emplacement, and they are most consistent with being melt-rich primary ballistic ejecta that were accumulated in local depressions. Considering that the youngest multiple-ring basin on the Moon, the Orientale basin, is capable of forming numerous PLPs across most parts of the entire Moon, this work predicts that both disrupted and sandwiched melt-rich ejecta deposits should be widespread in the lunar highland crust due to earlier-formed impact basins.