Modeling the Source of Impact Melt at the Apollo 14-17 Landing Sites

Impact cratering is the predominant geological process affecting the lunar surface. Because of this, many Apollo samples were formed from or modified by impacts, especially samples from missions that did not land on the mare (Apollo 14-17). If we know which craters produced which samples, we could construct a detailed chronology anchored by sample ages and crater counts. However, there is no direct way to correlate the oldest Apollo samples with a specific crater, so the lunar chronology prior to ~3.9 billion years (Ga) is poorly constrained.

Of the suite of lunar rocks returned by the Apollo missions, impact melts are important because the age of a melt should theoretically be the age of the crater that produced it. Because the largest craters, known as basins, produce much more melt and transport much more ejecta than smaller craters, they are theorized to be the source of most melt at the Apollo landing sites. Because many Apollo samples have radiometric ages of ~3.9 Ga, and all basins formed early in lunar history, many samples have been inferred to be sourced from the closest large basins to their collection sites.

The Apollo sites may have also been affected by craters smaller and slightly younger than basins. Deposits from these “Imbrian” craters, the largest of which is Iridum (D=250 km), should be either above or coeval with basin deposits in a stratigraphic sequence. These craters have not been extensively studied with respect to their provenance at the Apollo sites, but they should have emplaced some material at each site.

In this work, the Cratered Terrain Evolution Model (CTEM) is used to model the source of melt at the Apollo 14-17 landing sites. CTEM is a Monte Carlo impact bombardment model that emplaces and tracks melt and ejecta. A regolith mixing algorithm simulates gardening by small impacts via the homogenization of material to a calculated depth. We have enabled craters to be manually emplaced in CTEM with given sizes, locations, and times. We then manually emplaced a plausible sequence of known basins and several Imbrian craters that may have affected the Apollo sites. Material from each of these craters was tracked in a global bombardment simulation.

Preliminary results show that melt in the upper portion of the regolith at each site originates primarily in either large basins or smaller craters local to the region of interest. The local material can be of any age, including from the basin-forming period. There may be a significant amount of Iridum melt at the Apollo 15 site, but Imbrian melt is otherwise sparse. More work is needed to further refine our results, including performing simulations at higher resolutions.