TEMPORAL RELATIONSHIPS BETWEEN MARE FLOWS IN STEEP-WALLED LUNAR PITS

Mare volcanism on the Moon has been the architect of many fascinating features: expansive flows that have filled impact basins, sinuous rilles that meander and cut through surface deposits, and fields of glass beads, to name a few. The partially collapsed ceilings of sublunarean voids, of unknown extent, might be the most intriguing, considering their potential for future human exploration and habitation.

In order to begin to understand these lunar features, we are examining steep-walled lunar pits, possibly associated with lava tubes, which provide a rare opportunity to view lunar stratigraphy in cross-section thanks to oblique Lunar Reconnaissance Orbiter NAC imagery. Many pits show clear evidence of thin light and dark layering, which we interpret as individual mare flows and unconsolidated regolith, respectively. Detailed mapping of the units was performed at three steep walled locations:

• Tranquillitatis pit
• Mare Ingenii pit
• Marius Hills pit

Estimating the frequency of lunar mare basalt flows has been challenging as very few exposed and undisturbed cross-sections of lunar stratigraphy have been identified. Crater-counting techniques have been used in the past to estimate the ages of surface exposures when no other information is available, but these pits provide another opportunity. Utilizing regolith layer thicknesses in combination with modern regolith dust accumulation rates from Apollo data, we have estimated the duration of time between flow events. In the locations we examined, it appears that void formation and the last volcanic deposit are separated by up to tens of millions of years. Flows are observed to have been discrete events in a discontinuous eruptive period with meters of regolith accumulation occurring between each flow. And while regolith production is a much more complicated process than simply deposition of material at a constant rate; this method provides insightful first-order time relationship estimates that indicate episodic mare volcanism at these locations.