Duration: 6 months
Photographs of some lunar pit craters clearly show at least a localized cavern beneath the surface, whose horizontal extent is larger than the diameter of the pit itself. This has raised the exciting possibility that at least some lunar pits are "skylights" into extensive lava tubes. This possibility is plausible, based on extrapolation from terrestrial lava tubes and their skylights, and also on the proximity of some lunar pits to rille features on the Moon's surface; lava tubes may be "roofed-over" rilles. If this is the case, then an extensive lava-tube cave could be accessed through such a pit crater. However, there are also geologically plausible mechanisms for creating much smaller, much more-localized voids, for example melt chambers associated with impact-melt deposits, which may not be much larger in diameter than the pit itself. A mission to explore the interior of a lunar pit crater would be much more interesting and useful, if targeted to a pit whose associated subsurface cavern is large, rather than small. Optical imagery from lunar orbit cannot distinguish whether the void below any given pit is localized, or more extensive, and radar measurements from orbit are tantalizing but not definitive.
However, voids as large as those specified in this Lunar Caves Campaign will produce a "gravity low" signal at the lunar surface above such a void, which can be measured using a sensitive gravimeter. We propose that prospective lunar pits (such as the one in Marius Hills) first be "scouted" using a small, inexpensive lunar-surface rover mission carrying a small gravimeter instrument, which would survey the area around the pit to ascertain the likely geographic extent, size and depth of the associated cavern. This could then be followed-up, for pits with interestingly-large voids, using a (likely rather more expensive and risky) mission to actually drop a rover into the pit, to explore the void from its inside, maximizing scientific return per Euro spent.
