|Introduction: Martian and lunar pits with overhanging ledges might be connected to caves or lava tubes, and these caves could be used as safe havens for astronauts as protection from high radiation Solar events. Terrestrial pits found in volcanic terrains can form through several scenarios and still appear morphologically similar to the Martian and lunar pits. However, lava flow inflation (among other methods) also form pits with overhanging ledges that appear to connect to subsurface void space. Yet, field observations show that inflation pits do not connect to caves.
Analogs: We have conducted field work in basaltic environments in Hawaii, New Mexico, and Idaho and observed pits at each site. Not every pit we have observed leads to an extensive, interconnected system of tubes or void space. For example, within the McCartys lava flow circular pits form within the inflation plateaus and have overhanging ledges, rubbly bottoms, but drained-out tube systems are not present. In other terrains (e.g., Kilauea Southwest Rift Zone (SWRZ), HI; Cerro Rendija, NM), even though a lava tube is present beneath the surface, access to the tube has been blocked by debris. Ideal field sites (e.g. Mauna Loa SWRZ) exhibit visible layering in the upper walls and an accessible tube system.
Exploration: The discovery of subsurface voids on both the Moon and Mars could lead to potentially ground breaking missions. LiDAR can provide precise measurements of pit morphology and Ground Penetrating Radar (GPR) can provide reconnaissance of the subsurface around the pit. NASA has funded a study of volcanic pits via the SSERVI RIS4E team over the next 5 years to investigate this issue. Prior to any mission (human or robotic), it will be important to understand the geologic context of these pits. Differentiating tube or rille-fed from sheet-fed flows is crucial for developing informed predictions of which pits might be linked to tubes or caves. Furthermore, assessing the surface and subsurface expressions of pits and caves (lava tubes) is critical for future Human Exploration Operations Mission Directorate (HEOMD) strategies that might include these geologic features for radiation protection.