High resolution images from NASA’s Lunar Reconnaissance Orbiter (LRO) have revealed the existence of steep-walled pits on a number of locations on the surface of the Moon. These pits are believed to be surface entrances into subterranean voids of yet unknown lateral extent. Caverns on the Moon are particularly attractive for any long-term human presence on since they offer protection from surface hazards such as temperature cycling, radiation and micrometeoroids.
The absence of raised crater rims and ejecta fields rules out the possibility that these pits are created solely by objects impacting onto the lunar surface. Furthermore, the steep wall slopes and the high depth-to-diameter ratios of these pits are inconsistent with an impact origin. Nevertheless, it is possible that the origin of such a pit may have been initiated by an impact event which caused the surface to collapse into an underlying cavern.
Figure 1: Mare Tranquillitatis Pit. The maximum and minimum pit diameters are 100m and 86m respectively, and the maximum depth of the pit floor below the surface is 105m. (Location: 8.34N 33.22E) (A: M126710873R; B: M155016845R; C: M175057326R; D: M152662021R; E: M155023632R; F: M144395745L; Credit: NASA/GSFC/Arizona State University; M.S. Robinson, et al., 2012)
Figure 2: Marius Hills Pit. The maximum and minimum pit diameters are 57m and 48m respectively, and the maximum depth of the pit floor below the surface is 45m. (Location: 14.09N 303.23E) (A: M122584310L; B: M155607349R; C: M137929856R; D: M155614137R; Credit: NASA/GSFC/Arizona State University; M.S. Robinson, et al., 2012)