Numerical modelling of impact crater formation associated with isolated lunar skylight candidates on lava tubes

Abstract

Skylights are openings on subsurface voids as lava tubes and caves. Recently deep hole structures, possibly skylights, were discovered on lunar photo images by the JAXA SELenological and ENgineering Explorer (SELENE)-Kaguya mission, and successively confirmed by the NASA Lunar Reconnaissance Orbiter (LRO) mission. Vertical hole structures and possibly underlying subsurface voids have high potential as resources for scientific study, and future unmanned and manned activities on the Moon. One mechanism proposed for their formation is impact cratering. The collapse of craters is due to the back spallation phenomena on the rear surface of the lava tube roofs. Previous analysis in this topic was based on small-scales laboratory experiments. These have pointed out that (i) the target thickness-to-crater diameter ratio is 0.7, and (ii) the projectile diameter-to-target thickness ratio is 0.16, at the ballistic limit once extrapolated to planetary conditions.We investigate the impact process that might trigger the formation of lunar skylight candidates by numerically simulate the craters associated to the observed hole structures. The Marius Hills hole (MHh, located in a 26m thick target) was formed by a 4m projectile, which originated an impact crater of 40.0±1.6m in diameter. The Mare Tranquillitatis hole (MTh, located in a 47m thick target) was formed by a 7.2m projectile, which originated an impact crater of 75.6±3.0m in diameter. The target thickness-to-crater diameter ratio is 0.65 and 0.62, respectively for MHh and MTh. These values are smaller than the laboratory experiment ballistic limit ratio (0.7), supporting the hypothesis of collapse at the studied sites. The projectile diameter-to-target thickness ratio is 0.15 for both MHh and MTh, which is slightly smaller than we expected. The discrepancy could be due to the model assumption of vertical impact. We derived via modelling the ballistic limit for planetary scales and found that the target thickness-to-crater diameter ratio is 0.87.We discuss the implications of the proposed scenario for skylight formation in terms of future robotic or human exploration of the Moon, as evaluated in COSPAR and ILEWG.