INTERIOR MORPHOLOGY OF LUNAR PITS

The Lunar Reconnaissance Orbiter Narrow Angle Camera (NAC) has imaged much of the Moon at 0.5-2 m/px since launch in 2009 and collected extensive imagery (sometimes >50 images) at varied lighting and observation angles of some sites of interest. One class of feature that has been targeted extensively is pits, vertical-walled negative relief features first identified on the Moon by Kaguya and hypothesized to be openings into lava tubes or other sublunarian void spaces [1]. NAC images have expanded the number of pits known from 3 to over 300, mostly in impact melt ponds [2]. We have used the extensive multi-angle NAC coverage of several pits in the lunar maria to produce 3D models of sections of their interiors, with mean point spacings of ~5 m [3].

Overhangs: Our models extend 10 m under the east rim of the Mare Tranquillitatis pit (MTP), and 15 m under the southwest rim of the Mare Ingenii pit (MIP). In both cases, the floor slopes downward under the overhang, indicating that debris may not have fully filled in the original void space. The Marius Hills (MHP) and Lacus Mortis pits, by contrast, do not have significant overhangs on the modeled walls and have flat or upward-sloping floors, suggesting a more complete infilling. In the case of MHP, the rim shows no evidence of extensive mass wasting to produce that infill, suggesting that either the original void space was very small or that the as-yet-unimaged west wall should have a significant opening.

Layering: We have measured the 3D locations of apparent morphological layer boundaries within the walls of MIP and MTP. We found good correspondence between layers on east and west walls and found median layer thickness of ~3.7 m in both MTP and MIP [3]. Additionally, MTP has a >8 m deep recess on the west, north, and possibly east walls at 40 m depth, suggesting a strength discontinuity - possibly evidence of a buried pyroclastic or paleoregolith layer.

[1] Haruyama et al. (2009), Geophys. Res. Lett. 36, L21206. doi:10.1029/ 2009GL0406355.

[2] Wagner and Robinson (2014), Icarus, 237C, 52–60. doi:10.1016/j.icarus.2014.04.002.

[3] Wagner and Robinson (2019), 50^th LPSC, #2138.