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LROC NAC images reveal over 160 pits, with diameters greater than 5 m, in impact melt deposits of 23 Copernican and 2 Eratosthenian aged craters [1]. A pit is a negative relief feature (NRF) characterized by vertical walls, a flat or slightly concave floor, and the majority of the depth being contributed by vertical walls, rather than sloped floors. Pits are often round or elliptical, and can often be distinguished from impact craters by lack of ejecta, and usually having an inward-sloping, rather than raised, rim. In some cases pits are coincident with, or near linear NRFs (interpreted as fractures). We interpret the impact melt deposit pits to be collapses into subsurface voids.
The median pit diameter is 15 m, with the maximum pit diameter being 100 m. Impact craters found to contain pits occur almost exclusively in highland material, and the diameters of the host melt ponds range from 1-50 km. Typically craters with pits have only 1 to 4 pits identified thus far; with the exception of King, Copernicus, and five other craters that each have many pits. The largest concentration of pits is in the Al-tusi melt pond on the north flank of King Crater, with 50 pits. This deposit also contains a large number of fractures, often associated with pits, potentially caused by either extension or collapse of subsurface voids. The population of pits in the Al-tusi impact melt deposit is documented in [2], but it is an anomaly among the craters with pits, both by having nearly three times as many pits as any other crater, and by the fact that it is an external melt pond. All other impact melt pits have been found in impact melt deposits within the host crater. This abstract will focus on four typical craters with pits, covering a range of NRF densities.
Copernicus: One of the most visible craters (93 km diameter) on the near side (10°N, 20°W) with a distinctive central peak and abundant impact melt deposits on the crater floor. Currently only ~25% of the floor has been searched in NAC images, and 20 pits were found in two main groups. In the far east of the melt pond, there are six 10-20 m diameter pits, with no clear relation to any other features in the area. There are almost no other NRFs in this part of the impact melt deposit. In the northwest quadrant of the crater, there are numerous ~40 m wide sinuous depressions, and at least six 300-800 m wide NRFs that, while owing their depths primarily to concave floors rather than vertical walls, are otherwise similar to pits. NRFs of this size have not been seen in any other crater. There are also 14 smaller pits, often aligned within the sinuous depressions possibly indicating collapse into a linear tube (skylights).
Lalande: A 23 km crater on the eastern edge of Mare Procellarum (4.4°S, 8.5°W) with a small melt deposit. Most of the floor is covered with slumped wall material, but there is a 8 km diameter lumpy and fractured melt pond deposit, ~80% of which has been searched revealing 12 pits, generally distributed near the edge of the melt deposit.
Das: A 38 km crater in the farside highlands (26.6°S, 136.8°W) with interior melt distributed between two main ponds. There are very few fractures compared to most melt ponds, and those that do exist are smaller than typical. In both ponds, the pits (three in the east pond, a single small cluster in the west) are distributed near the pond boundaries with the crater wall.
Stevinus: A 70 km crater south of Mare Fecunditatis (32.4°S, 54.7°E) with a relatively smooth floor (no fractures). Only 20% of the floor has been searched so far, and already fifteen pits have been identified. Twelve of the pits occur in a group on the eastern side of the melt pond, with the other three scattered at other points around the pond.
The void spaces that become pits may be formed by flow within an impact melt deposit after a solid crust has formed. This flow could be caused by isostatic adjustment of the floor beneath the melt, localized drainage into porous ejecta, or slumping of crater walls [2]. In two of the craters described above, and several not described here, pits primarily occur near the margins of the melt deposits. A possible mechanism for this is central cooling stresses resulting in extension and weakening of the crust near the melt pond margins. The widespread nature of the pits identified thus far show that impact melt deposits evolve over significant periods of time after the parent impact event. Pits, in some cases, may offer entrances to extant subsurface voids that present unique geologic environments and engineering opportunities for future lunar explorers.
References:
[1] Wagner, R.V. et al. (2012) 43rd LPSC, #2266
[2] Ashley, J.W. et al. (2012) JGR, in revision
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