2nd International Planetary Caves Conference (2015) 9021.pdf

UPDATE: THE SEARCH FOR LUNAR PITS. R. V. Wagner and M. S. Robinson, Lunar Reconnaissance Orbiter Camera, School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-3603 ([email protected]).

Introduction: Since the late 19th century scientists speculated if open and accessible lava tubes exist on the Moon [reviewed in 1]. If extant, these sublunarean voids represent a bonanza for science and engineering investigations. The discovery of three vertical-walled pits by the Selene image team [2,3] fueled speculation that they were openings into lava tubes (skylights). Figure 1: Examples of pit morphologies. A) Mare Subsequent off-nadir imaging by the Lunar Tranquillitatis pit, high d:D ratio. B) Central Mare Reconnaissance Orbiter Camera (LROC) Narrow Fecunditatis pit, low d:D ratio. C) Lacus Mortis pit, Angle Camera (NAC) showed that two of these mare high d:D ratio, but with a collapse on the east wall that pits have overhangs extending at least 20 meters into may be part of a transition to a low d:D morphology. their host maria [1]. However there is still no conclusive evidence to the true nature of the mare pits. Recent discoveries: One pit in Sinus Iridum (70 × They may be openings to extant lava tubes, collapses 30 m diameter, 20 m depth), and two in northern into near surface magma chambers, or some other Oceanus Procellarum (both ~150 m diameter and ~40 tectono-volcanic void (vague wording intentional). We m depth). All three are of the low d:D ratio type (Fig. simply do not know their origin, and cannot know 2), and thus, assuming the morphology types are an short of a dedicated pit exploration mission. erosional continuum, they are less likely to have any The original NAC pit discoveries were mostly the remaining access to whatever void space they formed result of an automated image search method (PitScan) in relative to mare pits with high d:D. that located 5 new mare pits, 2 pits in highland The two Mare Procellarum pits may be related to intercrater materials, and 221 pits in impact melt the same subsurface feature, as they are only 2.5 km deposits of Copernican craters [4]. The impact melt apart, although there is no obvious surface expression pits were a surprise as no publications had predicted of any volcanic feature between them (rilles, ridges, their existence, likely because little thought was given subtle depressions, etc.; Fig. 2d). to organized melt flow within pooled impact melt.

Pit discoveries: Since the NAC-based pit discoveries reported in [4] we have discovered three new pits, all in mare terrain, bringing the total mare pit count to eleven. The known mare pits are spread across nine maria, and exhibit two distinct morphologies (Fig. 1): those with high depth-to-diameter (d:D) ratios and generally flat or slanted floors, with most of the depth coming from the vertical walls (n=7), and low d:D ratios with bowl-shaped floors, and much of the depth coming from the bowl shape of the floor, rather than the vertical walls (n=4). There is likely a continuum between these two types, with pits becoming wider, shallower, and more bowl-shaped over time as their edges are worn away by small impacts. A similar transition was documented on Earth in the Devil’s Throat pit crater in Hawaii [5], albeit on a much shorter timescale and with terrestrial Figure 2: Pits discovered since [4]. A) Sinus Iridum pit erosion mechanisms. On the Moon, two of the mare (45.623°N, 331.189°E). B) The northern, less-eroded pits we discovered have large collapses on one wall of the two northern Procellarum pits (35.408°N, (Fig. 1c), perhaps indicating a transition state between 314.360°E). C) The southern, more-eroded of the two the two forms. northern Procellarum pits (35.343°N, 314.345°E). D)

Context image of the two Procellarum pits. 2nd International Planetary Caves Conference (2015) 9021.pdf

PitScan has only detected a few (~5) not- While their host craters are significantly younger than previously-known impact melt pits in the four years the maria, the pits are correspondingly smaller (median since LRO was moved into a fuel-saving elliptical diameter 16 m), and so would have a shorter lifespan orbit, which reduced the NAC pixel scale over the before obliteration by micro- and macro-impacts [4]. latitude range in which PitScan can search (50°S to We have no evidence to definitively support or 50°N) to 0.6-1.5 m/pixel, compared to 0.5 m/pixel reject any of the remaining three options. LROC was during the circular mapping orbit that LRO was in for able to image a handful of pits (the four largest high the first two years of the mission. There are two likely d:D mare pits, and impact melt pits in Copernicus, factors driving this paucity of new discoveries: 1) King, and Stevinus craters) at oblique angles to check Impact melt pits have mostly been found in large (>10 for overhangs and found four with more than ten km diameter) Copernican craters, and these craters meters of overhang and no visible contact between were given high targeting priority during the mapping wall and floor, indicating subulaneran voids (the phase, so their 50-km altitude coverage is excellent, Tranquillitatis, Ingenii, and Marius Hills mare pits, and and thus we have likely already found most of the the pit containing the King Crater natural bridge [1,4]). impact melt pits within 50° of the equator. 2) PitScan has an internal size cut-off of 15 pixels in diameter, to Exploration: Exploring mare pits is likely to reduce the false positive rate to a manageable level. provide greater scientific value than exploring impact The larger pixel size from the elliptical orbit may put melt pits, as the former provide easy access to a cross- many impact melt pits (median diameter 16 m) below section of the upper layers of mare flows [1,6]. Of the the cut-off limit. Supporting this theory, PitScan has mare pits, the Mare Tranquillitatis and Lacus Mortis not “rediscovered” any already-known pits smaller pits are the best candidates for early exploration (Fig. than approximately 25 m in diameter. We have also 1a,c). Both are ~100 m deep, exposing a large cross- not found any additional pits in non-impact-melt- section of mare, and both are on the near side of the related highland terrain since the two reported in [4]. Moon. The Lacus Mortis pit has a smooth ~30° slope from rim to floor on one side, possibly allowing rover Geologic context of mare pits: There is no access, although it has no overhangs detectable from consistent geologic context in which mare pits occur. orbit, while the Tranquillitatis pit has a >20 m The Marius Hills pit is situated within an apparent overhang on the west side, and, unique among the volcanic rille near the Marius Hills volcanic complex, mare pits, has direct line-of-sight to Earth from parts of and the two northern Procellarum pits are about 40 km the floor [6]. away from an apparent volcanic vent, although there From an engineering perspective, impact melt pits are no closer apparent volcanic landforms (other than may be more useful, as they are smaller, more easily the mare itself). The Lacus Mortis pit is located less accessible, and occasionally occur in chains that could than a kilometer from a large graben, but there are no indicate access to a larger void space. morphologic clues that this proximity is anything but Pits are useful from both a science and an coincidental. The other seven mare pits are located in engineering perspective, and exploring several of them otherwise-featureless mare terrain, with no indication is critical for understanding mare and impact melt of what sort of sub-surface features produced them. deposition processes, and for characterizing any sublunarean void spaces that may exist. Genesis of mare pits: Several options have been proposed for the formation of mare pits: 1) skylights References: [1] Robinson et al., (2012) PSS 69,. into active lava tubes, 2) recent roof collapse into an doi:10.1016/j.pss.2012.05.008. [2] Haruyama, J. et al. ancient lava tube, 3) stoping of the ceiling of a near- (2009). Geophys. Res. Lett. 36, doi:10.1029/ surface magma chamber, and 4) collapse into tectonic 2009GL0406355. [3] Haruyama, J. et al. (2010). 41st voids [1,2,3,4]. LPSC #1285. [4] Wagner, R.V. and Robinson, M.S. Of these options, we can rule out the possibility that (2014) Icarus 237 (2014) 52–60. mare pits are original skylights formed in lava tubes at doi:10.1016/j.icarus.2014.04.002. [5] Okubo, C.H. and the time of lava tube emplacement. Since the maria, Martel, S.J. (1998) doi:10.1016/S0377- and thus any lava tubes that formed with them, formed 0273(98)00070-5. [6] Robinson et al. (2014), LEAG, >3 bya, craters near the mare pits should be in abstract #3025. equilibrium at diameters >200m [6]. Relatively small, crisp features like the Mare Tranquilitatis pit (Fig. 1b) are very unlikely to have survived intact for over 3 by. A similar argument can be made for impact melt pits.