Potential Caves and Sink Holes in Copernicus Crater
Lunar caves are very interesting for the standard reasons (they offer possible locations for radiation-proof, thermally-benign bases for future astronauts). Another reason of interest is that there are so few of them… there are only three officially recognized ones! The Japanese Space Agency’s Kaguya lunar orbiter had found these three caves using its imager and these were later confirmed by LRO images. LRO, having better resolution than Kaguya, provides the possibility of finding other smaller caves or simply caves illuminated at different sun angles.
I am a NASA engineer who works the design of space power systems and have done some lunar polar illumination analyses (papers here, here, and here) supporting the Constellation Lunar Surface Systems program. These analyses are used to optimally size the solar arrays and energy storage systems for spacecraft including landers, bases or rovers near the poles. During the acquisition of LRO NAC images to enhance the polar illumination analysis (by supplementing the LRO laser altimeter data), it occurred to me that it would be feasible and relatively easy to use the same set of images I was downloading to search for caves (essentially a spin-off of the illumination activity). Examining images of the three known caves, I devised a cave “fingerprint” and wrote a FORTRAN program to search all the LRO NAC images for features which matched this fingerprint.
After a number of false positives, I came across one good candidate on the floor of Copernicus Crater (I call it H1).
It is in LRO NAC image M135324446LC:
Another image at a slightly different sun angle is in M135317661RC
Wondering if it could have been captured during the old Lunar Orbiter days (~60s), I found it shown in the center of the following image (vhr_5154_h2), but not clearly enough to determine if it was a cave or hole.
The size of H1 is about 86 m across at its furthest points, 40 m at its closest and seems somewhat triangular. I estimate the depth to be 20m. Clearly, this feature seems like a collapse, certainly not a crater. It almost seems like a fissure/crack, but by not being “near” any rille or lava tube or other features normally associated with a crack, it seemed very odd. It’s as though there was some void under the surface and a weak spot collapsed for some reason, leaving a hole.
Looking around this cave-like feature, I tried to find any other feature that could help to explain it. The entire Copernicus floor does have a lot of cracks and ledges, but they did not seem to be directly associated with H1. One odd feature looked somewhat like a crater but had some unique aspects which tended to imply a sink hole or some other type of circular collapse. The characteristics of this feature include sharply defined edges, somewhat flat bottom, boulders inside the circular area but not outside. The implication is of a sheet of lava or rock which had a void underneath which at some time collapsed, possibly due to dust accumulation. The thickness of the rock sheet in these features seem substantial. The void could have been either under the flat surface or possibly the void was inside/under a dome/hill-like feature which also occur on the Copernicus crater floor. While the diameters range from 150-300, the depth of the sink hole-like features are hard to estimate, possibly 5-10 m. The height of the original voids are also difficult to estimate based on a collapse of flat surface or dome/hill surface (possibly 10-30m). In any event, the features differ from typical impact craters because they have more smooth edges, bowl-like shape and characteristic rock/debris distribution.
Examples of these include:
From LRO NAC image M104648293R (I call the sink hole feature C1)
From LRO NAC image M111728277R (C2)
From LRO NAC image M104648293L (C3), which shows the sharp edge of the feature on one side but dust/regolith drifting into the other side.
From LRO NAC image M104648293L (C4), which also shows a sharp edge on one side but dust/regolith drifting into the other side. Are C4 and C5 suggestive of a contributing cause of the collapse, namely dust accumulation?
From LRO NAC image M102293451L (C5), the excerpt shows not only the sink hole but a suggestive dome/hill-like feature. Could the hill hold an un-collapsed void?
Lunar scientists have made some observations about these images. IreneAnt commented (here and here) that the floor of Copernicus is covered by impact melt and when the melt sheets cool, they crack, which is why this floor has a number of them. But, the H1 feature is odd in that it does not align with a crack. Referencing Giordano Bruno crater it was suggested that if the melt drained away in a similar fashion in the Copernicus crater, then voids may occur under the melt sheet. Regarding the sinkhole features, this lunar scientist confirms that they were not craters since the features were bigger and fresher than other craters in the area, have no ejecta and have a spherical morphology (not conical).
Other cave-like features:
These were found in the course of the work on the Copernicus crater floor:
M122339925R (11m diameter, called H2).
M119978417L (25 m diameter, called H3)
M109358669L (H3 at a different sun angle)
M124708491L (9m diameter, called H4)
M109365462R (H4 at a different sun angle)
M124701702L (H4 at a different sun angle)
M109365462L (7 meter diameter, called H5), this seems associated with a tube, crack.
Map of the Features:
The following map shows the relative location of each of the above discussed features. Most seem in a particular area, but note that the entire floor of the crater has not yet been imaged with the high resolution camera.
In my free time, I am working to document and publish these results in a NASA report. It is my hope that these results can assist in finding caves in other locations on the Moon.
JFincannon is also a member of the Moon Zoo forum