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
Recently, a user found an enigmatic feature (see forum thread here) and several suggestions were given for how it formed, including (1) impact melt flows, (2) lava flow fronts / thick lava flows, and (3) wrinkle ridges.
Let’s address each hypothesis individually to see if we can determine which is the most likely scenario for the formation of the structures.
(1) Impact Melt Flows. A few things are important to note about impact melt flows. First, they are always associated with impact craters. Second, like water, the flows will always head downhill. Third, the flows often have lobate morphologies at their termini. Here are a few a beautiful examples of impact melt flows from the crater Giordano Bruno (Figs. 2). In these images, we see the characteristic lobate shape of flows. The features in the original post do not appear to either be near or closely associated with large impact craters (though the field of view afforded by NACs is admittedly small) or formed as the material flowed downhill. Evidence for this latter point comes from the observation that the structure has two steep scarps to it (one on either side) which would be difficult to get if there was a dominant flow direction.
Figure 2. Left: Image of impact melt flow on the southern flank of the crater Giordano Bruno. From Denevi et al. (2010). Portion of NAC frame M101476840LE. Illumination is to the west. The flow moves downhill to the lower right hand side of the image. North is up. Resolution 1.5 m/px. Right: Impact melt flows on the flanks of Giordano Bruno. Portion of NAC frame M110919730L (centered at 35.84°N, 102.72°E). Downhill is roughly towards the lower left corner of the image. Note the multiple different flows (if you are having trouble seeing these flows as positive relief features look at the white features – these are boulders – hopefully the impact melt flow should pop out into a flow!). Image resolution is 0.6 m/pixel. See the full frame here.
(2) Lava Flows. Similar morphological features are also characteristic of volcano lava flow fronts (flow downhill, lobate terminus), except lava flows do not typically emanate from impact craters. Lava flows on the Moon are typically much wider and longer (up to 800 km long and 20-40 km wide in places!) than the narrow impact melt flows seen in Fig. 2, as they are formed from highly effusive sources (vents) when there was lot of lava erupted to produce large-scale flows. However, there is a possibility that the feature could represent a localized, highly viscous flow unit, as suggested by Irene in the Image of the Week post. As much of the Moon has not been seen in such high-resolution before, we may well be viewing geological constructs that are completely new to lunar science!
(3) Finally, we should address the final formation hypothesis: wrinkle ridges. (See also a previous blog entry for some background structural geology.) So, what is a wrinkle ridge and how does it form? Since their discovery on the Moon (and elsewhere), there has been much debate about these questions. In the last 10 years or so, there has been a convergence in the community toward the “right answer:” that these structures are blind thrust fault anticlines. That’s structural geology speak for ridges or convex-upward shapes formed by a thrust fault that does not yet reach the surface. They form in response to horizontal squeezing (or compression) of the material they form in and this compression is usually related to regional-scale processes, like sagging of the crust under large impact craters filled with mare. Wrinkle ridges seem to preferentially form in layered material, where there are mechanically weaker layers (like ash or regolith) separating the mechanically stronger layers (the lava flows). This mechanical juxtaposition means that when the material is compressed, three things happen: folding, sliding, and faulting. As the thrust fault propagates toward the surface, the material folds above it and slip occurs along weak interfaces. Sometimes a thrust fault will form which slopes opposite to the main thrust fault (the backthrust, like this y) and this is manifested at the surface as the wrinkle.
The structures shown in the Image of the Week thread do not necessarily have what I would call typical wrinkle ridge (WR) morphology (except those shown in Tranquilitatis). This is what is interesting! If these are wrinkle ridges, it tells us something different is going on here and that difference will tell us about the material in which these structures formed and the geologic evolution of the area where they are found. When I first looked at these images, I wasn’t sure what to make of them. In the NAC frame to the left of the first image (see Figure 3 for both NAC frames montaged together), I saw a structure with more typical WR morphology and that really clinched the identification for me. I also took a look at the “WR” in Aitken crater from the LROC site. So, are they wrinkle ridges? Well, the answer is…sort of.
Figure 3. Section of montage of NAC frames M117732304RE and M117732304LE showing wider field of view of the area of interest (the image width is ~5 km). Note that the original section of image is at top right.
If you allow me to get on my soapbox for a few sentences, I can explain what I mean. To structural geologists like me, there is a connotation associated with terms like wrinkle ridge or lobate scarp telling us about the mechanical properties of the material, the morphology of the structures, and how they formed. Since these structures do not have the typical morphology of a wrinkle ridge, I would not call them wrinkle ridges, though they likely formed in a similar manner: compression of layered lava flows separated by weak layers, forming thrust faults and folds. The narrowness of the sinuous ridges implies that the interbedded basalt is fairly thin (maybe less than a kilometer). The lack of a broad ridge is something for which I do not yet have an explanation.
As Drew Enns suggests in his blog post on the LROC Picture of the Day site, the size of the ridges implies that the thickness of the basalt in both Aitken crater and near Bessarion W is thin. It may also suggest that the faulted layer is thin. That is, there may be more basalt at depth, but the weak mechanical layer may only be a few tens to hundreds of meters below the surface. It is in this way that structures like wrinkles ridges can tell us about the physical properties of the materials they deform and the geologic history of the areas where they form.
Dr. Amanda Nahm is a tectonic-fault expert at the Lunar and Planetary Institute