A Whole Lot of Shaking Going On

This week was insanely busy for me as I worked on a grant proposal with my CosmoQuest Moon Mappers colleagues. For the proposal, we are looking at very high resolution images from the NASA Lunar Reconnaissance Orbiter Narrow Angle Camera (LROC NAC). What always strikes me as so interesting is the strange kinds of surfaces we see on the Moon at these very high resolutions.

This interesting texture on the far side of the Moon (at approximately 57.6° S, 198.2°) shows many small degraded impact craters. Constant bombardment by later impacts shakes the layer of rubble (called regolith) that these craters rest on, causing them to slump and lose their shape. The largest crater in this image is only about 500 meters across. Image credit: NASA/GSFC.Arizona State University.

My favourites are these "taffy-like" textures that can be found in some places. Here the surface looks like some child has poked their fingers into a still-cooling pan of fudge or taffy. The resulting holes are actually impact craters that formed when a meteorite hit the surface at very large speeds. But, these craters don't look the way impact craters are supposed to look. They aren't the crisp and clean circular bowls we expect to see when we think of impact craters.

This is what we generally think impact craters are supposed to look like. These are crisp, perfectly circular crater in Mare Imbrium on the near side of the Moon (at approximately 32.3° N, 335.2°). These craters are relatively large, with the biggest one here being 5 km across. At this size, then impact mostly into solid rock and so retain their shape better. Image credit: NASA/GSFC.Arizona State University.

There are several reasons why these "taffy" craters look so different. For starters, they are much smaller than the craters we usually see when we look at images of the Moon. Bigger craters impact mostly into solid rock. Meanwhile, these smaller craters are often completely contained in the thick layer of loose rubble (called regolith) that covers almost every surface on the Moon. Because regolith is not as strong as solid rock, it is affected far more readily by subsequent impacts.

Each impact into the lunar surface creates a mini-moonquake. Calculations from the 1970's (by scientists Pete Schultz and Don Gault) show that each impact-induced moonquake shakes the surrounding regolith to a distance that is several times farther than the resulting crater. Looking at the number of craters that are visible on the lunar surface, it is clear that there's a whole lot of shaking going on!

Over time, this shaking works to modify craters; toppling their rims, filling in their centres, and eventually obliterating the craters completely. The various stages of this process can be clearly seen in high resolution images of the Moon.

Also, because moonquakes affect regolith more than solid rock, this kind of process will be more prevalent in areas where the regolith is thicker. This is usually found in older areas, since they have had more time to build up a thick layer of rubble from lots of different impacts. So, when I see this kind of "taffy-like" texture, I know I am looking at highly degraded craters in an old, thick regolith layer.

Explore this taffy-like texture for yourself at the ACT-REACT QuickMap online LROC image atlas tool. (A simple guide for using the ACT-REACT QuickMap can be found here).

References:
Schultz and Gault, Seismically induced modification of lunar surface features, Proceedings of the 6th Lunar Science Conference, 2845-2862, 1975.