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.

The Spectacular Giordano Bruno crater on the Moon

The other day, a friend and colleague from Russia asked me to send him an image of the crater Giordano Bruno from a popular science article I had written last year. The image I had didn't show the detail that he needed, so I made a better one for him from original data. And in doing so, I was reminded of all the spectacular features that this crater displays.

Giordano Bruno crater on the eastern far side limb of the Moon (35.9° N, 102.8° E) as revealed by the Lunar
Reconnaissance Orbiter Camera. The annotated image on the right shows the locations of the following images.
Image credit: NASA/GSFC/Arizona State University

This Slump Block in the upper right part of the crater slid
down the crater wall well after the crater was formed.
Image credit: NASA/GSFC/Arizona State University

Giordano Bruno is located on the far side of the Moon, just beyond the eastern limb. It's a relatively small crater as lunar crater go, only about 22 km in diameter. But at this size, it is large enough to exhibit some of the interesting features that show up in impact craters as they get bigger.

Many of these features are being revealed for the first time by very high resolution imagery from the Lunar Reconnaissance Orbiter's Narrow Angle Camera (LROC NAC). These NAC images have resolutions of about half a meter per pixel. This means that something the size of a chair (or you!) would take up one pixel, and your desk or kitchen table would show up as a 2 by 3 pixel rectangle. With those kinds of resolutions, there are some spectacular things to be seen.

Chaotic Impact Melts line the floor of Giordano Bruno, showing complex
structures that tell us of entrained boulders and folding chill crusts.
Image credit: NASA/GSFC/Arizona State University

For starters, there is a very interesting slump feature, located in the upper right part of the crater. It looks as if a whole block of the crater side slid down the crater wall in one piece. Such slumps are common in large impact craters, which tend to collapse at the end of their formation process. But, studies by Dr. Yuriy Shkuratov (from the Astronomical Institute of Kharkov in Ukraine) and his colleagues suggest that the slumping of this block occurred at a later time and so was not related to the crater formation process. Before this, it was thought that such big changes to craters only happened during their formation.

Ejected Melts beyond the crater rim can show channels
and rivulets where the hot melt flowed downhill from the crater rim crest.
Image credit: NASA/GSFC/Arizona State University

Next, the floor of Giordano Bruno crater is covered with chaotic impact melts. During the impact that formed this crater, temperatures got high enough to melt a lot of the crustal rock and this melt flowed down to the bottom of the crater, where it solidified. Now, high-resolution LROC NAC images are showing us that these melt are much more complex than had been previously believed. The melts entrained a lot of boulders and solid rock chunks, which can be seen poking above the melt surface. Ropy textures (that look like bunched up material) tell us that the top of the melts cooled quickly, forming what is called a chill crust, but the material underneath remained molten and mobile for some time, dragging the chill crust along and folding it as obstacles were encountered. Such images can help us learn how much melt formed and what happened to it during the crater-forming process

Not all melts end up on the crater floors. Some melt material gets ejected outside the crater. Again, high-resolution NAC images are showing us this process in amazing detail. Dark smooth patches show where melts were emplaced beyond the crater rim. In some cases, rivulets and channels were left behind as the hot melt flowed from the peak of the crater rim down the outside flanks of the crater.

Solid materials are also ejected beyond the crater rim. Many large boulders, ranging in size from 1 to 30 meters, can be seen at the edges of Giordano Bruno crater in this NAC image. To understand the scale of these boulders, consider that a meter-sized boulder is about as big as a desk, while a 30 meter-sized boulder is roughly equal to 9 houses bunched together. Sometimes, these boulders can roll down the slopes of the crater, either outside or inside the crater rim

Boulder Tracks on the slopes of the crater show how house-sized boulders on or
near the rim can roll down the crater walls.
Image credit: NASA/GSFC/Arizona State University

Lots of other spectacular features can be found in and around this interesting crater. Feel free to explore more of Giordano Bruno crater on your own at the ACT-REACT QuickMap on line LROC image atlas tool. A simple guide for using the ACT-REACT QuickMap can be found here.

References: Shkuratov et al., The lunar crater Giordano Bruno as seen with optical roughness imagery, Icarus 218, 525-533, doi:10.1016/j.icarus/2011.12.023, 2012.