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Erosion... on the Moon?
There are a lot of differences between our home planet Earth and its moon. I'm sure you could name a few, but one that's most obvious to astronomers is that Earth has very few impact craters, while the Moon is saturated with them.
Earth has so few because of several things. One is plate tectonics; continental drift and volcanism tend to recycle our surface, erasing very old craters.
Another is that we have an atmosphere, and in general that prevent impacts from cosmic objects smaller than a hundred or so meters across. And yet another reason is … our atmosphere. Seriously. It contributes to erosion, so older craters eventually get worn down by wind and rain.
The Moon has none of these, so you'd expect it to have a lot more craters, and it does. However, there is still erosion on the Moon! Over time, the solar wind and small micrometeorites sandblast the lunar surface, softening the edges of craters. Mind you, that's a slow process, but the Moon's had plenty of time.
Planetary scientists are interested in every process that could cause craters to erode. To better understand them, a team of them looked at small craters on the Moon in two different locations: near the crater Descartes, where Apollo 16 touched down, and near the Taurus-Littrow valley where Apollo 17 landed. Those two sites have been observed extensively by the Lunar Reconnaissance Orbiter, which takes very high-resolution images of the lunar surface. Not only that, but it's taken images at different times of the lunar day, so the shadows are different. That helps with contrast, and makes it easier to see how badly degraded craters are.
They divided the small impact craters into three classes: fresh, highly eroded, and intermediate. And they found something surprising: The two sites have different ratios of eroded craters. In the Apollo 16 area, heavily eroded craters made up 70% of the total, but at the Apollo 17 site they made up 88%.
Why?
They have two possible explanations. One is that the surface around the Apollo 17 landing area is made of a softer material, making it more prone to erosive events like movement. The other is that it's possible the Apollo 17 site is more prone to moonquakes.
Yup, moonquakes! Although there's no plate tectonic action on the Moon like there is on Earth, there is some movement of the crust. Not far from the Taurus-Littrow valley where Apollo 17 landed, there's a big scarp (cliff) indicating there can be slippage there, that could cause ground movement enough to degrade craters. They also posit — and this is incredible to me — that the impact that formed the giant crater Tycho 100 million or so years ago may have ejected enough material that landed in the Taurus-Littrow area that the ground shook, again allowing the soft material to flow, filling in smaller craters.
Tycho is over 2,000 kilometers from that part of the Moon! The idea that the enormous impact that formed Tycho lobbed huge boulders thousands of kilometers away which then fell from the sky hard enough to cause moonquakes is terrifying. I'm glad big impacts like that are rare!
… but seeing one hit the Moon would be amazing. The things we'd learn! From a safe distance, thankfully.
The evidence of the Moon's violent past is written in every single crater on its surface. But later violence can help erase past violence. That’s certainly not an aphorism I'd want to apply to life, but when applied to worlds it does seem to cover some ground.
[Note: I found this news on the Lunar Reconnaissance Orbiter Camera site, which is just a phenomenal parade of one incredible Moon photo after another. They have some fun pan-and-zoom images from the Apollo 16 and 17 sites you should play with.]
