The Lunar Reconnaissance Orbiter has been taking extreme close-ups of the Moon since September 2009, and since that time I've posted dozens of these images, including shots of the Apollo landing sites, images of Earth from the Moon, craters, lava tubes, and more. But among my favorite of all these are the ones showing action, something that's happened that you can actually see. And the best of these, I think, are from boulders rolling around on the Moon's surface. Like this one!
[Click to embiggen and get a somewhat wider view.]
That boulder you see there is about 30 meters across -- 100 feet -- so it's pretty big, the size of a house. It's sitting inside the 110-km-wide crater Gassendi, and rolled down from the crater's central peak. The trail is obvious, as are several smaller ones around it.
Funny-- it seems obvious in retrospect (as so many things do), but I wouldn't have predicted boulders rolling down slopes on the Moon. In my mind, even now, it's hard to shake the prejudice that the Moon is dead, dead, dead. But it still has some activity; for example, the changing gravity it feels from the Earth as it loops around us in its elliptical orbit causes the Moon to stretch and compress. This can create seismic activity -- moonquakes! And that can cause rocks to tumble downslope.
There are lots of images from LRO of this, too. Here's one with rolling rocks and trails in Tsiolkovsky crater, and one of my all-time favorites shows a boulder that rolled down a slope and into a small crater: a lunar hole in one!
Back to that big guy up top; Gassendi was a landing target for Apollo 17 for a while, but the uncertain terrain (lunain?) made the planners decide to go elsewhere. Too bad: boulders that roll down from higher locations make it easier to get rock samples from different areas, since the rock already did the work of coming to you!
But maybe, just maybe, we're not quite done with the Moon yet. I hope to once again see people walking on its surface, and instead of seeing pictures like this from above, we'll have shots of astronauts living and working there, taken by their own hands.
Image credit: ASA/GSFC/Arizona State University