About 80 percent of the stones are set in motion by impacts.
On Earth, it happens regularly: rock chunks breaking loose and rolling miles down a valley. The same phenomenon occurs on the moon. During their fall, the rolling stones leave impressive traces that were first spotted in the 1960s, when unmanned probes traveled to the moon. But how widespread are such phenomena? And where exactly on the moon are they for?
During the Apollo missions, astronauts examined a number of tracks left behind by rolling stones. Samples of rock chunks were also taken and sent back to Earth. Yet many things about rockfall on the moon remain shrouded in mists. In the meantime, however, we have a bulging archive consisting of numerous images of the lunar surface. And so researchers decided to analyze more than two million photos in an attempt to map the rockfall on the moon.
Thanks to the analysis, the researchers have clarified a lot. For example, they discovered that more than 136,000 rock chunks have been moved on the moon relatively recently. This happened between 80 degrees north and south latitude. “Our map allows us for the first time to systematically analyze the occurrence and causes of rock fall on another celestial body,” concludes researcher Urs Mall.
The team also managed to determine the size of the rolling stones. “The vast majority of the displaced rock chunks on the moon have a diameter between seven and ten meters,” explains researcher Valentin Bickel. The fact that we can figure this out now is quite an achievement. “Previous space probes that have studied the moon have not been able to detect such small objects,” Bickel continues. That was first made possible in 2010 when NASA’s Lunar Reconnaissance Orbiter was launched. This probe managed to shoot razor-sharp images of the entire lunar surface, making even the smallest details visible.
We now know that rock chunks regularly roll around on the moon. But why are the stones actually moving? Scientists previously suspected that mainly lunar tremors were responsible for the displacement of the rotb skirts. But the researchers from the current study come up with a different explanation. They argue that impacts of asteroids play a much more important role. In fact, the researchers state that as many as 80 percent of the stones have been set in motion by impacts. “Moving stones are mainly found near crater walls,” explains researcher Simon Loew. Some rock chunks come right after an impact in motion, others only start rolling much later. The researchers hypothesize that the impacts cause cracks that penetrate the underlying rock. For example, parts of the lunar surface can become unstable even after a very long time.
Surprisingly, even in the oldest regions of the moon – which were formed 4 billion years ago or even earlier – traces of rolling stones can be found. And that’s pretty striking. Such traces usually disappear after a few million years. The fact that we can still see the prints now means that these areas are apparently still subject to erosion caused by the displacement of the rock chunks. And that billions of years after they were formed. “Apparently, the impacts affect a certain area over very long time scales,” Bickel concludes. The results of the study also suggest that very ancient areas on other celestial bodies – such as on Mercury or on the large asteroid Vesta – may also still be subject to change.
Whether that’s true, the Space Probe BepiColombo may unravel. This vessel was launched in 2018 and is currently heading towards its final destination: Mercury. That will take some time, by the way; In all likelihood, BepiColombo will not arrive at the innermost planet of our solar system until 2025. Once arrived, the probe will release two orbiters orbiting the planet. Among other things, the mission is to reveal how Mercury was created, what geological processes are at play on the planet and exactly how the planet works.