The result is a map of the lunar surface between 80 degrees northern and southern latitude that shows 136,610 rockfalls with diameters of more than two and a half meters. "For the first time, this map enables us to systematically analyze the occurrence and causes of rockfalls on another celestial body", says Dr. Urs Mall from MPS.
Previously, scientists had assumed that lunar quakes in particular were responsible for the displacement of boulders. The new global map of rockfalls indicates that impacts from asteroids may play a much more important role. They are apparently - directly or indirectly - responsible for more than 80 percent of all observed rockfalls.
"Most of the rockfalls are found near crater walls," says Prof. Dr. Simon Loew of ETH Zurich. Some of the boulders are displaced soon after the impact, others much later. The researchers hypothesize that impacts cause a network of cracks that extend in the underlying bedrock. Parts of the surface can thus become unstable even after very long periods of time.
Surprisingly, even in the oldest lunar landscapes, which formed up to 4 billion years ago or even earlier, traces of rockfall events can be found. Since such imprints would typically disappear after a few million years, these surfaces are apparently still subject to erosion through rockfall, even billions of years after they were formed.
"Apparently, impacts influence and modify the geology of a region over very, very long time scales," says Bickel. The results also suggest that very old surfaces on other airless bodies such as Mercury or the large asteroid Vesta may still be evolving as well.