Swiss and German researchers have created the first map of rockfalls on the Moon.

After analysing more than two million images of the lunar surface they documented 136,610 incidents of boulders of at least 2.5 metres in diameter travelling down slopes, often leaving impressive tracks behind them.

And the vast majority were actually between seven and 10 metres, according to Valentin Bickel, first author of a paper in the journal Nature Communications.

Bickel, who is affiliated with the Max Planck Institute for Solar System Research (MPS) and ETH Zurich, collaborated with colleagues from both institutions.

Their resulting map charts the lunar surface between 80 degrees northern and southern latitude. “For the first time, this map enables us to systematically analyse the occurrence and causes of rockfalls on another celestial body”, says co-author Urs Mall from MPS.  

Lunar rockfalls have been observed since the first unmanned flights to the Moon in the 1960s. Later the Apollo astronauts examined a number of sites and brought displaced samples to Earth.

However, it was not until 2010, with the launch of NASA’s Lunar Reconnaissance Orbiter, that imagery of the entire lunar surface with the necessary spatial resolution and coverage became available.

And it has changed some thinking. Scientists had assumed displacement of boulders was largely caused by lunar quakes, but the researchers now suggest impacts from asteroids may be directly or indirectly responsible for more than 80% percent of all observed rockfalls.

“Most of the rockfalls are found near crater walls,” says Simon Loew of ETH Zurich. “Some of the boulders are displaced soon after the impact, others much later.”

He and his colleagues hypothesise 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.

Traces of rockfall events can be found in even the oldest lunar landscapes, which formed around four billion years ago.

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.