Huge chunks rumble down the slopes – rock falls obviously occur not only on earth, but also on their satellite. This now documents the first comprehensive map of the traces of rock exits on the moon. The information shows that most of the rockfalls are caused by instabilities due to asteroid impacts. However, some also seem to be the result of moonquakes and the former volcanism, the scientists report.
The continual shifts in the earth’s crust, quakes and the effects of water and temperature fluctuations make earth’s rock fragile until it finally loses its hold: Again and again, spectacular rock falls occur in mountainous regions of the world. In the Swiss Alps, for example, a huge boulder crashed from the summit of Mel de la Niva in 2015 and cut a 1.4-kilometer path into the mountain forest on its way into the valley. Already in the 1960s, recordings of unmanned probes showed that rocks always roll on the moon and leave distinctive structures in the underground. During the later Apollo missions, astronauts also examined these interesting places on site.
Artificial intelligence identifies the traces
So far, however, there has not been a comprehensive overview of the traces of rock falls on the moon. “Earlier space probes that examined the moon could not make such comparatively small structures visible everywhere,” says Valentin Bickel from the Max Planck Institute for Solar System Research (MPS) in Göttingen. More precise information is only accessible through the images of NASA’s Lunar Reconnaissance Orbiter (LRO), which has mapped the entire lunar surface with high spatial resolution and coverage since 2010. Bickel and his colleagues have now systematically searched the archive, which contains more than two million recordings, for traces of lunar rock outcrops.
To this end, they developed a search algorithm that provides computers with a sharp recognition ability: on the basis of neural networks, the system learns to capture the typical traces of the rockfalls in the satellite images. This is how a global map of these special structures on the moon was created – which is apparently teeming with: The map reports 136,610 traces of rockfalls with a diameter of more than two and a half meters, the scientists report. The data show that the diameters of the once mobile boulders range from around three to 25 meters. Most of the rolling boulders on the moon are between seven and ten meters wide, the scientists say.
Asteroid impacts are the main cause
“For the first time, the map offers us the opportunity to investigate the occurrence of rock falls on another celestial body and their causes,” says MPS co-author Urs Mall. So far, it was assumed that seismic effects in particular in the moon’s subsurface would release boulders and set them in motion. As the researchers report, it is now becoming apparent that asteroid impacts play a significantly more important role. According to your data, these triggers are directly or indirectly responsible for more than 80 percent of the rock exits. “A large part of the rockfalls are found near crater walls,” says co-author Simon Löw from the Swiss Federal Institute of Technology in Zurich. Some of the chunks will probably dissolve immediately or soon after impact – but others will only clear up significantly later. The scientists see a fundamental weakening of the structures in the impact area as the reason for the sometimes extremely delayed effect: a network of cracks is likely to arise, which make the material unstable, which means that parts can become mobile even after very long periods of time.
In this way, the researchers explain the finding that traces of relatively fresh rockfalls can be seen on ancient craters in landscapes of the moon that are up to four billion years old. The fact that some of them were only created in the current era or in the last millions of years is due to the slight traces of weathering on the structures, which also arise on the moon due to the influence of micrometeorites and the effects of temperature fluctuations. “Asteroid impacts obviously influence and change the geology of a region over very, very long periods of time,” says Bickel. This finding on the moon suggests that very old surface structures can still change on other celestial bodies without an atmosphere, the researchers say.
As they report further, there are also traces of rock falls on the moon, which are not to be found on crater structures and thus have no connection with asteroid impacts. This is presumably due to instabilities in the subsurface, which are linked to the former volcanism on the moon or also with quakes. The seismic activities could have to do with the moon’s shrinkage processes, a study showed last year. Apparently they cause tremors along fault lines from time to time. To this end, the researchers found the rock outcrops on presumably seismically active tectonic trenches and on old volcanic vents. The new overview map can thus also help to identify unknown, seismically active regions on our satellite, the scientists say.
Source: Max Planck Institute for Solar System Research, specialist article: Nature Communications, doi: 10.1038 / s41467-020-16653-3