Using an artificial intelligence and big data-driven approach, a team of planetary researchers from the Max Planck Institute for Solar System Research and ETH Zurich has analyzed an archive of more than 2 million high-resolution images and created the global map of 136,610 lunar rockfall events.
Heat maps of lunar global rockfall distribution in both equirectangular (70°N to 70°S) and polar projection (60°N to 80°N and 60°S to 80°S). Mapping results above 75°N and below 75°S might be less complete due to challenging illumination conditions (indicated by transparent white areas). Rockfall density is given in rockfalls per square degrees, i.e., the number of rockfalls per 2° latitude by 2° longitude quadrangle. Inset shows examples of rockfalls and their tracks with various sizes. Image credit: LROC / ASU / NASA / Bickel et al, doi: 10.1038/s41467-020-16653-3.
Over multi-billion-year timescales, erosion processes smooth topography and shape surfaces of terrestrial planets and their satellites.
On airless bodies, such as the Moon, erosion has primarily been thought to occur through space weathering. However, recently acquired, high-resolution images have revealed a startling feature on lunar slopes: mass-wasting features.
These include granular flows, slides, slumps, and creeps, as well as rockfalls, a process where boulders are released or ejected from topographic highs and fall, roll, bounce, and slide to topographic lows.
Rockfalls carve tracks into the lunar surface, which provide a record of the dynamic emplacement process. The characteristic combination of a displaced boulder or rock fragment and its track enables unambiguous detection from satellite images.
“The vast majority of displaced boulders on the Moon have a diameter of between 7 and 10 m,” said lead author Dr. Valentin Bickel, a scientist in the Department Planets and Comets at the Max Planck Institute for Solar System Research and the Department for Earth Sciences at ETH Zurich.
“Earlier space probes that have studied the Moon were unable to detect such small features on a global scale.”
An example of the lunar rockfall. Image credit: NASA’s Goddard Space Flight Center / ASU.
In the new study, Dr. Bickel and colleagues analyzed an archive of over 2 million high-resolution images taken by the narrow angle camera (NAC) aboard NASA’s Lunar Reconnaissance Orbiter.
They used a neural network in combination with advanced cloud computing capabilities to create a map of the location and size of lunar rockfalls.
They identified 136,610 rockfall events across the lunar surface in the latitude range from 80°N to 80°S, with a mean spatial density of 2 rockfalls per 1° by 1° quadrangle.
“For the first time, this map enables us to systematically analyze the occurrence and causes of rockfalls on another celestial body,” said co-author Dr. Urs Mall, from the Department Planets and Comets at the Max Planck Institute for Solar System Research.
The team found that asteroid impacts are — directly or indirectly — responsible for more than 80% of all observed rockfalls.
“Most of the rockfalls are found near crater walls. Some of the boulders are displaced soon after the impact, others much later,” said co-author Professor Simon Loew, from the Department for Earth Sciences at ETH Zurich.
Surprisingly, the researchers found traces of rockfall events even in the oldest lunar landscapes, which formed up to 4 billion years ago or even earlier.
“Apparently, impacts influence and modify the geology of a region over very, very long time scales,” Dr. Bickel said.
“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.”
The team’s paper was published in the journal Nature Communications.
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V.T. Bickel et al. 2020. Impacts drive lunar rockfalls over billions of years. Nat Commun 11, 2862; doi: 10.1038/s41467-020-16653-3
