Scientists have long believed that the near-Earth asteroid Bennu has a sandy beach-like surface abundant in fine sand and pebbles, as well as broad bands of fine-grained material less than a few inches called fine regolith.
However, NASA’s OSIRIS-REx mission, which arrived in Bennu in late 2018, saw an area covered with rocks. The mysterious lack of fine regolith became even more surprising when scientists on the mission observed evidence of processes potentially capable of crushing boulders into fine regolith, the US space agency said.
To gain insight, a team of scientists from the University of Arizona conducted research using machine learning and surface temperature data.
The results, published in the journal Nature, showed that the very porous rocks of Bennu are responsible for the surprising lack of fine regolith at the surface.
âWhen the first images of Bennu arrived, we noticed some areas where the resolution was not high enough to see if there were small rocks or a fine regolith. We have started using our machine learning approach to distinguish fine regolith from rocks using thermal (infrared) emission, âsaid Saverio Cambioni of the university’s Lunar and Planetary Laboratory, in a statement.
The thermal emission of fine regolith is different from that of larger rocks, because the size of its particles controls the former, while the latter is controlled by the porosity of the rock. The team first built a library of thermal emissions associated with fine regoliths mixed in different proportions with rocks of various porosities.
Then they used machine learning techniques to teach a computer how to âconnect the dotsâ between the examples, Cambioni said. They analyzed 122 areas on Bennu’s surface, which were observed day and night.
Cambioni also found that the fine regolith was not distributed randomly on Bennu. Instead, it reached several tens of percent in the very few areas where rocks are not porous, and consistently lower where rocks have higher porosity, which is most of the surface.
The team concluded that very little fine regolith is produced from the very porous rocks of Bennu because they are compressed rather than fragmented by meteorite impacts. Like a sponge, voids in rocks cushion the blow of incoming meteorites. These results are also in agreement with the lab experiments of other research groups, NASA said.
Additionally, the team showed that the cracking caused by heating and cooling Bennu’s rocks as the asteroid rotated day and night occurred more slowly in porous rocks than in denser rocks, further frustrating the production of fine regolith.
The OSIRIS-REx space probe is expected to reach Earth on September 24, 2023, after making two orbits around the Sun.
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