Planetary researchers have discovered an inverse correlation between the local abundance of fine-grain material and the porosity of rocks on the carbonaceous asteroid (101955) Bennu.
Planetary scientists thought the surface of the asteroid Bennu would be abundant in fine sand and pebbles, which would have been perfect for collecting samples. Telescopic data had suggested the presence of fine regolith that’s smaller than a few centimeters.
But when NASA’s OSIRIS-REx spacecraft arrived at Bennu in 2018, the mission team saw a surface covered in boulders.
The mysterious lack of fine regolith became even more surprising when the researchers observed evidence of processes capable of grinding boulders into fine regolith.
“OSIRIS-REx collected very high-resolution data for Bennu’s entire surface, which was down to 3 mm per pixel at some locations,” said OSIRIS-REx principal investigator Professor Dante Lauretta, a planetary scientist at the University of Arizona.
“Beyond scientific interest, the lack of fine regolith became a challenge for the mission itself, because the spacecraft was designed to collect such material.”
OSIRIS-REx successfully made contact with Bennu to collect sample material in October 2020.
“When the first images of Bennu came in, we noted some areas where the resolution was not high enough to see whether there were small rocks or fine regolith,” said OSIRIS-REx team member Dr. Saverio Cambioni, a researcher in the Lunar and Planetary Laboratory at the University of Arizona and the Division of Geological and Planetary Sciences at Caltech.
“We started using our machine learning approach to separate fine regolith from rocks using thermal emission (infrared) data.”
The thermal emission from fine regolith is different from that of larger rocks, because the former is controlled by the size of its particles, while the latter is controlled by rock porosity.
The study authors first built a library of examples of thermal emissions associated with fine regolith mixed in different proportions with rocks of various porosity.
Next, they used machine learning techniques to teach a computer how to ‘connect the dots’ between the examples.
Then, they used the machine learning software to analyze the thermal emission from 122 areas on the surface of Bennu observed both during the day and the night.
“Only a machine learning algorithm could efficiently explore a dataset this large,” Dr. Cambioni said.
The scientists found that the fine regolith was not randomly distributed on Bennu but instead was lower where rocks were more porous, which was on most of the surface.
They concluded that very little fine regolith is produced by Bennu’s highly porous rocks because these rocks are compressed rather than fragmented by meteoroid impacts.
Like a sponge, the voids in rocks cushion the blow from incoming meteors.
These findings are also in agreement with lab experiments from other research groups.
“Basically, a big part of the energy of the impact goes into crushing the pores restricting the fragmentation of the rocks and the production of new fine regolith,” said Dr. Chrysa Avdellidou, a postdoctoral researcher at CNRS and the Lagrange Laboratory at the Côte d’Azur Observatory and University.
Additionally, cracking caused by the heating and cooling of Bennu’s rocks as the asteroid rotates through day and night proceeds more slowly in porous rocks than in denser rocks, further frustrating the production of fine regolith.
“When OSIRIS-REx delivers its sample of Bennu to Earth in September 2023, scientists will be able to study the samples in detail,” said OSIRIS-REx project scientist Dr. Jason Dworkin, a researcher at NASA’s Goddard Space Flight Center.
“This includes testing the physical properties of the rocks to verify this study.”
The findings appear in the October 7, 2021 issue of the journal Nature.
S. Cambioni et al. 2021. Fine-regolith production on asteroids controlled by rock porosity. Nature 598, 49-52; doi: 10.1038/s41586-021-03816-5