Why do some asteroids have rocky surfaces?

NASA mission helps solve a mystery.

Asteroids are rocky, airless remnants left over from the early formation of our solar system. Most asteroids consist of regolith, which includes dust, broken rocks, and other related materials.

Earlier observations of Asteroid Bennu revealed that the asteroid’s surface was like a sandy beach, abundant in fine sand and pebbles. Telescopic observations also revealed the presence of large swaths of fine-grained material smaller than a few centimeters called fine regolith.

But when NASA’s OSIRIS-REx mission arrived at Bennu in late 2018, the mission saw a surface covered in boulders. The mysterious lack of fine regolith became even more surprising when mission scientists observed evidence of processes capable of grinding boulders into fine regolith.

New research helps solve this mystery. Using machine learning algorithms and surface temperature data, a team of scientists found that the surface of asteroid Bennu has highly porous rocks. These rocks are responsible for the surface’s surprising lack of fine regolith.

Study co-author and OSIRIS-REx Principal Investigator Dante Lauretta said, “The ‘REx’ in OSIRIS-REx stands for Regolith Explorer, so mapping and characterizing the surface of the asteroid was the main goal.”

“The spacecraft collected very high-resolution data for Bennu’s entire surface, which was down to 3 millimeters per pixel at some locations. Beyond scientific interest, the lack of fine regolith became a challenge for the mission itself because the spacecraft was designed to collect such material.”

Saverio Cambioni of the University of Arizona said, “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. We started using our machine learning approach to distinguish fine regolith from rocks using thermal emission (infrared) data.”

“Only machine learning could efficiently explore a dataset this large.”

After completing data analysis: scientists found something strange: the fine regolith was not randomly distributed on Bennu. Instead, it was up to several tens of percent in those very few areas where rocks are non-porous and systematically lower where rocks have higher porosity, which is most of the surface.

Study co-author Chrysa Avdellidou, a postdoctoral researcher at the French National Centre for Scientific Research (CNRS), said, “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.”

It was also found that the cracks on the surface are caused by the heating and cooling of Bennu’s rocks. When an asteroid rotates through day and night proceeds more slowly in porous rocks than in denser rocks, further frustrating the production of fine regolith.

The team predicts that large swaths of fine regolith should be uncommon on carbonaceous asteroids based on the finding. In contrast, terrains rich in fine regolith to be common on S-type asteroids.

Cambioni said“This is an important piece in the puzzle of what drives the diversity of asteroids’ surfaces. Asteroids are thought to be relics of the early solar system, so understanding the evolution they have undergone in time is crucial to comprehend how the solar system formed and evolved. Now that we know this fundamental difference between carbonaceous and S-type asteroids, future teams can better prepare sample collection missions depending on the nature of the target asteroid.”

Journal Reference:
  1. Cambioni, S., Delbo, M., Poggiali, G. et al. Fine-regolith production on asteroids controlled by rock porosity. Nature 598, 49–52 (2021). DOI: 10.1038/s41586-021-03816-5

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