NASA Mission helps solve a mystery: why are some asteroid surfaces rocky?

Mon, 25 Oct 2021 01:47:17 GMT
Space Daily

Tucson AZ (SPX) Oct 25, 2021 Scientists thought Bennu's surface was like a sandy beach, abundant in...

The mysterious lack of fine regolith became even more surprising when mission scientists observed evidence of processes potentially capable of grinding boulders into fine regolith.

"The 'REx' in OSIRIS-REx stands for Regolith Explorer, so mapping and characterizing the surface of the asteroid was a main goal," said study co-author and OSIRIS-REx Principal Investigator Dante Lauretta, a Regents Professor of Planetary Sciences at the University of Arizona.

"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."

A Rocky Start and Solid Answers "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 data," Cambioni said.

The thermal emission from fine regolith is different from that of larger rocks, because the size of its particles controls the former, while the latter is controlled by rock porosity.

Cambioni and colleagues showed that 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.

The Japan Aerospace and Exploration Agency Hayabusa2 mission to Ryugu, a carbonaceous asteroid like Bennu, found that Ryugu also lacks fine regolith and has high-porosity rocks.

Conversely, JAXA's Hayabusa mission in 2005 revealed abundant fine regolith on the surface of asteroid Itokawa, an S-type asteroid with rocks of a different composition than Bennu and Ryugu.

"The most indisputable evidence that these small asteroids could have substantial fine regolith emerged when spacecraft visited S-type asteroids Eros and Itokawa in the 2000s and found fine regolith on their surfaces."

The team predicts that large swaths of fine regolith should be uncommon on carbonaceous asteroids, the most common of all asteroid types observed, and which the team expects to have high-porosity rocks like Bennu.

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