Scientists figure out why asteroid Bennu has rocks instead of regolith

(NASA/University of Arizona)

The almost complete absence of small regolith on asteroid Bennu may be due to the high porosity of the rocks, which are better able to withstand cracking or impacts of meteoroids.

This conclusion was reached by scientists who analyzed the data of spectroscopic observations of the asteroid station OSIRIS-REx.

Near-Earth 500-meter asteroid (101955) Bennu became one of the few small bodies in the solar system, which was studied in detail using a spacecraft, and the second asteroid from which a soil sample was taken.

The research was carried out by the OSIRIS-REx mission, which for 2.5 years of work mapped the asteroid, determined its properties, found out that Bennu was active

The probe also found fragments of the asteroid West and organic matter on its surface, and traced the course of its evolution and collected 400 grams its substance, which in 2023 will deliver to Earth.

Now, a group of planetary scientists led by Saverio Cambioni from the University of Arizona decided to figure out why Bennu's surface is practically devoid of small regolith, but is strewn with boulders of various sizes.

For this purpose, they analyzed infrared spectra from the asteroid's surface, obtained at different local times of the day for 122 sites on Bennu between April and June 2019.

These data make it possible to relate the surface temperature to its roughness and thermal inertia.

To distinguish shallow regolith from rocks at Bennu, scientists have applied machine learning techniques to possible combinations of shallow regolith and rock spectrum based on surface content, roughness, and thermal inertia.

Scientists have found an inverse correlation between the local content of regolith particles less than a centimeter in size and the porosity of rocks.

They concluded that the accumulation of regolith is ineffective where the rocks are highly porous.

In this case, falling meteoroids cannot crush them strongly, and thermal cracking occurs more slowly than in denser rocks.

The researchers concluded that regolith deposits are rarely found on carbonaceous asteroids (C-type), which are the most abundant type of asteroids

However, regolith will be abundant on rocky asteroids (S-type), which are the second most abundant group of asteroids.

Sources:

• Nature: https://www.nature.com/articles/s41586-021-03816-5
• NASA: https://www.nasa.gov/feature/goddard/2021/nasa-mission-helps-solve-a-mystery-why-are-some-asteroid-surfaces-rocky/

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