Researchers solve the mystery of Bennu's lack of regolith

The NASA OSIRIS-REx mission ran into a surprising discovery when it visited asteroid Bennu in late 2018. Mission planners believed the asteroid's surface would have been abundant in fine sand and pebbles, similar to a sandy beach here on Earth. However, when the mission arrived, mission controllers discovered the surface of the asteroid was covered in boulders and lacked the fine regolith expected.The team was even more surprised when they observed on the asteroid's surface that there were processes that were potentially capable of grinding large boulders into the regolith. The research has been published by a team led by Saverio Cambioni from the University of Arizona. In the research, the team used machine learning and surface temperature data to determine why the surface of Bennu was so different than expected.

Researchers discovered that the highly porous rocks on the asteroid's surface were responsible for the lack of fine regolith. OSIRIS-REx captured high-resolution data for the entire surface of the asteroid down to a resolution of three millimeters per pixel in some locations. However, project researchers say when the first images from the asteroid were received in some areas, they lacked the resolution to see small rocks and fine regolith on the surface.

The team began using machine learning to distinguish fine regolith from rocks using thermal emission data. Thermal emission from fine regolith is different from the thermal emission of large rocks. They built a library of thermal emissions associated with fine regolith mixed with different portions of rock of various porosity. Machine learning allows the researchers to explore such a massive data set, allowing them to "connect the dots" between their examples.

Ultimately, project researchers discovered the regolith wasn't randomly distributed on the asteroid. They discovered it was several tens of percent more common in areas where rocks are non-porous than in areas with higher porosity. Less regolith occurs in areas with highly porous rock because when meteoroids impact the rock, it compresses rather than fragmenting.