Scientists thought Bennu’s surface was like a sandy beach, abundant in fine sand and pebbles, which would have been perfect for collecting samples. Past telescope observations from Earth had suggested 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 potentially capable of grinding boulders into fine regolith.
New research, published in Nature and led by Saverio Cambioni, of the University of Arizona, used machine learning and surface temperature data to solve the mystery. Cambioni conducted the research at the university’s Lunar and Planetary Laboratory. He and his colleagues ultimately found that Bennu’s highly porous rocks are responsible for the surface’s surprising lack of 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.”