Highly porous rocks are responsible for asteroid Bennu's surprisingly craggy surface

OSIRIS-REx mission researchers thought that sampling Bennu would be similar to walking on the beach. However, the unusually rocky surface proved more challenging. Credit: NASA/Goddard/University of Arizona
Scientists believed that Bennu's surface was a sandy beach with plenty of fine sands and pebbles. This would have made it ideal for sampling. Extensive telescope observations of Earth's orbit have indicated the existence of fine regolith, a fine-grain material that is smaller than a few millimeters.

The surface was covered with boulders when the OSIRIS-REx, a NASA-led OSIRIS asteroid sample return mission to Bennu, arrived on Bennu in 2018 by the University of Arizona-led OSIRIS NASA spacecraft. Mission scientists discovered evidence that boulders can be ground into fine regolith, making the mystery of Bennu's lack of fine regolith even more shocking.

SaverioCambioni, a member of the mission team, led new research that used machine learning and surface temperatures data to solve the mystery. Cambioni, who was a graduate student in the UArizona Lunar and Planetary Laboratory at the time of the research, is now a distinguished postdoctoral fellow in the Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology. His colleagues and he eventually discovered that Bennu's porous rocks were responsible for the surprising lack of fine regolith.

Dante Lauretta is a Regents Professor of Planetary Sciences and study co-author. "The 'REx" in OSIRIS–REx stands to represent Regolith Explorer. So mapping and characterizing asteroid's surface was a major goal," said Dante Lauretta. The spacecraft collected extremely high-resolution data on Bennu's entire surface. Some locations had a resolution of 3 millimeters per pixels. The mission was not only of scientific interest but also faced challenges due to the absence of fine regolith.

OSIRIS-REx was designed to collect samples to send back to Earth. It navigated within a Bennu area roughly equal to a 100-space parking garage. Due to numerous boulders, however, the safe sampling area was reduced to five parking spaces. In October 2020, the spacecraft made contact with Bennu and collected sample material.

Solid answers after a difficult start

"When we received the first images of Bennu, we noticed some areas where the resolution wasn't high enough to distinguish between small rocks and fine regolith. "We used our machine learning approach to seperate fine regolith and rocks using thermal emission (infrared data") data," Cambioni stated.

Fine regolith's thermal emission is distinct from larger rocks. This is because the size of the particles controls the temperature, while rock porosity regulates the temperature. First, the team created a database of examples of thermal emission associated with fine regolith in various proportions with different rocks of different porosity. Then, they used machine-learning techniques to teach a computer to "connect" the dots between the examples. Next, they used machine learning software to analyze thermal emission from 122 locations on Bennu's surface during day and night.

"Only a machine-learning algorithm could efficiently explore this dataset," Cambioni stated.

Cambioni and his colleagues discovered something unexpected when they completed the data analysis. The fine regolith wasn't distributed randomly on Bennu, but was rather lower in areas where rocks are more porous. This was the case for most of the surface.

Bennu's highly porous rock is compressed by meteoroid impact and produces very little fine regolith. The voids in rocks act as a sponge and absorb the impact of meteors. These results are in line with other research groups' laboratory experiments.

"Basically, a large part of energy from the impact goes into crushing pores restricting fragmentation of rocks and the production new fine regolith," explained Chrysa Avdellidou (study co-author), a postdoctoral researcher at France's National Centre for Scientific Research. She is a member of the Lagrange Laboratory of Cte d'Azur Observatory, University of France.

Cracking that occurs as Bennu's rock rotates through day/night is also more slow in porous rocks. This further frustrates the production of fine regolith.

Jason Dworkin (OSIRIS-REx project scientist, NASA Goddard Space Flight Center), stated that scientists will be able "to study the samples in detail" when OSIRIS–REx delivers Bennu's sample to Earth in September 2023. This includes testing the physical properties to confirm this study.

The team's findings are supported by evidence from other missions. Hayabusa 2 was a Japanese Aerospace Exploration Agency mission to Ryugu. Ryugu is a carbonaceous, similar asteroid to Bennu. It also found that Ryugu has a lack of fine regolith, and highly porous rocks. JAXA's 2005 Hayabusa mission was to Itokawa. Itokawa is an S-type asteroid and has fine regolith. Cambioni and colleagues previously found that Itokawa rocks are more porous than Bennu and Ryugu, using observations taken from Earth.

Astronomers have been disputing for decades that small, near Earth asteroids could have bare rock surfaces. Marco Delbo (research director at CNRS and also at the Lagrange Laboratory), provided the most convincing evidence that these asteroids might have significant fine regolith.

The team predicts that large areas of fine regolith will be rare on carbonaceous asteroids. These asteroid types are the most widespread and have high-porosity rocks such as Bennu. S-type asteroids are second most common and have more fine regolith than carbonaceous. They are also thought to have denser and less porous rocks.

This is an important piece of the puzzle that explains the diversity of asteroids’ surfaces. Cambioni stated that asteroids are believed to be fossils from the solar system. Understanding their evolution over time is essential to understand how the solar systems formed and evolved. Future teams will be able to better prepare sample collection missions based on the nature of target asteroid.

The University of Arizona is the science team leader for OSIRIS-REx. It also handles the science observation planning and data processing. NASA's Goddard Space Flight Center, Greenbelt (Maryland) provides mission management, systems engineering and safety and mission assurance for OSIRIS–REx. The spacecraft was built by Lockheed Martin Space, Littleton, Colorado. They also provide flight operations. Goddard and KinetX are responsible for the navigation of the OSIRIS–REx spacecraft. OSIRIS-REx, the third mission in NASA’s New Frontiers Program is managed by NASA’s Marshall Space Flight Center Huntsville, Alabama for the agency’s Science Mission Directorate, Washington, D.C.

More information: Saverio Cambioni and al, Fine-regolith formation on asteroids controlled rock porosity, Nature (2021). Information from Nature Saverio Cambioni et al. Fine-regolith production of asteroids controlled rock porosity (2021). DOI: 10.1038/s41586-021-03816-5