Highly porous rocks responsible for Bennu’s surprisingly steep surface
By Mikayla Mace Kelley, University Communications
Scientists believed the surface of Asteroid Bennu would resemble a sandy beach, abundant in fine sand and pebbles, which would have been perfect for collecting samples. Previous telescope observations from Earth’s orbit had suggested the presence of broad bands of fine-grained material called fine regoliths, smaller than a few centimeters in size.
But when the spacecraft from NASA’s University of Arizona asteroid sample return mission OSIRIS-REx arrived in Bennu in late 2018, the mission team saw a rock-covered surface. The mysterious lack of fine regolith became even more surprising when mission scientists observed evidence of processes capable of crushing boulders into fine regolith.
New research, published in Nature and led by mission team member Saverio Cambioni, used machine learning and surface temperature data to solve the mystery. Cambioni was a graduate student of the Arizona Lunar and Planetary Laboratory when the research was conducted and is now a Distinguished Postdoctoral Fellow in the Department of Earth, Atmospheric, and Planetary Sciences at the Massachusetts Institute of Technology. He and his colleagues eventually discovered that Bennu’s highly porous rocks were responsible for the surprising lack of fine regolith on the surface.
“The ‘REx’ in OSIRIS-REx stands for Regolith Explorer, so mapping and characterizing the asteroid’s surface was a primary focus,” said study co-author and OSIRIS-REx principal investigator. Dante Laurette, a Regents professor of planetary science at the University of Arizona. “The spacecraft collected very high-resolution data for Bennu’s entire surface, which had gone down to 3 millimeters per pixel in some places. Beyond scientific interest, the lack of fine regolith became a challenge for the mission itself, because the spacecraft was designed to collect such material. “
To collect a sample to return to Earth, the OSIRIS-REx spacecraft was built to navigate an area in Bennu roughly the size of a 100-space parking lot. However, due to numerous boulders, the safe sampling site was reduced to approximately the size of five parking spaces. The spacecraft managed to make contact with Bennu to collect material samples in October 2020.
A difficult start and solid responses
“When the first images of Bennu arrived, we noticed some areas where the resolution was not high enough to see if there were small rocks or a fine regolith. We have started using our machine learning approach to separate fine regolith from rocks using thermal emission (infrared) data, ”Cambioni said.
The thermal emission of fine regolith is different from that of larger rocks, as the former is controlled by the size of its particles, while the latter is controlled by the porosity of the rock. The team first built a library of examples of thermal emissions associated with fine regoliths mixed in different proportions with rocks of varying porosity. Then, they used machine learning techniques to teach a computer how to “connect the dots” between the examples. Then, they used machine learning software to analyze thermal emission from 122 areas on Bennu’s surface observed both during the day and at night.
“Only a machine learning algorithm could effectively explore such a large data set,” Cambioni said.
Once the data analysis was complete, Cambioni and his collaborators found something surprising: the fine regolith was not distributed randomly over Bennu but was rather lower where the rocks were more porous, i.e. – say over most of the surface.
The team concluded that very little fine regolith is produced by the very porous rocks of Bennu because these rocks are compressed rather than fragmented by meteorite impacts. Like a sponge, voids in rocks cushion the blow of incoming meteors. These results are also in agreement with the laboratory experiments of other research groups.
“Basically a lot of the impact energy goes to crush pores limiting rock fragmentation and the production of new fine regoliths,” said study co-author Chrysa Avdellidou, postdoctoral researcher at the Center French national of science. Research (CNRS) – Lagrange Laboratory of the Observatory and University of the Côte d’Azur in France.
Additionally, the cracking caused by the heating and cooling of Bennu’s rocks as the asteroid rotates day and night occurs more slowly in porous rocks than in denser rocks, further frustrating the production of fine regolith.
“When OSIRIS-REx delivers its sample to Bennu (to Earth) in September 2023, scientists will be able to study the samples in detail,” said Jason Dworkin, OSIRIS-REx project scientist at NASA’s Goddard Space Flight Center. “This includes testing the physical properties of rocks to verify this study.”
Other missions have evidence to confirm the team’s findings. The Japan Aerospace Exploration Agency’s Hayabusa 2 mission to Ryugu, a carbonaceous asteroid like Bennu, found that Ryugu also lacked fine regolith and had very porous rocks. Conversely, JAXA’s Hayabusa mission to the asteroid Itokawa in 2005 revealed an abundance of fine regolith on the surface of Itokawa, an S-type asteroid with rocks of a different composition than Bennu and Ryugu. A previous study by Cambioni and his colleagues proved that the Itokawa rocks are less porous than those of Bennu and Ryugu, using observations from Earth.
“For decades, astronomers have contested that small asteroids close to Earth could have bare rocky surfaces. The most indisputable evidence that these small asteroids could have substantial fine regolith appeared when spacecraft visited asteroids in type S Eros and Itokawa in the 2000s and found regolith on their surfaces, ”said study co-author Marco Delbo, research director at CNRS, also at the Lagrange laboratory.
The team predicts that large bands of fine regolith should be rare on carbonaceous asteroids, which are the most common of all asteroid types and are believed to have high porosity rocks like Bennu. In contrast, terrains rich in fine regolith should be common on S-type asteroids, which are the second most common group in the solar system, and are believed to have denser and less porous rocks than carbonaceous asteroids. .
“This is an important piece in the puzzle of what determines the diversity of asteroid surfaces. Asteroids are believed to be fossils of the solar system, so understanding the evolution they have undergone over time is crucial to understanding how the solar system formed and evolved, ”Cambioni said. “Now that we know this fundamental difference between carbonaceous and S-type asteroids, future teams will be able to better prepare sample collection missions depending on the nature of the target asteroid.”
The University of Arizona is leading the OSIRIS-REx science team and planning the scientific observation and data processing of the mission. NASA’s Goddard Space Flight Center in Greenbelt, Maryland provides overall mission management, systems engineering, and mission security and assurance for OSIRIS-REx. Lockheed Martin Space in Littleton, Colorado, built the spacecraft and operates flight operations. Goddard and KinetX Aerospace are responsible for the navigation of the OSIRIS-REx spacecraft. OSIRIS-REx is the third mission in NASA’s New Frontiers program, managed by NASA’s Marshall Space Flight Center in Huntsville, Alabama, for the agency’s Science Missions Directorate in Washington, DC