Science & Technology

Scientists surprised by the surface of the asteroid Bennu – this is the reason for the mysterious lack of wonderful regolith

OSIRIS-REx mission scientists thought sampling a portion of Bennu was like walking on the beach, but found the surprisingly rugged surface to be even more difficult.Credits: NASA / Goddard / University of Arizona

Use data from NASA OSIRIS-REx A mission, a team of scientists led by the University of Arizona, concluded that asteroids with highly porous rocks like Bennu should have no fine-grained material on their surface.

Scientists believed that the surface of the asteroid Bennu was sandy and rich in fine sand and pebbles, making it ideal for collecting samples. Past telescope observations from Earth’s orbit have suggested the existence of large bands of fine particles called fine regoliths smaller than a few centimeters.

However, when the NASA University of Arizona-led OSIRIS-REx asteroid sample-return mission spacecraft arrived in Bennu in late 2018, the mission team saw the surface covered with rocks. The mysterious lack of fine regoliths became even more surprising when mission scientists observed evidence of the process by which rocks could be crushed into fine regoliths.

New research published in Nature Led by a mission team member, Saverio Cambioni, he solved the mystery using machine learning and surface temperature data. Cambioni was a graduate student at the UArizona Lunar and Planetary Laboratory at the time of the study and is now a postdoctoral fellow at the Massachusetts Institute of Technology’s Faculty of Earth, Atmospheric and Planetary Sciences. He and his colleagues finally discovered that Bennu’s highly porous rocks were responsible for the surprising lack of surface fine regoliths.

“Since” REx “in OSIRIS-REx stands for Regolith Explorer, our main goals were to map and characterize the surface of the asteroid,” said Dante Lauretta, co-author of the study and senior researcher at OSIRIS-REx. (Professor of Planetary Science) said. University of Arizona. “The spacecraft has collected very high resolution data about the entire surface of Benne. This has dropped to 3 millimeters per pixel in some places. The spacecraft collects such material. Beyond scientific interest, the lack of fine-grained legoris became a challenge for the mission itself. “

To collect samples for returning to Earth, the OSIRIS-REx spacecraft was built to navigate within the area of ​​Bennu, the size of a parking lot for approximately 100 cars. However, due to the large amount of rocks, the safe sampling site has been reduced to approximately five parking spaces. The spacecraft successfully contacted Benne in October 2020 to collect sample material.

Rocky start and solid answer

“When the first image of Benne came in, I noticed an area that wasn’t high enough resolution to see if there were small rocks or fine regoliths. Using a machine learning approach, thermal radiation (infrared) We started using the data to separate fine regoliths from rocks, “Cambioni said.

The heat radiation from a fine regolith is different from the heat radiation from a large rock. The former is controlled by the size of the particles and the latter by the porosity of the rock. The team first built a library of examples of heat release associated with fine regoliths mixed in different porosity rocks and in different ratios. Then they used machine learning techniques to teach computers how to “connect dots” between examples. Next, machine learning software was used to analyze thermal radiation from 122 regions of Benne’s surface observed both day and night.

“Only machine learning algorithms can efficiently explore such large datasets,” says Cambioni.

When the data analysis was complete, Cambioni and his collaborators discovered something amazing. The fine regoliths were not randomly distributed in Bennu, but instead the rocks were more porous and lowered where they were on most of the surface.

The team concluded that there are few fine regoliths produced by Benne’s highly porous rocks, as these rocks are compressed rather than fragmented by the impact of meteoroids. Like a sponge, the rock crevices soften the impact of incoming meteors. These findings are consistent with laboratory experiments by other research groups.

“Basically, most of the impact energy is spent crushing the pores that limit the fragmentation of rocks and the formation of new fine regoliths,” said the study’s co-author, the Center National de la Recherche Scientifique. Chrisa Avdelidu, a postdoctoral researcher at the Research Center, said. Study (CNRS) – Côte d’Azur Observatory in France and Lagrange Institute at the University.

In addition, the cracks caused by the heating and cooling of Bennu’s rocks as the asteroid rotates day and night progress more slowly in porous rocks than in denser rocks, producing fine regoliths. It’s even more frustrating.

Jason Dwarkin, OSIRIS-REx Project Scientist at NASA Goddard Space Flight Center, said: “This includes testing the physical properties of the rock to validate this study.”

Other missions have evidence to confirm the team’s findings. The Hayabusa2 mission of the Japan Aerospace Exploration Agency to Ryugu, a carbonic asteroid like Bennu, revealed that Ryugu also lacks fine regoliths and has very porous rocks. vice versa, JAXAIn the Hayabusa mission to Itokawa in 2005, fine regoliths, which are S-type asteroids with rocks of different composition from Bennu and Ryugu, were found abundantly on the surface of Itokawa. Previous studies by Cambioni and his colleagues have used observations from Earth to provide evidence that Itokawa rocks are less porous than Bennu and Ryugu rocks.

“For decades, astronomers have argued that small near-Earth asteroids may have bare rock surfaces. Spacecraft visited the S-type asteroids Eros and Itokawa in the 2000s, and they The most obvious evidence that these small asteroids could have fairly fine legoris emerged when they discovered the fine legoris on their surface, “said CNRS, research director, co-author of the study. Marco Delvo also states. At the Lagrange Institute.

The team predicts that large bands of fine regoliths should be rare on carbonic asteroids, which are the most common of all asteroid types and are thought to have highly porous rocks like Bennu. doing. In contrast, fine regolith-rich terrain is common with S-type asteroids, the second most common group in the solar system, with denser, less porous rocks than carbonic asteroids. It is believed that there is.

“This is an important part of the puzzle of what promotes asteroid surface diversity. Understanding that asteroids have evolved over time is an important part of the puzzle, as asteroids are considered to be fossils of the solar system. It’s important to understand how the solar system was formed and evolved, “says Cambioni. “Now that we know this fundamental difference between carbonaceous and S-type asteroids, future teams can better prepare sample collection missions, depending on the nature of the target asteroid.”

Reference: S. Cambioni, M. Delbo, G. Poggiali, C. Avdellidou, AJ Ryan, JDP Deshapriya, E. “Generation of fine regoliths on asteroids controlled by rock porosity” by Asphaug, R.-L Ballouz, MA Barucci, CA Bennett, WF Bottke, JR Brucato, KN Burke, E. Cloutis, DN DellaGiustina, JP Emery, B. Rozitis, KJ Walsh, DS Lauretta, October 6, 2021, Nature.
DOI: 10.1038 / s41586-021-03816-5

The University of Arizona leads the OSIRIS-REx science team and mission scientific observation planning and data processing. NASA’s Goddard Space Flight Center in Greenbelt, Maryland provides OSIRIS-REx’s overall mission management, system engineering, and safety and mission assurance. Lockheed Martin Space, located in Littleton, Colorado, manufactures spacecraft and provides flight operations. Goddard and KinetX Aerospace are responsible for navigating the OSIRIS-REx spacecraft. OSIRIS-REx is NASA’s third mission in the New Frontiers program and is managed by NASA’s NASA Marshall Space Flight Center in Huntsville, Alabama.

Scientists surprised by the surface of the asteroid Bennu – this is the reason for the mysterious lack of wonderful regolith Scientists surprised by the surface of the asteroid Bennu – this is the reason for the mysterious lack of wonderful regolith

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