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This image shows four views of the asteroid Bennu along with a corresponding global mosaic. The images were taken on December 2, 2018 by the OSIRIS-REx probe’s PolyCam camera, which is part of the OCAMS suite of tools designed by Arizona scientists and engineers. Credits: NASA / Goddard / University of Arizona
The impact craters left by space debris in the boulders on the bumpy surface of asteroid Bennu allowed researchers to reconstruct the history of the near-Earth object in unprecedented detail.
By studying the impact marks on the surface of the asteroid Bennu – the goal of NASA‘S OSIRIS-REx mission – a team of researchers led by the University of Arizona uncovered the asteroid’s past and revealed that, despite having formed hundreds of millions of years ago, Bennu has only recently wandered into the vicinity of Earth.
The study, published in the journal Nature, provides a new benchmark for understanding the evolution of asteroids, offers insights into a little-known population of space debris dangerous to spacecraft, and improves scientists’ understanding of the solar system.
The researchers used laser-based images and measurements acquired during a two-year survey phase in which the 15-passenger van-sized OSIRIS-REx spacecraft orbited Bennu and broke the space probe record. smaller in orbit around a small body.
Presented at the opening day of the American Astronomical Society’s Division for Planetary Sciences meeting on October 26, the paper details the first observations and measurements of impact craters on individual boulders on an airless planetary surface since the Apollo missions on the moon 50 years ago, according to the authors.
The publication comes just days after an important milestone for NASA’s OSIRIS-REx mission led by the University of Arizona. On October 20, the spacecraft successfully descended on asteroid Bennu to grab a sample from its boulder-strewn surface, a first for NASA. The sample has now been successfully stored and will be returned to Earth for study in 2023, where it could provide scientists with information on the early stages of the formation of our solar system.
The impact craters on the rocks tell a story
Although the Earth is hit by more than 100 tons of space debris every day, it is virtually impossible to find a rocky surface hit by impacts of small objects at high speed. Courtesy of our vibe, we can enjoy any object smaller than a few feet as a shooting star rather than having to fear being hit by what essentially amounts to a bullet from outer space.
Planetary bodies without such a protective layer, however, bear the full weight of a perpetual cosmic barrage, and have the scars to prove it. The high-resolution images taken by the OSIRIS-REx spacecraft during its two-year survey campaign allowed the researchers to study even tiny craters, with diameters ranging from one centimeter to one meter, on the Bennu boulders.
This composite image of a boulder on the surface of Bennu shows the cascading edge of one of the asteroid’s ancient craters that originated while Bennu resided in the asteroid belt. The image combines photos of OSIRIS-REx and reconstructed shape models built by the OSIRIS-REx laser altimeter instrument. The overlapping colors highlight the topography of the boulder (warmer colors are higher elevations). Credits: University of Arizona / Johns Hopkins APL / York University
The team found that boulders 1 meter or larger were scarred, on average, from one to 60 pits, hit by space debris ranging in size from a few millimeters to tens of centimeters.
“I was surprised to see these features on Bennu’s surface,” said lead author Ronald Ballouz, a postdoctoral researcher at the Arizona Lunar and Planetary Laboratory and a scientist with the OSIRIS-REx working group. on the development of regolith. “The rocks tell their story through the craters accumulated over time. We haven’t seen anything like this since astronauts walked on the moon. “
For Ballouz, who grew up in the 1990s in postwar Beirut, Lebanon, the image of a rocky surface dotted with small impact craters evoked childhood memories of building bullet-hole riddled walls in his home country ravaged by war.
“Where I grew up, buildings have bullet holes all over them, and I never thought about them,” he said. “It was just a fact. So when I looked at the images of the asteroid, I was very curious and immediately thought it was impact characteristics. “
The observations made by Ballouz and his team bridge a gap between previous studies of space debris larger than a few centimeters, based on impacts on the moon, and studies of objects smaller than a few millimeters, based on observations of meteors entering the atmosphere. land and spacecraft impacts.
“The objects that formed the craters on the Bennu boulders fall into this space that we don’t know much about,” Ballouz said, adding that rocks in that size range are an important field of study, mainly because they pose hazards to vehicles. spacecraft orbiting the Earth. “An impact from one of these millimeter to centimeter-sized objects at speeds of 45,000 miles per hour can be dangerous.”
Ballouz and his team have developed a technique to quantify the strength of solid objects using remote observations of craters on the surface of boulders: a mathematical formula that allows researchers to calculate the maximum impact energy that a boulder of a given size and strength it could endure before being smashed. In other words, the distribution of the crater on Bennu today preserves a historical record of the frequency, size and speed of impact events that the asteroid has undergone throughout its history.
“The idea is actually quite simple,” Ballouz said, using a building exposed to artillery fire as an analogy to boulders on an asteroid. “We ask, ‘What is the largest crater you can create on that wall before the wall disintegrates?’ Based on observations of multiple walls of the same size, but with craters of different sizes, you can get an idea of the strength of that wall. “
The same goes for a boulder on an asteroid or other airless body, said Ballouz, who added that the approach could be used on any other asteroid or airless body that astronauts or spacecraft might visit in the future.
“If a boulder is hit by something larger than an object that would leave a certain size, it would simply disappear,” he explained. In other words, the size distribution of the boulders that have persisted on Bennu serve as silent witnesses to its geological past.
A newcomer to Earth’s neighborhood
By applying the technique to boulders of varying sizes, from cobblestones to parking lots, the researchers were able to make inferences about the size and type of impactors the boulders were exposed to and for how long.
The authors conclude that the largest craters on the Bennu boulders were created while Bennu resided in the asteroid belt, where impact velocities are slower than in the near-Earth environment, but are more frequent and often near the limit of what the boulders could bear. Smaller craters, on the other hand, were acquired more recently, during the Bennu period in near-Earth space, where impact speeds are higher but potentially destructive impactors are far less common.
Based on these calculations, the authors determine that Bennu is a relatively newcomer to Earth’s vicinity. Although it is thought to have formed in the main asteroid belt more than 100 million years ago, it is estimated that it was ejected from the asteroid belt and migrated to its current territory only 1.75 million years ago. Extending the results to other near-Earth objects, or NEOs, the researchers also suggest that these objects likely come from parent bodies that fall into the category of asteroids, which are mostly rocky with little or no ice, rather than comets, which have more ice than rock.
While theoretical models suggest that the asteroid belt is the reservoir for the NEOs, there was no observational evidence of their provenance other than the meteorites that fell to Earth and were collected, Ballouz said. With this data, researchers can validate their patterns of NEO provenance, according to Ballouz, and get an idea of how strong and solid these objects are – crucial information for any potential future asteroid-targeting mission for research, the resource extraction or Earth protection from impact.
Reference: “Life near Bennu Earth of 1.75 million years deduced from the craters on its boulders” by R.-L. Ballouz, KJ Walsh, OS Barnouin, DN DellaGiustina, M. Al Asad, ER Jawin, MG Daly, WF Bottke, P. Michel, C. Avdellidou, M. Delbo, RT Daly, E. Asphaug, CA Bennett, EB Bierhaus, HC Connolly Jr, DR Golish, JL Molaro, MC Nolan, M. Pajola, B. Rizk, SR Schwartz, D. Trang, CWV Wolner and DS Lauretta, 26 October 2020, Nature.
DOI: 10.1038 / s41586-020-2846-z
Funding: NASA, French Space Agency, European Union Horizon 2020 Research and Innovation Program, Academies of Excellence: Complex Systems and Space, Environment, Risk, Resilience, Italian Space Agency
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