NASA brought samples from the Bennu asteroid to Earth: the finding that baffled scientists

The exploration of carbonaceous asteroids, a class of celestial bodies rich in carbon, poses fundamental challenges for planetary science. These objects, considered remnants of the early formation of the solar system, can offer clues about the origins of the planets and about the impact risks for Earth. When the OSIRIS-REx mission arrived at Bennu, an asteroid 500 meters in diameter, it surprised experts with a surface full of bumps and large blocks, in contrast to the previous expectations of finding smooth and regular areas. The scientific community could not explain why it presented such a marked and rugged relief.

A study published in Nature Communications, conducted by teams from the University of Arizona, the NASA Johnson Space Center, and Nagoya University, analyzed samples brought to Earth by the NASA mission. This research allowed for a detailed observation of the internal structure of the asteroid and provided a new insight into how these bodies behave in space.

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The True Face of Bennu's Surface

For decades, astronomers analyzed the composition and structure of these asteroids through remote observations, guided mainly by thermal inertia, a measure of how quickly the surface of an asteroid absorbs and releases heat. This physical characteristic became essential for interpreting the nature of asteroids.

Before OSIRIS-REx arrived at Bennu, telescopes like the Spitzer Space Telescope had detected a low thermal inertia on the asteroid. In simple terms, this means that the surface heats up and cools down quickly, a behavior similar to that of sand. Therefore, it was expected to find a layer of small loose grains.

The reality turned out to be very different. The images from OSIRIS-REx showed that Bennu is almost completely covered in large blocks of rock, with very few smooth sectors. The published article explains that “the surface of Bennu is covered by blocks of different physical properties, with the most abundant population exhibiting very low thermal inertia compared to carbonaceous meteorites”. This means that, although they expected dense rocks and compact, many turned out to be surprisingly porous and, above all, very cracked. The analysis of the samples in the laboratory allowed to distinguish three main types of particles: the hummocky, which have a rough surface and rounded shape; the angular, which have flat faces and greater density; and the mottled, less common and covered with shiny minerals. According to the authors of the study, the angular particles "present greater thermal inertia, greater hardness and fewer cracks, although these are longer and favor a more efficient fragmentation". The hummocky particles, on the other hand, have "a tortuous network of cracks that reduces thermal inertia and hinders disintegration". The study states that the low thermal inertia of the Bennu asteroid is due to cracks in the rocks, resulting from internal geological processes or, more recently, from impacts of micrometeorites and thermal fatigue. In other words, the surface rocks are full of fissures formed by the passage of time, the collision with small particles, and the extreme temperature changes in space.

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How the inside of space rocks was revealed

The possibility of analyzing materials brought directly from Bennu represented an advance for science. To investigate the internal structure of the fragments, the team applied techniques of X-ray computed tomography (XCT), which allows seeing inside objects in three dimensions, and laser thermography, which helps to measure how heat is transmitted through the materials. "The thermal inertia measured in the laboratory samples turned out to be much greater than that recorded by the spacecraft's instruments, which coincides with similar findings obtained by the OSIRIS-REx associated mission team, the Hayabusa-2 of JAXA (Japan Aerospace Exploration Agency)" highlighted Andrew Ryan, scientist at the Lunar and Planetary Laboratory of the University of Arizona, principal investigator of the study, in an official statement. This means that, when observing the small fragments on Earth, the measurements changed compared to those taken on the asteroid's surface, which led scientists to investigate how to scale these results to larger rocks. During the procedure, the researchers used a special box at the NASA Johnson Space Center to avoid any terrestrial contamination. Scientist Nicole Lunning explained that “the sample enters its own ‘space suit’, receives a tomography and then returns to its pristine environment, all without exposure to the terrestrial environment”. The images obtained allowed the creation of digital files of each particle and, with computer simulations, scientists managed to reproduce the thermal behavior of Bennu's large rocks. Thus, they confirmed that the networks of cracks, visible in the samples, explain the strange ability of the surface to lose heat rapidly. Understanding this process is key because the way an asteroid absorbs and emits heat affects its evolution, its resistance to impacts, and its possible trajectory deviation, fundamental aspects for anticipating collision risks with Earth and for planning exploration or deviation missions.

What these findings show about the origin and evolution of asteroids

Identifying that Bennu's low thermal inertia is largely due to internal cracks allows us to understand both the formation of the asteroid and its potential risks to Earth. Based on the data obtained, models on the resistance and evolution of the surface of asteroids can become more precise, allowing for a better prediction of how an asteroid would fragment or disintegrate when interacting with the atmosphere or during an attempt to divert it. This is relevant for designing planetary defense strategies in the event of a possible collision.

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As co-author Ron Ballouz explained, “we can finally ground our understanding of the telescopic observations of the thermal properties of an asteroid by analyzing these samples from that same asteroid.” Furthermore, the analysis of porosity and cracks sheds light on processes that occurred in Bennu's parent asteroid, such as water alteration. Comparing these fragments with meteorites and other asteroids helps reconstruct the early history of the solar system and the formation of planets.