Cracked Asteroid Bennu: How Hidden Fractures Explained a Thermal Mystery

Bennu’s Boulders and a Rethinking of Asteroid Surfaces

For years, scientists have studied asteroids from afar, building expectations about their surfaces based on telescope data. But NASA’s OSIRIS-REx mission to the near-Earth asteroid Bennu revealed a startling discrepancy: what appeared to be a relatively smooth, sandy surface was, in reality, a chaotic landscape dominated by large boulders. This unexpected finding isn’t just a matter of aesthetics; it’s forcing researchers to re-evaluate how they interpret data from these distant worlds and understand how asteroids store and release heat.

The OSIRIS-REx spacecraft arrived at Bennu in 2018, tasked with collecting a sample of the asteroid’s surface material. Initial observations from Earth-based telescopes had suggested a mix of rocks and smoother areas, potentially including patches of fine-grained regolith – the loose surface material found on many airless bodies. Still, the reality was far different. Instead of easily accessible sampling sites, the spacecraft encountered a surface almost entirely covered in boulders, presenting a significant challenge to the mission’s sample collection plans. As Andrew Ryan, a scientist with the University of Arizona Lunar and Planetary Laboratory, explained, “We expected some boulders, but we anticipated at least some large regions with smoother, finer regolith that would be easy to collect. Instead, it looked like it was all boulders, and we were scratching our heads for a while.”

Thermal Inertia: A Conflicting Signal

The puzzle deepened when scientists compared the visual observations with earlier thermal data. Thermal inertia refers to a material’s ability to resist changes in temperature. A low thermal inertia indicates that a surface heats up and cools down quickly, characteristic of loose, fine-grained material like sand. Previous observations of Bennu, conducted by NASA’s Spitzer Space Telescope in 2007, had indicated a low thermal inertia. This suggested a surface composed of small particles. However, the presence of large boulders, which should retain heat much longer like concrete, contradicted this finding. The spacecraft’s instruments registered a thermal signature that didn’t align with the observed rocky terrain.

The key to resolving this discrepancy lay in the internal structure of the boulders themselves. After the OSIRIS-REx mission successfully returned samples to Earth in September 2023, scientists were able to conduct detailed laboratory analyses. These analyses revealed that the rocks weren’t as dense as expected; they were riddled with cracks, and pores. This internal structure dramatically altered how heat moved through the rocks, explaining the unexpected thermal readings.

Cracked Interiors and Heat Transfer

Researchers hypothesized that the porous nature of the rocks allowed heat to escape more rapidly than anticipated, even within the larger boulders. To test this idea, they employed a technique called lock-in thermography. This method involves heating a small spot on a sample with a laser and then tracking how the heat dissipates. The results confirmed their suspicions: the thermal inertia of the lab samples was significantly higher than what the spacecraft had initially measured on Bennu’s surface. This finding echoed similar observations made by Japan’s Hayabusa-2 mission during its exploration of the asteroid Ryugu. NASA’s OSIRIS-REx mission page provides further details on the mission’s objectives and findings.

To bridge the gap between the small-scale lab measurements and the large-scale observations from the spacecraft, scientists used advanced imaging techniques, including X-ray computed tomography (CT) scans. These scans allowed them to visualize the internal structure of the rocks in three dimensions without causing any damage. Nicole Lunning, the lead OSIRIS-REx sample curator at NASA’s Johnson Space Center, described the meticulous process of preserving the samples: “The sample goes into its own ‘spacesuit,’ gets a CT scan, and then comes back to its pristine environment, all without having any exposure to the terrestrial environment.” The CT scans revealed a network of cracks running throughout the rocks, confirming the porous structure.

Using computer models, researchers were able to simulate heat transfer through rocks with these internal cracks. Scaling up the models to represent the size of the boulders on Bennu, they found that the cracked interiors allowed heat to escape quickly enough to explain the low thermal inertia readings. “It turns out that they’re really cracked too, and that was the missing piece of the puzzle,” Ryan stated.

Implications Beyond Bennu

This discovery has significant implications for how scientists interpret data from asteroids and other airless bodies. For years, thermal measurements have been used to infer surface properties, such as grain size and composition. However, the Bennu findings demonstrate that these measurements can be influenced by hidden internal structures, like cracks and pores. This means that previous interpretations based solely on thermal data may require to be revisited. SciTechDaily’s coverage of the study highlights the impact of this discovery on our understanding of asteroid surfaces.

Ron Ballouz, a scientist with the Johns Hopkins University Applied Physics Laboratory, emphasized the transformative nature of this work: “We can finally ground our understanding of telescope observations of the thermal properties of an asteroid through analyzing these samples from that very same asteroid.” This ability to connect remote observations with direct sample analysis will be crucial for future asteroid exploration missions.

What Comes Next: Refining Asteroid Models

The findings from the Bennu samples are already being incorporated into new models of asteroid surfaces. These models will account for the effects of internal cracking and porosity on thermal inertia, leading to more accurate interpretations of remote sensing data. Further research will focus on understanding the processes that create these cracks in asteroids – whether they are the result of impacts, thermal stress, or other factors. The full study detailing these findings was published in the journal Nature Communications.

The OSIRIS-REx spacecraft, now renamed OSIRIS-APEX, is continuing its journey through the solar system, with a planned encounter with asteroid Apophis in 2029. Wikipedia’s OSIRIS-REx page provides a comprehensive overview of the mission’s history and future plans. This next mission will provide further opportunities to test and refine our understanding of asteroid surfaces and their thermal properties, building on the valuable lessons learned from Bennu.