Asteroid Bennu’s Surface Contains Cracked Boulders Instead of Smooth Beaches, Making It Rapidly Lose Heat
Recent inspections of Asteroid Bennu have revealed many surprises, all wrapped up in a rugged blend of space boulders. Initially, most of Bennu’s surface was thought to be almost beach-like in appearance, with smooth patches of sand and gravel scattered about. But as NASA’s OSIRIS-REx mission would find in 2018, the asteroid turned out to be much rockier than anticipated.
A new study published in Nature Communications has discovered that Bennu’s surface, laden with boulders, loses heat in an unexpected way. Bennu is known to have low thermal inertia, meaning it heats up and cools down rapidly as it rotates in and out of sunlight. While the porous nature of Bennu’s boulders partially explains this outcome, an extensive network of cracks is the main reason why the asteroid loses heat so rapidly.
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Boulders, Not Beaches
Samples from Asteroid Bennu, taken by the OSIRIS-REx spacecraft in October 2020, made it to Earth in 2023. Scientists have been closely studying them since then, believing they could provide key information about the theorized delivery of life-forming compounds to Earth billions of years ago.
According to NASA, researchers uncovered a rich assortment of life’s building blocks within Bennu samples in 2025, including 14 of the 20 amino acids that life on Earth uses to make proteins and the five nucleobases that life uses to store and transmit genetic information.
But in addition to Bennu’s compounds, the rocky texture of its surface has also piqued researchers’ interest.
“When OSIRIS-REx got to Bennu in 2018, we were surprised by what we saw,” said Andrew Ryan, a scientist with the University of Arizona Lunar and Planetary Laboratory, in a statement. “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.”
Bennu’s Crack Networks
Bennu’s thermal inertia was found to be exceptionally low based on observations made in 2007 by NASA’s now-deactivated Spitzer Space Telescope. But Bennu’s boulders, researchers thought, should promote gradual heat loss, not the rapid heat loss that Bennu was actually experiencing.
Data collected during the OSIRIS-REx mission suggested that the boulders could be porous, and while this proved true, it didn’t tell the whole story. In the new study, researchers examined rock particles of samples and found that cracks in the boulders drive most of the heat loss.
The researchers confirmed this by subjecting the sample material to lock-in thermography, which involves illuminating a tiny spot on the sample’s surface with a laser to measure how heat diffuses through it.
They also determined that the type of particle plays a role in heat loss; rough “hummocky” particles have lower thermal inertia and larger networks of cracks, whereas smoother “angular” particles have higher thermal inertia and fewer but longer cracks. The cracks, researchers propose, may have been caused by impacts from smaller micrometeoroids or prior geological processes within Bennu’s parent body.
A Spacesuit for Samples
To scale up the sample measurements and predict how larger boulders on Bennu might behave, the researchers used a glove box to seal the particles in air-tight containers under a protective nitrogen atmosphere before scanning them with X-ray computed tomography.
“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,” said co-author Nicole Lunning, a sample curator at NASA Johnson Space Center. “We can image right through these airtight containers to visualize the shape and internal structure of the rock that’s inside.”
The researchers modeled heat flow and thermal inertia using data from these scans, and the results aligned with measurements taken by the OSIRIS-REx mission in space. Similar work may help to interpret thermal properties of other asteroids in the future.
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