“When OSIRIS-REx arrived at Bennu in 2018, we were surprised by what we saw,” said Andrew Ryan, a scientist at the University of Arizona Lunar and Planetary Institute who led the mission’s working group responsible for analyzing the physical and thermal properties of the samples. “We expected there to be some rocks, but at least in some large areas we expected smoother, finer regolith that would be easier to collect. Instead, everything seemed to be rocks, which puzzled us for a while.”

The discovery was especially confusing because astronomers had previously learned that Bennu could heat up and cool down rapidly. Measurements by NASA’s Spitzer Space Telescope in 2007 showed that the asteroid’s surface changes temperature quickly as it rotates between sunlight and darkness.

Scientists describe this property as low thermal inertia. Materials with low thermal inertia, such as beach sand, gain heat quickly in sunlight and lose it rapidly when the Sun disappears. Larger, harder rocks behave differently. Like concrete, they typically retain heat and release it more gradually.

Bennu’s rocky surface appeared to contradict those earlier measurements, raising a central question: How can an asteroid covered in huge boulders cool down so quickly?

Hidden Cracks Explain Bennu’s Unusual Thermal Behavior

Observations collected by OSIRIS-REx during its exploration of Bennu provided the first clues. The asteroid’s rocks appeared to contain far more cavities than scientists had assumed, suggesting they could lose heat faster than ordinary solid rocks.

When samples from Bennu returned to Earth, researchers were finally able to examine that possibility directly.

Ryan and his colleagues used several experimental techniques to study small rock fragments collected from the asteroid. Their findings, reported in Nature Communications, confirmed that the material was highly porous. However, porosity alone could not fully explain Bennu’s unusually rapid heat loss.

Researchers discovered another important feature: Many of the rock particles contained extensive networks of cracks running through them.

To investigate how these cracks affect heat transfer, researchers at Japan’s Nagoya University examined the samples using a technique called lock-in thermography. The method uses a laser to heat a small area of a sample while measuring how heat spreads through the surrounding material, similar to ripples moving across the surface of a pond.

“That’s when things got really interesting,” Ryan said. “The thermal inertia measured in the laboratory samples was found to be much higher than that recorded by the spacecraft’s instruments, mirroring similar findings obtained by the team on OSIRIS-REx’s partner mission, JAXA’s Hayabusa2.”

The difference between laboratory measurements and spacecraft observations suggested that the size and internal structure of Bennu’s rocks were essential to understanding the asteroid’s thermal behavior.

Because the returned sample was much smaller than the giant boulders covering Bennu, scientists needed a way to determine how heat moves through the material on a larger scale.

NASA Uses 3D X-Ray Scans to See Inside Bennu Rocks

At NASA’s Johnson Space Center in Houston, researchers prepared the samples for detailed X-ray imaging while taking special precautions to prevent contamination.

Inside a glovebox filled with protective nitrogen gas, team members sealed individual rock particles in airtight containers. The containers were then transported to a laboratory for X-ray computed tomography, or XCT, scanning. After imaging, the particles were returned to the glovebox without exposure to Earth’s environment.

“The sample is encased in a unique ‘spacesuit,’ undergoes a CT scan, and then returns to its original environment without exposure to the Earth’s environment,” said Nicole Lanning, lead OSIRIS-REx sample curator at NASA Johnson’s Astromaterials Research and Exploration Sciences Division and one of the study’s co-authors. “We can take images through these airtight containers and visualize the shape and internal structure of the rocks inside.”

The scans gave scientists a detailed look inside the samples, revealing structures that could not be investigated by examining their surfaces alone.

“X-ray computed tomography allows us to see inside objects in three dimensions without damaging them,” said study co-author Scott Eckley, an X-ray scientist at NASA Johnson.

Each scan also produced a permanent three-dimensional digital record of the particle’s external and internal features. These records were added to a publicly accessible database, allowing the samples to be studied without repeatedly handling the original material.

Ryan’s team then incorporated the X-ray scan data into a computer simulation designed to track how heat travels through Bennu’s rocks. By extending their calculations from individual particles to the much larger rocks found on the asteroid, the researchers were able to reproduce the low thermal inertia observed by OSIRIS-REx.

Why Bennu’s Cracked Rocks Matter for Asteroid Science

The results solved an important mystery about Bennu’s unusual surface. Scientists had previously thought that the asteroid’s rocks might be highly porous, with a soft, fluffy, or spongy internal structure.

Although Bennu’s rocks contain significant pore space, the researchers found that extensive internal cracks are just as important for understanding how the material stores and releases heat.

“It turned out there was actually a crack in it as well. That was the missing piece of the puzzle,” Ryan said.

The discovery has implications beyond Bennu. Astronomers often rely on measurements of how asteroids absorb and emit heat to estimate the physical properties of their surfaces. Until now, interpreting those observations was difficult without examining actual asteroid rock samples.

Ron Balluz, a scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, and the study’s second author, explained that the discovery provides a valuable link between remotely collected measurements and the true internal structure of asteroid material.

“By analyzing these samples from the very same asteroid, we can finally establish a telescopic understanding of the asteroid’s thermal properties,” Burrows said.