When NASA's OSIRIS-REx spacecraft reached asteroid Bennu in 2018, mission scientists were in for a surprise. Instead of the relatively smooth terrain suggested by earlier observations, they found a rugged landscape dominated by enormous boulders, with few areas suitable for sample collection.
"When OSIRIS-REx got to Bennu in 2018, we were surprised by what we saw," said Andrew Ryan, a scientist at the University of Arizona's Lunar and Planetary Laboratory who led the mission's working group on the physical and thermal properties of its samples. "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."
The discovery was puzzling because of what astronomers already knew about Bennu's heat retention. Measurements from NASA's Spitzer Space Telescope in 2007 indicated that the asteroid's surface warmed and cooled rapidly as it rotated through sunlight and darkness, a property known as low thermal inertia. Materials with low thermal inertia, such as beach sand, quickly gain heat in sunlight and lose it once the sun sets. Large, solid rocks typically hold on to warmth and release it gradually, so Bennu's boulder-covered surface appeared to contradict those earlier measurements.
Clues emerged while OSIRIS-REx was still surveying the asteroid. The boulders might contain far more empty space than assumed, allowing them to lose heat faster. When samples were returned to Earth, researchers had the chance to test that idea directly.
Ryan and his colleagues used several laboratory techniques to examine tiny rock fragments. Their findings, published in Nature Communications, confirmed the rocks were highly porous. But porosity alone could not fully explain the rapid heat loss. The researchers also discovered extensive networks of cracks running through the interiors of many rock particles.
To understand how these fractures affect heat movement, researchers at Nagoya University in Japan used a technique called lock-in thermography, which uses a laser to heat a tiny area while measuring how the heat spreads. "That's when things became really interesting," Ryan said. "The thermal inertia measured in the lab samples turned out to be much higher than what the spacecraft's instruments had recorded."
The difference between laboratory and spacecraft observations suggested that the size and internal structure of Bennu's rocks were crucial. Because the returned samples were much smaller than the enormous boulders on Bennu, scientists needed a way to model how heat would move through the material on a larger scale.
At NASA's Johnson Space Center in Houston, researchers prepared the samples for X-ray computed tomography (XCT) scanning, taking special precautions to prevent contamination. Inside a glove box with protective nitrogen gas, team members sealed individual rock particles in airtight containers before transporting them to the XCT lab.
"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 Nicole Lunning, lead OSIRIS-REx sample curator at NASA Johnson and a co-author of the study.
"X-ray computed tomography allows us to look at the inside of an object in three dimensions, without damaging it," added Scott Eckley, a NASA Johnson X-ray scientist and co-author. Each scan produced a permanent 3D digital record of the particle's exterior and internal features, added to a publicly accessible database.
Ryan's team incorporated the scan data into computer simulations that tracked how heat travels through asteroid rocks. By extending calculations from individual particles to the much larger boulders on Bennu, they were able to reproduce the low thermal inertia observed by the spacecraft.
"It turns out that they're really cracked too, and that was the missing piece of the puzzle," Ryan said.
The discovery has implications beyond Bennu. Astronomers often rely on measurements of how an asteroid absorbs and releases heat to estimate the physical properties of its surface. Until now, interpreting those observations has been difficult without actual rock samples.
"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," said Ron Ballouz, a scientist at the Johns Hopkins University Applied Physics Laboratory and the study's second author.