NASA's OSIRIS-REx mission encountered an unexpected mystery when it arrived at asteroid Bennu in 2018. Instead of the relatively smooth terrain that earlier observations had suggested, the spacecraft found a rugged landscape dominated by enormous boulders, with very few areas suitable for collecting samples. "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 responsible for analyzing 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 especially confusing because of what astronomers had previously learned about Bennu's ability to retain heat. 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.
Scientists describe this property as low thermal inertia. Materials with low thermal inertia, such as beach sand, quickly gain heat in sunlight and lose it once the Sun disappears. Large, solid rocks behave differently.
Much like concrete, they typically hold on to warmth and release it gradually. Bennu's boulder-covered surface appeared to contradict those earlier measurements, leaving researchers with a puzzling question: How could an asteroid covered in massive rocks cool so quickly? Hidden Cracks Reveal the Secret of Bennu's Boulders Observations collected by OSIRIS-REx while surveying Bennu offered an early clue.
The asteroid's boulders might contain far more empty space than scientists had assumed, allowing them to lose heat faster than ordinary solid rocks. When samples from Bennu were returned to Earth, researchers finally had an opportunity to examine that possibility directly. Ryan and his colleagues used several laboratory techniques to investigate tiny rock fragments collected from the asteroid.
Their findings, published in Nature Communications, confirmed that the rocks were indeed highly porous. However, porosity alone could not fully explain Bennu's unusually rapid heat loss. The researchers discovered another important feature.
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