Hidden cracks inside Bennu’s boulders explain why the asteroid sheds heat so fast, NASA scientists found

Asteroid Bennu’s boulders are laced with hidden cracks, and those cracks explain why the asteroid sheds heat so quickly, according to a team led by Andrew Ryan of the University of Arizona’s Lunar and Planetary Laboratory. The finding resolves a puzzle that began when the OSIRIS-REx spacecraft reached Bennu in 2018 and found something nobody expected. The result, published in Nature Communications, ties together telescope data, spacecraft imagery and laboratory work on material that was physically brought back to Earth.

AI, explained every day: The Deep View is the go-to daily newsletter for 600,000+ founders, knowledge workers and professionals who want to stay up to date with artificial intelligence.

Get The Deep View →

We may earn a commission if you sign up.

Before the probe arrived, telescope measurements suggested a surface of fine, sandy material that warms quickly in sunlight and cools quickly in darkness. Close up, the spacecraft saw the opposite landscape: a rugged world dominated by large boulders with very few smooth patches. Solid rock should hold its heat well, so the boulders seemed to contradict the telescope readings they were supposed to explain. For years that left planetary scientists with two trustworthy datasets that refused to agree, and with no obvious way to decide which one misled them.

Why thermal inertia was the puzzle

Scientists describe this behavior as thermal inertia. A material with low thermal inertia, like beach sand, heats and cools fast, while a material with high thermal inertia, like concrete, soaks up heat and releases it slowly. A 2007 observation from NASA’s Spitzer Space Telescope indicated that Bennu’s surface had low thermal inertia, according to the ScienceDaily report on the new study, which is why the sight of so many boulders was such a surprise.

Laboratory tests on returned samples deepened the mystery at first. Their thermal inertia came out much higher than the readings taken from the spacecraft, a mismatch that echoes results reported by the team behind Japan’s Hayabusa-2 mission. Porosity, the many tiny voids in the rock, accounted for part of the difference but not all of it, and ScienceDaily reports that the team needed something more to bring the laboratory numbers down to what the spacecraft saw. The samples themselves had come from an asteroid whose chemistry has already drawn attention, as an earlier ScienceDaily story on Bennu and the chemistry of life showed.

What the X-ray scans and laser tests found

The missing piece turned out to be cracking. In the University of Arizona’s own account of the work, Ryan said the samples turned out to be really cracked too, and that was the missing piece of the puzzle. Widespread networks of fractures inside the rock particles interrupt the flow of heat, so the stone behaves more like loose material even though it is solid. In practical terms, a boulder full of cracks lets sunlight warm its outer layer quickly and offers heat no easy path inward, so the surface gives up its warmth soon after the Sun goes down.

Getting to that answer required keeping the samples pristine. NASA Johnson Space Center staff held the material in airtight containers filled with nitrogen inside a glove box and ran X-ray computed tomography scans without exposing it to Earth’s air. Nicole Lunning, the lead OSIRIS-REx sample curator at Johnson, described the process as a sample going into its own spacesuit, getting a CT scan, and then returning to its pristine environment. The three-dimensional scans were also added to a public database.

Researchers at Nagoya University in Japan performed lock-in thermography, a technique in which a laser heats a small spot and sensors follow how the warmth spreads through the sample. The team then fed the X-ray data into heat-transfer simulations and scaled the results up from small particles to boulder-sized masses. Those simulations reproduced the low thermal inertia that the spacecraft had measured, which is the step that closes the loop between the lab and the asteroid. In effect, the cracks seen in a few grains were shown to be enough to explain the behavior of whole boulders, a jump in scale that the researchers had to model rather than measure directly.

Who did the work and where it was published

The study, titled “Low thermal inertia of carbonaceous asteroid Bennu driven by cracks observed in returned samples,” appeared in Nature Communications, with a DOI record listed on the ScienceDaily page. Ron Ballouz of the Johns Hopkins University Applied Physics Laboratory is the second author, and Lunning and Scott Eckley of NASA Johnson are named co-authors. D. S. Lauretta is listed as the last author in the reference list.

Ballouz said researchers can finally ground their understanding of telescope observations of an asteroid’s thermal properties by analyzing samples from that very same asteroid. That matters for planetary defense work, because heat radiated by an asteroid shapes how its orbit drifts over time, and telescope-based estimates of thermal behavior are often the only data available for most space rocks. A calibration against real Bennu material gives future observers a better yardstick for reading those distant measurements correctly.

The coverage reviewed here does not give numerical thermal inertia values, so the exact size of the effect of cracking is left to the paper. Bennu has been a rich source of results since the capsule returned, and ScienceDaily’s earlier story on Bennu’s surface and structure shows how the asteroid’s rugged terrain first came into focus.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

AI, explained every day: Get The Deep View →

We may earn a commission if you sign up.