Asteroid Bennu looked like a smooth, sandy body when NASA’s Spitzer Space Telescope measured its thermal properties years ago. That reading suggested fine grains covering the surface, the kind of terrain a spacecraft could scoop easily. When the OSIRIS-REx probe arrived and photographed the asteroid up close, the surface turned out to be covered in boulders, some as large as a house. Scientists have now resolved that contradiction: the boulders themselves are laced with dense networks of internal fractures that make them conduct heat as poorly as loose sand would.
How hidden cracks fooled every telescope pointed at Bennu
Before OSIRIS-REx reached Bennu, ground-based and space-based infrared observations measured its thermal inertia, a property that describes how quickly a surface absorbs and releases heat. Low thermal inertia typically signals fine-grained material like beach sand. That interpretation shaped mission planning and led engineers to expect a relatively smooth landing zone. The actual surface, documented in a 2019 Nature paper, was far rockier and more hazardous than anticipated, forcing the team to redesign its sampling approach on the fly.
The disconnect between thermal readings and physical reality persisted as a scientific puzzle for years. A peer-reviewed study published in Nature Communications now provides the answer. Researchers used X-ray computed tomography, or XCT, on particles collected by the OSIRIS-REx TAGSAM mechanism and found extensive crack networks inside the returned Bennu samples. Those fractures reduce thermal conductivity within each boulder, mimicking the thermal signature of loose regolith even though the surface is solid rock.
The new work builds directly on OSIRIS-REx’s close-up reconnaissance. Mission scientists had already mapped Bennu’s chaotic, rubble-pile terrain and cataloged its boulder populations, but without physical samples they could only infer what lay inside those rocks. The low thermal inertia inferred from telescopic data had suggested a blanket of dust, yet high-resolution images showed almost none. By tying XCT-derived fracture patterns to measured heat flow, the Nature Communications team closed that gap and showed that the “sandy” signal was coming from the interior of the boulders themselves.
XCT scans at Johnson Space Center revealed the fracture networks
The scanning work took place at NASA’s Johnson Space Center, where curation teams imaged the returned stones under pristine conditions. The samples remained sealed in a nitrogen environment to prevent terrestrial contamination, and XCT data were acquired at voxel-level resolution to map internal textures without destroying the material. Technical documentation of this process is recorded in a NASA workshop record that details the curation protocols, scanning setups, and the range of internal fabrics visible in the Bennu stones.
The XCT images revealed cracks running through the samples at multiple scales, from hairline partings to millimeter-wide voids. Rather than isolated flaws, the fractures form connected networks that interrupt heat flow through the rock. In effect, each boulder behaves like a composite of solid mineral and insulating gaps, similar to a foam or pumice stone. Thermal models built from these XCT observations, using specific TAGSAM sample identifiers, demonstrated that cracked boulders produce the same low thermal inertia that telescopes had measured from millions of miles away. The finding eliminates the need to invoke a thick blanket of fine dust or sand to explain the readings.
Separate peer-reviewed work published in Geophysical Research Letters had already proposed that thermal-fatigue fractures could account for Bennu’s low apparent thermal inertia without requiring a sand-like surface layer. Those models treated boulders as progressively damaged materials that lose thermal conductivity over time as cracks accumulate. The returned-sample XCT data now confirm that hypothesis with direct physical evidence rather than modeling alone, showing that even small fragments are riddled with voids that would strongly suppress heat transport.
Sunlight-driven temperature swings created the cracks
The fractures did not form from a single catastrophic event. Bennu rotates roughly once every 4.3 hours, and each cycle exposes its boulders to sharp temperature swings as sunlit faces heat up and shadowed faces cool rapidly. Over thousands to hundreds of thousands of years, that repeated thermal stress produces fatigue cracks, much the way bending a metal paperclip back and forth eventually snaps it. On an airless world with no atmosphere to buffer these swings, the temperature contrast between day and night can be extreme, making thermal fatigue especially efficient.
OSIRIS-REx cameras had already spotted signs of this process while orbiting the asteroid. Mission imagery documented linear fractures and disaggregation features on boulder surfaces consistent with thermal fatigue, as described in a NASA summary of Bennu’s rugged surface and its puzzling heat signature. A separate fracture-orientation study published in Nature Geoscience found that cracks on Bennu’s boulders show preferential alignment consistent with diurnal thermal cycling, ruling out random impact damage as the primary cause. Peer-reviewed modeling placed the formation timescales at geologically rapid intervals, consistent with a global geologic map that dates Bennu’s heterogeneous resurfacing to the past 500,000 years.
A companion study published in Icarus examined how petrology-the mineral structure and composition of the rock-controls the spacing and propagation of fatigue cracks. Different minerals expand and contract at different rates when heated and cooled, so rocks with strong contrasts between grain types develop internal stresses along their boundaries. That work provides the mechanical explanation for why fractures should be pervasive across Bennu’s surface rather than confined to a few weak boulders. The implication is that thermal fatigue is a dominant resurfacing process on small, airless rubble-pile asteroids, not a minor secondary effect.
Open questions about fracture scaling and future asteroid surveys
The returned samples are tiny fragments, each only millimeters to centimeters across. Whether fracture density inside Bennu’s boulders scales predictably with boulder size is still an open question. If smaller boulders crack more densely than larger ones, there could be a size threshold below which thermal inertia drops sharply enough to mimic a sand-like surface even more strongly. Conversely, if large boulders share similar internal damage, then the entire asteroid may be thermally “fluffy” at scales far beyond what the samples directly record.
Answering that question will require combining XCT data with detailed shape models and thermal observations from the OSIRIS-REx spacecraft. By correlating boulder sizes, surface textures, and local temperature curves, researchers can test whether the fracture networks seen in the lab extend to meter- and tens-of-meters-scale rocks. Future missions that carry higher-resolution thermal imagers or in situ heat-flow probes could push this comparison even further, directly measuring how quickly individual boulders warm and cool over a rotation.
The findings also have implications for planetary defense. Many proposed asteroid-deflection techniques, from kinetic impactors to surface ablation, assume that target bodies behave like coherent rocks. A rubble-pile asteroid whose boulders are internally fractured and thermally insulating could respond very differently to an impact, absorbing and redistributing energy in complex ways. Understanding how widespread Bennu-like fracture networks are among near-Earth asteroids will help refine models of how these objects might fragment or deform under stress.
For remote-sensing astronomers, Bennu’s cracked boulders are a cautionary tale. Thermal inertia has long served as a proxy for surface grain size, but OSIRIS-REx shows that internal structure can complicate that relationship. Telescopic surveys of other small bodies may need to account for the possibility that low thermal inertia reflects damaged boulders rather than loose dust. Combining thermal data with radar, polarimetry, and high-phase-angle imaging could help distinguish between these scenarios, improving interpretations of asteroid surfaces before spacecraft ever arrive.
Ultimately, the Bennu samples demonstrate the power of sample return to ground-truth decades of remote observations. A handful of grains, scrutinized with XCT in a controlled laboratory, have rewritten the story told by entire-asteroid thermal measurements. As more sample-return missions target diverse small bodies, scientists will be able to test whether Bennu is typical or an outlier-and, in the process, refine the tools used to read the subtle thermal signatures of worlds too distant to touch.
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*This article was researched with the help of AI, with human editors creating the final content.
