Researchers from the University of Arizona, working with scientists from NASA’s Johnson Space Center, have solved a longstanding mystery about the unexpectedly rocky surface of the asteroid Bennu that they say has left the astronomy community “baffled” since 2017.
According to the team’s new experiments, an unexpectedly high number of large boulders found on the asteroid’s surprisingly jagged surface during the 2018 OSIRIS-REx mission are riddled with tiny cracks, resulting in the heat signature detected by NASA that mimicked the readings expected from an asteroid with a much smoother surface.
Baffling Heat Readings Create Asteroid Bennu Mystery
Over a decade before NASA’s OSIRIS-Rex mission linked up with Bennu in 2018, NASA’s Spitzer Space Telescope measured low thermal inertia coming from the distant space rock. These readings suggested that Bennu was heating up and cooling down rapidly as it rotated in and out of the sunlight. The researchers compared the effect to a sandy beach on Earth, heating up quickly in the Sun and cooling down rapidly after sunset.
According to Andrew Ryan, a scientist with the University of Arizona Lunar and Planetary Laboratory and leader of the mission’s sample physical and thermal analysis working group, when OSIRIS-Rex finally reached Bennu, they expected to find some boulders. However, they also anticipated “at least some large regions with smoother, finer regolith that would be easy to collect.”
Instead, Ryan said that scientists examining the images were “surprised” by what they saw. Rather than a smooth, sandy beach, the mission target’s shockingly jagged and pockmarked surface was also covered with huge rocks.
“It looked like it was all boulders,” the U of A scientist explained, before adding that “we were scratching our heads for a while.”
When absorbing and releasing heat, scientists said these large boulders should act more like “blocks of concrete,” continuously shedding heat long after the sun has set. Instead, these massive stone formations were rapidly expelling heat, like a smooth, sandy beach. These incompatible readings have left researchers searching for an explanation. Now, the joint U of A/NASA team believes they have found the answer.
“That’s When Things Became Very Interesting”
To explain this discrepancy, the researchers began by taking another look at the OSIRIS-Rex data. As hoped, this data suggested that the boulders may be much more porous than predicted, which could account for the excess heat loss.
After procuring pebble samples collected by OSIRIS-Rex and returned to Earth, the team examined them for evidence of excessive porosity. Although these analyses determined that the samples are porous enough to account for some of the heat loss, these surface features could not account for “all of it.”
Fortunately, the team’s hunt for pores resulted in an entirely separate discovery. Along with their porous surfaces, the Bennu samples were “riddled with extensive networks of cracks.” This suggested that the cracks were contributing to the material’s heat loss, explaining the contradictory heat loss observations.
To test the idea, researchers at Nagoya University in Japan analyzed the Bennu samples using lock-in thermography. According to the research team, this process allowed them to “hit a tiny spot on the surface of the sample” and observe how the resulting heat moves through the material. They compared the effect to how ripples move across the surface of a pond.
According to Ryan, that’s when the process “became really interesting.” The thermal inertia from the samples was even higher than what OSIRIS-Rex’s instruments had recorded. The U of A scientists said these echoed “similar findings” obtained by the team of JAXA’s (Japan Aerospace Exploration Agency) Hayabusa-2,” which was the partner mission to OSIRIS-Rex.
How X-ray Tomography and Computer Simulations ‘Cracked’ the Case
Although the successful experiments appeared to solve a mystery that had baffled scientists for nearly a decade, the study used only small pebbles from Bennu, which may yield a different result than the large boulders on the asteroid’s surface. This required the team to devise a way to ‘scale up’ the measurements to better approximate Bennu’s surface features, using only the small samples available to them.
NASA/Scott Eckley.
Fortunately, scientists at Johnson Space Center had previously gathered critical information about the samples that could help. Specifically, each sample was previously examined by X-ray tomography, which produced an X-ray CT scan of each pebble.
“X-ray computed tomography allows us to look at the inside of an object in three dimensions, without damaging it,” explained study co-author and NASA Johnson X-ray scientist Scott Eckley.
Because these detailed scans are archived in a publicly available database, Ryan’s team used them in computer simulations to model heat flow and thermal inertia. Critically, the computer models allowed the team to scale up the samples to boulder size, resulting in a more accurate representation of Bennu’s actual surface.
According to the study authors, when they performed their simulation with the scaled-up samples, the thermal inertia results “fell into agreement with what the spacecraft had measured” during its 2018 encounter.
“It turns out that they’re really cracked too, and that was the missing piece of the puzzle,” Ryan explained.
The study “Low thermal inertia of carbonaceous asteroid Bennu driven by cracks observed in returned samples” was published in Nature Communications.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.