In one of the biggest surprises of NASA’s OSIRIS-REx mission, its target asteroid, Bennu, turned out to be a jagged, rugged world covered in large boulders, with few of the smooth patches that earlier observations from Earth-based instruments had indicated.
“When OSIRIS-REx got to Bennu in 2018, we were surprised by what we saw,” said Andrew Ryan, a scientist with the University of Arizona Lunar and Planetary Laboratory, who led the mission’s sample physical and thermal analysis working group. “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.”
Asteroid Bennu’s rugged surface explained
Particularly puzzling were observations made in 2007 by NASA’s Spitzer Space Telescope, which measured low thermal inertia, indicative of an asteroid whose surface heats up and cools down rapidly as it rotates into and out of sunlight, like a sandy beach on Earth. This was at odds with the many large boulders that OSIRIS-REx found upon arrival, which should act more like blocks of concrete, shedding heat long after the Sun has set.
Data collected by the OSIRIS-REx spacecraft during its survey campaign at the asteroid suggested a possible explanation: the boulders could be much more porous than expected. Once the samples were delivered to Earth, researchers were able to investigate this further.
Ryan’s team scrutinized rock particles collected from Bennu’s surface using a variety of laboratory analysis techniques. In a study published in Nature Communications, the authors reported that the boulders are indeed porous enough to account for some of the observed heat loss, but not all of it. Rather, many of the rocks turned out to be riddled with extensive networks of cracks.
To test whether the cracks could be the reason for the asteroid’s surface losing heat, a team at Nagoya University in Japan analyzed Bennu sample material using lock-in thermography. This laser-based technique allows researchers to hit a tiny spot on the surface of the sample and measure how the heat diffuses through it, similar to how ripples move across a pond.
“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, echoing similar findings obtained by the team of OSIRIS-REx’s partner mission, JAXA’s (Japan Aerospace Exploration Agency) Hayabusa-2.”
To make meaningful predictions about how the material would behave in the large boulders on the asteroid, the team had to find a way to scale up the measurements obtained with the small sample particles.
Using a glove box, team members at NASA’s Johnson Space Center in Houston sealed sample particles in air-tight containers under a protective nitrogen atmosphere, then transferred them to a lab where they could perform X-ray computed tomography, or XCT scans. Once a particle was scanned, it went back into the glove box.
“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 within the Astromaterials Research and Exploration Science division at NASA Johnson and one of the study’s co-authors. “We can image right through these airtight containers to visualize the shape and internal structure of the rock that’s inside.”
“X-ray computed tomography allows us to look at the inside of an object in three dimensions, without damaging it,” said study co-author and NASA Johnson X-ray scientist Scott Eckley.
Once mapped in this way, a permanent three-dimensional digital archive of a sample particle’s shape and interior is created, and the data are entered into a public database. Ryan’s team used the X-ray CT scan data for computer simulations modeling heat flow and thermal inertia. When scaled up to boulder size, the thermal inertia results fell into agreement with what the spacecraft had measured at the asteroid.
Where scientists once expected the boulders of Bennu to be extremely porous and fluffy, perhaps even spongy, the sample analysis revealed something unexpected.
“It turns out that they’re really cracked too, and that was the missing piece of the puzzle,” Ryan said.
Ron Ballouz, a scientist with the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, and the paper’s second author, said this work transforms how scientists interpret the structure of an asteroid based on its thermal properties seen from Earth.
“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,” Ballouz said.
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