A detailed examination of Bennu’s rocks uncovered a surprising hint that altered the narrative.
One of NASA’s OSIRIS-REx mission’s most surprising discoveries was Bennu’s actual surface. Contrary to earlier Earth-based predictions of many smooth regions, the asteroid revealed itself as a rugged, uneven terrain strewn with massive boulders.
“When OSIRIS-REx arrived at Bennu in 2018, the surface surprised us,” said Andrew Ryan of the University of Arizona’s Lunar and Planetary Laboratory, who led the mission’s sample physical and thermal analysis team. “We expected to find some boulders, but also hoped for sizable areas of smoother, finer material that would be easy to collect. Instead, boulders covered almost the entire asteroid, leaving us puzzled for a while.
Another puzzle stemmed from 2007 data collected by NASA’s Spitzer Space Telescope. That data indicated low thermal inertia, suggesting Bennu’s surface heated and cooled quickly when exposed to sunlight—similar to sand on a beach. This contradicted OSIRIS-REx’s observations, since large boulders should retain heat like concrete, staying warm long after sunset.
Bennu’s Samples Offer New Clues
OSIRIS-REx’s survey of Bennu suggested a potential explanation: the asteroid’s boulders could be much more porous than anticipated. Once the samples arrived on Earth, researchers could directly test this hypothesis.
Ryan’s team analyzed rock particles from Bennu’s surface using multiple laboratory techniques. In a study published in Nature Communications, they discovered that while the boulders’ porosity accounted for some of the asteroid’s heat loss, it didn’t explain all of it. Many rocks also featured extensive networks of cracks.
To determine if these cracks contributed to the heat loss, researchers at Nagoya University in Japan applied lock-in thermography to Bennu samples. This laser-based technique allows scientists to focus on a small area of a sample and observe how heat propagates through it, much like ripples spreading across a pond.
Examining How Heat Moves Through the Rocks
“That’s when things got really intriguing,” Ryan said. “The thermal inertia measured in the lab samples was much higher than what the spacecraft had recorded, mirroring similar results from OSIRIS-REx’s partner mission, JAXA’s Hayabusa‑2.”
To predict how this material would behave in Bennu’s larger boulders, the researchers needed to scale up their findings from the tiny returned particles.
At NASA’s Johnson Space Center in Houston, team members placed the sample particles in airtight containers within a glove box under a protective nitrogen atmosphere. They then transported them to a lab for X‑ray computed tomography (XCT) scans, returning each particle to the glove box afterward.
“The sample is placed in its own ‘spacesuit,’ scanned with a CT machine, and then returned to its pristine environment—all without ever being exposed to Earth’s atmosphere,” said Nicole Lunning, lead OSIRIS-REx sample curator in NASA Johnson’s Astromaterials Research and Exploration Science division and a co-author of the study. “Through these airtight containers, we can see the rock’s shape and internal structure.”
“X-ray computed tomography lets us examine the interior of an object in three dimensions without causing any damage,” added study co-author and NASA Johnson X-ray scientist Scott Eckley.
Cracking the Heat Loss Puzzle
This process produced a permanent 3D digital record of each sample particle’s shape and internal structure, with the data added to a publicly accessible database. Ryan’s team then fed the X‑ray CT scan data into computer simulations of heat flow and thermal inertia. When these results were scaled to the size of Bennu’s boulders, they closely matched the spacecraft’s measurements.
Previously, scientists had expected Bennu’s boulders to be extremely porous and soft, almost spongy. The sample analysis, however, revealed a more complex picture.
“It turns out they’re heavily cracked as well, and that was the missing piece of the puzzle,” Ryan said.
Ron Ballouz, a scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, and the study’s second author, noted that the findings change how researchers interpret asteroid structure based on thermal properties measured from Earth.
“By analyzing these samples directly, we can finally connect telescope observations of an asteroid’s thermal properties with its actual physical structure,” Ballouz said.
Read the original article on: SciTechDaily
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