Asteroid Bennu’s Rugged Surface Baffled NASA, We Finally Know Why

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 rocks, with some of the smooth areas indicated by earlier observations from Earth-based instruments.

“When OSIRIS-REx arrived at Bennu in 2018, we were surprised by what we saw,” said Andrew Ryan, a scientist at the University of Arizona Lunar and Planetary Laboratory in Tucson, who led the working group on the physical and thermal analysis of the mission’s samples. “We expected a few rocks, but we anticipated at least a few large areas with smoother, finer regolith that would be easy to collect. Instead, it seemed like it was just rocks, and we scratched our heads for a while.”

Particularly puzzling are 2007 observations by NASA’s Spitzer Space Telescope, which measured low thermal inertia, indicative of an asteroid whose surface heats and cools rapidly as it rotates in and out of sunlight, like a sandy beach on Earth. This was at odds with the many large boulders discovered by OSIRIS-REx upon arrival, which are expected to act more like concrete blocks, giving off heat long after sunset.

Data collected by the OSIRIS-REx space probe during its study campaign on the asteroid suggests a possible explanation: the rocks could be much more porous than expected. Once the samples were delivered to Earth, the researchers were able to further their research.

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 rocks are indeed porous enough to explain some, but not all, of the observed heat loss. On the contrary, many rocks were found to be riddled with extensive networks of cracks.

To test whether the cracks could be causing heat loss from the asteroid’s surface, a team from Nagoya University in Japan analyzed the Bennu sample using lock-in thermography. This laser-based technique allows researchers to touch a small spot on the surface of the sample and measure how heat diffuses through it, similar to how ripples move across a pond.

“That’s when things got really interesting,” Ryan said. “The thermal inertia measured in the laboratory samples was found to be much higher than that recorded by the spacecraft’s instruments, echoing similar results obtained by the team on OSIRIS-REx’s partner mission, JAXA (Japan Aerospace Exploration Agency) Hayabusa-2.”

To make meaningful predictions about how the material in the asteroid’s large rocks would behave, the team had to find a way to augment the measurements obtained with the small particle samples.

Using a glove box, team members at NASA’s Johnson Space Center in Houston sealed particle samples in airtight containers under a protective nitrogen atmosphere, then transferred them to a laboratory where they could perform X-ray CT or XCT scans. Once a particle was scanned, it returned to the glove compartment.

“The sample goes into its own ‘spacesuit,’ passes a scanner, and then returns to its pristine environment, all without any exposure to the Earth’s environment,” said Nicole Lunning, principal curator of OSIRIS-REx samples in NASA Johnson’s Astromaterials Research and Exploration Science division and one of the study’s co-authors. “We can visualize through these airtight containers the shape and internal structure of the rock inside.”

“X-ray CT allows us to look at the interior of an object in three dimensions, without damaging it,” said Scott Eckley, study co-author and X-ray scientist at NASA Johnson.

Andrew Ryan

Scientist at the Lunar and Planetary Laboratory at the University of Arizona

Once mapped in this way, a permanent three-dimensional digital archive of the shape and interior of a sample particle is created, and the data is entered into a public database. Ryan’s team used the X-ray CT data for computer simulations modeling heat flow and thermal inertia. When scaled up to the size of a rock, the thermal inertia results fell into agreement with what the spacecraft had measured on the asteroid.

While scientists once expected Bennu’s rocks to be extremely porous and fluffy, perhaps even spongy, analysis of the samples revealed something unexpected.

“Turns out they are also very cracked, 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 paper’s second author, said the work transforms the way scientists interpret an asteroid’s structure based on its thermal properties as seen from Earth.

“We can finally deepen our understanding of telescope observations of an asteroid’s thermal properties by analyzing these samples from that same asteroid,” Ballouz said.

NASA’s Goddard Space Flight Center provided overall mission management, systems engineering, and mission safety and assurance for OSIRIS-REx. Dante Lauretta, of the University of Arizona in Tucson, is the principal investigator. The university leads the science team as well as planning the science observation and data processing of the mission. Lockheed Martin Space in Littleton, Colorado, built the spacecraft and provided flight operations. Goddard and KinetX Aerospace were responsible for navigation of the OSIRIS-REx spacecraft. OSIRIS-REx curation takes place at NASA’s Johnson Space Center in Houston. International partnerships for this mission include the CSA (Canadian Space Agency) OSIRIS-REx laser altimeter instrument and the asteroid sample science collaboration with the JAXA (Japan Aerospace Exploration Agency) Hayabusa2 mission. OSIRIS-REx is the third mission in NASA’s New Frontiers program, managed by NASA’s Marshall Space Flight Center in Huntsville, Alabama, for the agency’s Science Mission Directorate in Washington.

By Daniel Stolt
University of Arizona

For more information on the OSIRIS-REx mission, visit:

https://www.nasa.gov/osiris-rex

Karen Fox / Molly Wasser
Headquarters, Washington
202-285-5155 / 240-419-1732
karen.c.fox@nasa.gov / molly.l.wasser@nasa.gov

Victoria Segovia
Johnson Space Center, Houston
281-483-5111
victoria.segovia@nasa.gov