Bennu

Photographs of asteroid Bennu's surface
Asteroid Bennu is blanketed by rocks and huge boulders. And now that the OSIRIS-REx spacecraft is taking a close look at those rocks, researchers are able to see something surprising for an airless body: the rocks have tiny cracks and fissures.

The cause?

Most likely the cracks are from wide variations in temperatures, with sunlight heating up the rocks during the day, and then cooling off quickly at night. This process happens quickly and frequently, as Bennu makes one full rotation on its axis every 4.3 hours.

"This is the first time evidence for this process, called thermal fracturing, has been definitively observed on an object without an atmosphere," said Jamie Molaro of the Planetary Science Institute in Tucson, Arizona. Molaro is the lead author of a paper published today in Nature Communications. "It is one piece of a puzzle that tells us what the surface used to be like, and what it will be like millions of years from now."

On Earth, the primary cause of rocks cracking is weathering. Rainwater can seep into small faults or cracks in the rock, and if the temperature drops below freezing, the water turns to ice and expands, forcing the cracks to widen.

But of course, there's no weather on the atmosphere-less Bennu. However, swings in temperatures can have a similar effect. Anytime a rock is exposed to a change in temperature, the surface will either expand or contract. And since rock is a poor conductor of heat, the surface of the rock will expand or contract more than the rock's center. This will cause stress cracks in the rock's surface.

Bennu
© NASA/Goddard/University of Arizona.
Exfoliation features on a cliff face (a) and on boulders (b-f) with varying size and location on asteroid Bennu from images taken by NASA's OSIRIS-REX spacecraft. The bright dome on the horizon of panel (a) is a boulder behind the exfoliating cliff.
"Like any weathering process, thermal fracturing causes the evolution of boulders and planetary surfaces over time - from changing the shape and size of individual boulders, to producing pebbles or fine-grained regolith, to breaking down crater walls," said OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) principal investigator Dante Lauretta of the University of Arizona, Tucson. "How quickly this occurs relative should to other weathering processes tells us how and how quickly the surface has changed."

Scientists have tossed around the idea that thermal fracturing could be an important weathering process on airless objects like asteroids because so many experience extreme temperature differences between day and night, compounding the stress.

The daytime highs on Bennu can reach almost daytime highs on Bennu can reach to approximately 400 degrees Kelvin (260 degrees Fahrenheit), and nighttime lows plummet to 200 degrees Kelvin (-100 degrees Fahrenheit).

The cracks in Bennu's rocks are fairly small and difficult to see, especially before OSIRIS-REx began performing close flyovers of the asteroid, in preparation for landing on the surface in October 2020 for sample collection. But within the past couple of months, the spacecraft has dipped to within 75 meters (246 ft) of Bennu's surface, and previous maneuvers saw it approach within 250 meters (820 ft.) OSIRIS-REx is taking pictures of the primary and backup sites to gain even high-resolution photos to prepare for the sample collection portion of the mission.

Bennu
© NASA Visualization Technology Applications and Development VTAD
3-D model of Bennu.
Other processes can produce similar features, but the team's analysis ruled them out. Besides from weathering, rocks squeezed by tectonic activity can also crack, but Bennu is too small for such activity. Meteoroid impacts do occur on Bennu and can certainly crack rocks, but they would not cause the types of cracking and other erosion seen by the spacecraft. Also, there's no sign of impact craters where the cracking it is occurring.

"We've observed evidence of thermal fracturing on Earth and on Mars, both environments where chemical weathering may play a role," said Molaro. "Therefore, while it was theoretically possible for thermal fracturing on an airless body to occur alone, it was not clear whether or not the stresses would be strong enough to cause crack growth in absence of the chemical effects."

Bennu
© NASA/Goddard/University of Arizona
This global map of asteroid Bennu’s surface is a mosaic of images collected by NASA’s OSIRIS-REx spacecraft between Mar. 7 and Apr. 19, 2019.
Molaro said that the spacecraft's observations are consistent with scientific models of thermally induced rock breakdown with no other weathering mechanisms.

"Boulders on Bennu exhibit many possible signs of thermal fracturing, but the clearest is images showing exfoliation, where thin layers of material flake off boulder surfaces," Molaro said. "These findings provide substantive and compelling evidence that thermal fracturing plays an important role on airless body surfaces, which has major implications for understanding the evolution of asteroid surfaces, orbits, and populations."

The mission team found features consistent with thermal fracturing using the spacecraft's OSIRIS-REx Camera Suite (OCAMS), which can see features on Bennu smaller than one centimeter (almost 0.4 inches). It found evidence of exfoliation, where thermal fracturing likely caused small, thin layers (1 - 10 centimeters) to flake off of boulder surfaces. The spacecraft also took images of cracks running through boulders in a north-south direction, along the line of stress that would be produced by thermal fracturing on Bennu.

This finding on Bennu will help future explorations of other airless bodies.

"We don't have good constraints yet on breakdown rates from thermal fracturing," Molaro said, "but we can get them now that we can actually observe evidence for it for the first time in-situ."

Bennu
© NASA/Goddard/University of Arizona
This artist's concept shows NASA's OSIRIS-REx spacecraft descending towards asteroid Bennu to collect a sample of the asteroid's surface. The sampling procedure will take place on October 20th.
OSIRIS-REx will briefly touchdown on Bennu's surface in the attempt to extract surface samples. This is currently scheduled for October 20, 2020. If the first attempt is unsuccessful, OSIRIS-Rex is equipped for multiple sample collection attempts. Once the mission is complete in 2021, OSIRIS-REx will head back home, bringing its valuable, pristine cargo to Earth in September 2023. If all goes as planned, the samples will be studied on Earth for decades to come.