Close inspection of asteroid Bennu reveals rocks cracking from sunlight
As NASA’s OSIRIS-REx spacecraft homes in on Bennu ahead of its sample collection later this year, the probe is continuing to reveal fascinating insights about the attributes of the primitive asteroid. Mission scientists are now reporting a long-suspected but never-before-seen phenomenon whereby extreme temperature fluctuations are fracturing rocks on the surface, a process that could offer clues about the body’s past and possible future.
This process is known as thermal fracturing and is commonly seen in rocks as they age, with heat from the Sun causing them to expand during the day, before they cool down at night to create stress fractures that gradually grow larger over time.
“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,” says OSIRIS-REx principal investigator Dante Lauretta of the University of Arizona, Tucson. “How quickly this occurs relative to other weathering processes tells us how and how quickly the surface has changed.”
While this process can be seen playing out on planets where temperature fluctuations are softened by the presence of an atmosphere, there is reason to suspect that its effects could be even greater on objects without an atmosphere, such as an asteroid like Bennu. Here, the temperature can reach 127 °C (260 °F) in the day and -73 °C (-99 °F) at night, but getting close enough to tease out the subtle clues of thermal fracturing hasn’t been possible until now.
The OSIRIS-REx probe has now traveled as close as 75 m (246 ft) to the surface of Bennu as preparations continue for its sampling effort later this year, and through these close inspections the team has been able to gain a fresh, intimate perspective. Using the spacecraft’s high-res onboard camera, it has been able to observe features smaller than a centimeter (0.4 in) in size.
In doing so, the scientists found various evidence of thermal fracturing, including exfoliation, where the cracks cause thin layers to flake off boulder surfaces, and fractures running through boulders in a north-to-south direction. This is the direction that the team would expect to find lines of stress driven by thermal fracturing on Bennu.
“This is the first time evidence for this process, called thermal fracturing, has been definitively observed on an object without an atmosphere,” says Jamie Molaro of the Planetary Science Institute, Tucson, Arizona, lead author of the study. “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.”
How thermal fracturing fits in with the overall picture of aging is complex. While a weathered surface featuring many impact craters might be indicative of an older body, the rate of thermal fracturing could be driven by its proximity to the Sun or the length of its day. A rapid deterioration of the surface through thermal fracturing could therefore make a body seem a lot older than it is. Or just as likely, the opposite could be true. The scientists plan to continue studying thermal fracturing on bodies like Bennu to better understand this relationship.
“We don’t have good constraints yet on breakdown rates from thermal fracturing, but we can get them now that we can actually observe it for the first time in situ,” says OSIRIS-REx project scientist Jason Dworkin of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Laboratory measurements on the properties of the samples returned by the spacecraft in 2023 will help us learn more about how this process works.”
The team’s research was published in the journal Nature Communications.