NASA’s OSIRIS-REx Finds Heat, Cold Fracturing Rocks on Asteroid Bennu

Exfoliation features on a cliff face (a) and on boulders (b-f) with varying size and location. The bright dome on the horizon of panel (a) is a boulder behind the exfoliating cliff. (Credit: NASA/Goddard/University of Arizona)

TUCSON (Planetary Science Institute PR) — Close-up observations of asteroid Bennu by NASA’s OSIRIS-REx spacecraft contain the first evidence of thermal fracturing of rocks on an airless body, a Nature Communications paper by Planetary Science Institute Research Scientist Jamie Molaro says. 

Thermal fracturing or thermal stress weathering occurs as rocks heat and cool each day, and mechanical stresses build up that can cause cracks to develop and grow. Over time the cracks grow larger and cause the rock to disaggregate or split into multiple pieces. For example, daytime highs on Bennu can reach  about  400 degrees Kelvin (260 degrees Fahrenheit), and nighttime lows plummet to 200 degrees Kelvin (-100 degrees Fahrenheit). 

“This is the first time evidence for thermal fracturing has been definitively observed on an object without an atmosphere,” said Molaro, lead author of the paper “In situ evidence of thermally induced rock breakdown widespread on Bennu’s surface“ (http://dx.doi.org/10.1038/s41467-020-16528-7) published June 9, 2020. “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.” 

“This thermally induced breakdown has long been known on Earth. The OSIRIS-REx Camera Suite (OCAMS) orbiting as close as 0.6 km (0.4 mi) has obtained images of the surface of Bennu at pixel scales down to about 1 centimeter per pixel, providing an opportunity to search over a wide range of scales for evidence of thermal breakdown occurring in situ,” Molaro said. 

“On Earth there are chemical weathering processes that help make thermal fracturing more efficient. The presence of air and moisture within cracks makes them easier to grow, and so on Earth this effect really cannot be decoupled from the effect of the thermal stresses themselves. We’ve observed evidence of thermal fracturing on Earth and on Mars, both environments where chemical weathering may play a role. 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,” Molaro said. 

Examples of disaggregation (top) and linear fractures (bottom) in boulders of varying sizes on Bennu. (Credit: NASA/Goddard/University of Arizona)

“Like any weathering process, thermal fracturing can cause 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,” Molaro said. “How quickly this occurs relative to other weathering processes tells us how quickly the surface has changed. It is one piece of a puzzle that tells us what the planetary surface used to be like, and what it will be like millions of years from now. We don’t have good constraints yet on breakdown rates from thermal fracturing, but we can get them now that we can actually observe evidence for it for the first time in-situ. 

“We show observations of boulder morphologies and fractures on Bennu that are consistent with models of thermally induced rock breakdown, and not easily explained by 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.” 

Molaro’s research was funded by a grant to PSI from NASA’s Participating Scientist program.