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OSIRIS-REx Images Allow Closer Look at Boulder Breakup on Bennu

March 19, 2019

crumbling boulders on Bennu

The boulders on Bennu’s surface sport a variety of surface textures, from smooth, to hummocky, striated, and crumbly “cauliflower” in nature. The large boulder in the image center is approximately 28 meters across and has a somewhat round shape, though many smaller boulders surrounding it are very angular. Some of these appear to be fragments that may have disaggregated from the central boulder, and display layering effects that may reflect some of the properties of their mineral composition. Other boulders show signs of surface exfoliation and fractures that may have been caused by impacts, mechanical weathering, and other forms of rock breakdown active on Bennu’s surface.

Credit: NASA/Goddard/University of Arizona.



Tucson, Ariz. -- High-resolutions images of asteroid Bennu from NASA’s OSIRIS-REx spacecraft offer scientists the opportunity to analyze the processes that break down boulders on the surfaces of airless bodies, Planetary Science Institute Research Scientist Jamie Molaro said. 

Breakdown can occur due to impacts on the surface, movement of boulders in landslides, and thermal fracturing. Thermal fracturing is a process where cracks can form in boulders due to heating and cooling from the Sun over time. This process is believed to be very important on certain objects in the Solar System. However, it is hard to observe because it happens at small scales. 

“Based on what we’re seeing in these images, we believe rock breakdown due to thermal fracturing is happening on Bennu. What is really exciting is that this is the first time we’re observing direct evidence for this process on a planetary surface,” Molaro said during a presentation at the 50th Lunar and Planetary Science Conference held this week in The Woodlands, Texas. 

Until OSIRIS-REx arrived at Bennu, scientists relied primarily on numerical models and laboratory studies to understand the nature of this process. We have learned that boulder size and composition play a big role in how thermal fracturing operates because it changes the boulder’s response to heating and cooling. For example, it may erode small dust grains from the surface of one rock but develop larger fractures in another that can split a boulder into many pieces. Now, the results from these computer models and laboratory studies can be compared directly to observations from the spacecraft, allowing us to better understand how it works to break down rocks and produce dust on asteroid surfaces. 

“The OSIRIS-REx mission to Bennu offers an unprecedented opportunity to search for evidence of thermal breakdown occurring in-situ on its surface. Characterizing this process is key to understanding Bennu’s surface evolution and properties,” Molaro said. “This analysis may also assist with sample site selection and eventually sample analysis.” 

Molaro’s research was funded by a grant to PSI from NASA’s OSIRIS-Rex Participating Scientists program. 



Alan Fischer

Public Information Officer


fischer [at]




Jamie Molaro

Senior Scientist

jmolaro [at]



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