Asteroids are not inert bodies silently drifting through space. They are dynamic objects constantly being shaped by their harsh environment. Scientists have long known about the effects of meteoroid impacts and space radiation on asteroids. Now, groundbreaking research from NASA’s OSIRIS-REx mission has revealed a new weathering process at play: sunshine is causing rocks on asteroid Bennu to Crack Rocks.
Images captured by the OSIRIS-REx spacecraft provide the first definitive evidence of thermal fracturing on an airless celestial body. Thermal fracturing is a process where temperature fluctuations, specifically the extreme temperature swings between day and night on asteroids, cause rocks to weaken and fracture. This discovery, detailed in a Nature Communications paper led by Jamie Molaro of the Planetary Science Institute, sheds light on the long-term evolution of asteroid surfaces.
Examples of rock fractures on asteroid Bennu caused by thermal stress, showing disaggregation and linear cracks as observed by NASA's OSIRIS-REx spacecraft.
Dante Lauretta, principal investigator for OSIRIS-REx at the University of Arizona, explains the broader implications: “Thermal fracturing is a fundamental weathering process that drives the evolution of boulders and entire planetary surfaces. It alters everything from the shape of individual rocks to the breakdown of crater walls and the production of fine regolith.” Understanding the rate of thermal fracturing relative to other weathering mechanisms is crucial to deciphering the history and changes of asteroid surfaces.
The mechanism behind this rock-cracking phenomenon is straightforward yet powerful. During the daytime on Bennu, surface temperatures can soar to a scorching 127 degrees Celsius (260 degrees Fahrenheit) under the intense sunlight. As the asteroid rotates into night, temperatures plummet to a frigid minus 73 degrees Celsius (minus 100 degrees Fahrenheit). This extreme temperature variation causes the rocks to expand when heated and contract when cooled. This repeated expansion and contraction induces stress within the rocks, leading to the formation and gradual growth of cracks, ultimately causing them to crack rocks.
While scientists theorized about thermal fracturing as a significant weathering process on airless bodies due to these extreme temperature shifts, direct observational evidence was lacking before OSIRIS-REx. The high-resolution cameras of the OSIRIS-REx Camera Suite (OCAMS) provided the necessary detail, capable of detecting features smaller than a centimeter.
The OCAMS images revealed distinct features consistent with thermal fracturing. One such feature is exfoliation, where thin layers of rock, ranging from 1 to 10 centimeters thick, are seen peeling away from boulder surfaces. This flaking is a direct result of the stress induced by thermal cycling cracking rocks layer by layer. Furthermore, the spacecraft captured images of linear cracks running through boulders, predominantly in a north-south direction. This orientation aligns perfectly with the expected stress patterns caused by thermal fracturing on Bennu.
Exfoliation features on asteroid Bennu cliffs and boulders, revealing layers peeling off due to thermal fracturing, captured by NASA's OSIRIS-REx mission.
The research team carefully considered other potential weathering processes that could mimic thermal fracturing. Processes like rain and chemical weathering were ruled out due to Bennu’s lack of atmosphere. Tectonic activity, which can also cause exfoliation, is not feasible on a small asteroid like Bennu. Meteoroid impacts can certainly fracture rocks, but they would not produce the uniform, layered erosion observed across boulder surfaces. Moreover, the absence of impact craters in areas exhibiting exfoliation further strengthens the case for thermal fracturing as the primary cause of these crack rocks features.
Ongoing studies of Bennu and analysis of samples returned by OSIRIS-REx in 2023 are expected to provide more precise estimates of the rate at which thermal fracturing is reshaping the asteroid. Jason Dworkin, OSIRIS-REx project scientist at NASA’s Goddard Space Flight Center, emphasizes the importance of future research: “Laboratory analysis of the returned samples will give us valuable insights into the mechanics of thermal fracturing and help us better understand how this process works.”
Furthermore, understanding thermal fracturing is crucial for accurately determining the age of asteroid surfaces. The degree of surface weathering is often used to estimate age, with heavily cratered surfaces typically considered older. However, thermal fracturing, by accelerating the breakdown of features like crater walls, can complicate these age estimations. The efficiency of thermal fracturing varies depending on factors like an asteroid’s distance from the Sun, rotation period, and rock properties. Therefore, a deeper understanding of how thermal fracturing crack rocks on different asteroids is essential for refining our methods of dating planetary surfaces.
This discovery underscores the dynamic nature of asteroids and highlights the subtle yet powerful ways in which sunshine can act as a force of erosion in space, causing even seemingly solid rocks to crack rocks over time. The OSIRIS-REx mission continues to provide invaluable insights into these fascinating celestial bodies, rewriting our understanding of asteroid evolution and the processes that shape them.
