A combination of laboratory experiments and modelling shows that diurnal temperature variations are the main cause of rock breakdown and the ensuing formation of powdery rubble on the surface of small asteroids. See Letter p.233
The surfaces of asteroids orbiting the Sun are known to be covered in particulate debris called regolith, rather like the non-living components of soil on Earth. Impacts by meteoroids — meteorites when they reach the ground — and micrometeoroids are usually thought to be the major agents of regolith formation on rocky bodies that have no atmosphere, such as asteroids. Meteoroid impacts rupture boulders, producing smaller debris, whereas micrometeoroids (up to a few millimetres in diameter) cause the gradual erosion of boulders and debris into finer particles. On page 233 of this issue, Delbo et al.1 use data from laboratory experiments and modelling to test the alternative hypothesis that temperature cycling is the major cause of regolith production on small (kilometre-sized and smaller) asteroids and may contribute to it on larger asteroids (Fig. 1).
The ability of diurnal temperature cycling to cause the mechanical breakdown of surface rocks and boulders on Earth and other planetary bodies has been heavily debated for more than a century. In essence, directional heating and cooling by solar radiation can cause large temperature changes at the surface of rocks and boulders. Depending on the thermal conductivity of the materials involved, large temperature gradients can occur between the surface and inner portions of the boulders, with resultant mechanical stresses that, when repeated over time, can lead to cracking and deterioration of the boulders.
Studies2,3,4 based on field observations, laboratory experiments and modelling have confirmed the effectiveness of such thermal weathering in a range of settings (Earth's deserts, on Mars and on other planetary bodies), but have not evaluated whether it is the dominant process. Delbo et al. have made a major advance by linking results from experimental weathering of meteorites to a micromechanical model. This allowed them to quantify the rate of breakdown from crack growth caused by thermal cycling and to compare it with the rate of attrition by micrometeoroid impact.
The authors performed experiments on samples of two chondrites (stony meteorites) approximately 1 centimetre in diameter — one an ordinary (silicate) chondrite and the other a carbonaceous chondrite, which is dark and has low reflectivity. They exposed them to more than 400 diurnal temperature cycles representative of conditions found on near-Earth asteroids, each cycle having a temperature range of 190 kelvins and lasting 2.2 hours. Using the technique of X-ray computed tomography scanning to calculate rates of crack growth in the samples, Delbo et al. observed a measurable increase in the size of pre-existing cracks in both types of meteorite even after 76 temperature cycles. Their micromechanical model simulates the measured growth of cracks and shows how growth from an initial crack 30 micrometres in length leads to rapid breakdown of centimetre-sized boulders and the production of finer-grained particulate material.
For silicate asteroids at 1 astronomical unit (AU), where 1 AU is the mean Earth–Sun distance, the authors' model predicts breakdown rates that are at least tenfold quicker than those caused by micrometeoroid impact, with larger boulders predicted to break down faster than smaller ones. The disparity is even larger for the carbonaceous asteroids at 1 AU: a 10-cm boulder is predicted to survive less than 1,000 years under thermal cracking, compared with 10 million years under micrometeoroid attrition. These findings may explain why carbonaceous asteroids in near-Earth orbits are rare. Even for boulders on asteroids farther from the Sun, for example in the inner parts of the main asteroid belt, which lies between the orbits of Mars and Jupiter, the model's results show that thermal cycling causes faster breakdown than micrometeoroid impacts.
Although Delbo and colleagues' findings throw interesting light on the role of diurnal heating and cooling in the breakdown of rocks on planetary bodies, they require further testing and development. Experiments on meteorites under controlled laboratory conditions are necessarily imperfect analogues of conditions and materials on asteroid surfaces. It would be useful to have a wider range of experimental results with which to compare model predictions. Longer experiments than those performed by the authors would be particularly helpful, to try to capture the shift from crack development to boulder breakdown, which is predicted by their model to occur after between 107 and 108 experimental cycles. Furthermore, it would be good to develop more-sophisticated modelling to include situations in which multiple cracks develop in single boulders and produce more rapid breakdown. It might also be fruitful to investigate potential synergies between micrometeoroid impact and thermal-weathering processes.
Delbo and co-workers' insightful work should lead to further advances in understanding rock breakdown and regolith formation on asteroids and other rocky bodies that have no atmosphere. Their work has obvious ramifications for developing the study of the geomorphology of asteroid surfaces. Diurnal temperature cycling has previously been neglected as a potential contributor to surface modification and regolith production on asteroids. It should now be considered alongside other processes of space weathering that can produce debris of various sizes and characteristics, and thus affect the interpretation of remotely sensed spectroscopic data from asteroids5,6,7.
The work is also relevant for the investigation of rock breakdown and regolith formation on Earth and Mars. Although conditions are more complex on planetary surfaces with an atmosphere, the methodology developed by Delbo et al., which links laboratory experimental quantification of crack growth to micromechanical modelling, could be used to good effect to explore the role of diurnal temperature cycling in the breakdown of surface boulders under arid conditions.