Space missions and thermal infrared observations have shown that small asteroids (kilometre-sized or smaller) are covered by a layer of centimetre-sized or smaller particles, which constitute the regolith. Regolith generation has traditionally been attributed to the fall back of impact ejecta and by the break-up of boulders by micrometeoroid impact. Laboratory experiments and impact models, however, show that crater ejecta velocities are typically greater than several tens of centimetres per second, which corresponds to the gravitational escape velocity of kilometre-sized asteroids. Therefore, impact debris cannot be the main source of regolith on small asteroids. Here we report that thermal fatigue, a mechanism of rock weathering and fragmentation with no subsequent ejection, is the dominant process governing regolith generation on small asteroids. We find that thermal fragmentation induced by the diurnal temperature variations breaks up rocks larger than a few centimetres more quickly than do micrometeoroid impacts. Because thermal fragmentation is independent of asteroid size, this process can also contribute to regolith production on larger asteroids. Production of fresh regolith originating in thermal fatigue fragmentation may be an important process for the rejuvenation of the surfaces of near-Earth asteroids, and may explain the observed lack of low-perihelion, carbonaceous, near-Earth asteroids.

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The distributions of size and chemical composition in the regolith on airless bodies provides clues to the evolution of the solar system. Recently, the regolith on asteroid (25143) Itokawa, visited by the JAXA Hayabusa spacecraft, was observed to contain millimeter to centimeter sized particles. Itokawa boulders commonly display well-rounded profiles and surface textures that appear inconsistent with mechanical fragmentation during meteorite impact; the rounded profiles have been hypothesized to arise from rolling and movement on the surface as a consequence of seismic shaking. We provide a possible explanation of these observations by exploring the primary crack propagation mechanisms during thermal fatigue of a chondrite. We present the in situ evolution of the full-field strains on the surface as a function of temperature and microstructure, and observe and quantify the crack growth during thermal cycling. We observe that the primary fatigue crack path preferentially follows the interfaces between monominerals, leaving them intact after fragmentation. These observations are explained through a microstructure-based finite element model that is quantitatively compared with our experimental results. These results on the interactions of thermal fatigue cracking with the microstructure may ultimately allow us to distinguish between thermally induced fragments and impact products.

We investigate the rate-dependent compressive failure and fragmentation of a hot-pressed boron carbide, under both uniaxial and confined biaxial compression, using quantitative fragment analysis coupled with quantitative microstructural analysis. Two distinct fragmentation regimes are observed, one of which appears to be more sensitive to the microstructural length scales in the material, while the second is more sensitive to the structural character and boundary conditions of the compressed sample. The first regime, which we refer to as "microstructuredependent," appears to be associated with smaller fragments arising from the coalescence of fractures initiating from graphitic inclusions. This regime appears to become more dominant as the strain rate is increased and as the stress-state becomes more confined. The second regime generates larger fragments with the resulting fragment size distribution dependent on the specific structural mechanisms that are activated during macroscopic failure (e.g., buckling of local columns developed during the compression). The average fragment size in the latter regime appears to be reasonably predicted by current fragmentation models. This improved understanding of the effects of microstructure and confinement on fragmentation then provides new insights into prior ballistic studies involving boron carbide.

Regolith generation by thermal fatigue has been identified as a dominant mechanism for the breakdown of small (cm-sized) rocks on certain airless bodies. Simple numerical models for thermal fatigue seemed to indicate that this breakdown occurs faster in the larger decimeter-sized rocks, which is in contrast to the predictions of disruption models through successive micrometeorite impacts. The observation is justified by the existence of larger temperature gradient in bigger rocks, but it is not clear that this conclusion can be extrapolated or scaled to meter-sized boulders. Here we reveal a transition in the rock disaggregation rates by thermal fatigue when rock sizes rise above a critical length scale. A simple analytic model is formulated to predict the time to fracture of rocks on small airless bodies. We consider an uncoupled approach consisting of a one-dimensional thermal model, and a two-dimensional fracture model. The solution of the heat equation is used as input to the thermomechanical crack growth problem. This new