Grain-scale thermoelastic stresses and spatiotemporal temperature gradients on airless bodies, implications for rock breakdown

Thermomechanical processes such as fatigue and shock have been suggested to cause and contribute to rock breakdown on Earth, and on other planetary bodies, particularly airless bodies in the inner solar system. In this study, we modeled grain-scale stresses induced by diurnal temperature variations on simple microstructures made of pyroxene and plagioclase on various solar system bodies. We found that a heterogeneous microstructure on the Moon experiences peak tensile stresses on the order of 100 MPa. The stresses induced are controlled by the coefficient of thermal expansion and Young’s modulus of the mineral constituents, and the average stress within the microstructure is determined by relative volume of each mineral. Amplification of stresses occurs at surface-parallel boundaries between adjacent mineral grains and at the tips of pore spaces. We also found that microscopic spatial and temporal surface temperature gradients do not correlate with high stresses, making them inappropriate proxies for investigating microcrack propagation. Although these results provide very strong evidence for the significance of thermomechanical processes on airless bodies, more work is needed to quantify crack propagation and rock breakdown rates.

💡 Research Summary

The paper investigates how diurnal temperature cycles on air‑less planetary bodies generate grain‑scale thermoelastic stresses that could contribute to rock breakdown. Using a two‑dimensional finite‑element framework, the authors construct simple microstructures composed of the silicate minerals pyroxene and plagioclase, interspersed with a prescribed fraction of pores. Material properties—coefficient of thermal expansion (α), Young’s modulus (E), and Poisson’s ratio (ν)—are taken from the literature for each mineral. Separate thermal boundary conditions are applied to represent the solar insolation and radiative cooling regimes of the Moon, Mercury, and Mars, thereby reproducing realistic surface temperature histories for each body.

The simulations reveal that the largest tensile stresses arise at interfaces where the two minerals meet, especially along boundaries that run parallel to the surface and at the tips of pore spaces. On the lunar case, peak tensile stresses reach roughly 100 MPa, a magnitude comparable to or exceeding the typical tensile strength of silicate rocks (tens of MPa). The magnitude of the stress field is governed primarily by the contrast in α and E between the constituent minerals; the average stress within the microstructure scales linearly with the volume fraction of the higher‑α, higher‑E mineral (pyroxene). Consequently, a microstructure richer in pyroxene experiences higher mean stresses, while a plagioclase‑dominated assemblage shows lower values.

A key finding is that spatial and temporal surface temperature gradients—often used as proxies for thermal fatigue—do not correlate with the locations of maximum stress. High temperature gradients are dictated by the external heating/cooling cycle, whereas stress concentrations are dictated by internal material heterogeneity and geometric discontinuities. This decoupling suggests that temperature gradient alone is an insufficient indicator of microcrack initiation or propagation.

The authors argue that these results provide strong quantitative evidence that thermomechanical processes on air‑less bodies can generate stresses capable of driving fatigue cracking, especially over long timescales. However, the study stops short of modeling crack nucleation, growth, or the cumulative damage that would translate into measurable rock breakdown rates. The paper calls for future work that couples the presented stress fields with fracture mechanics models and validates the predictions against laboratory thermal cycling experiments on analog materials. Such extensions would enable estimation of fatigue lifetimes, erosion rates, and the overall contribution of thermal fatigue to the evolution of regolith on the Moon, Mercury, Mars, and other air‑less bodies.