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

Abstract

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.

Cracks in Martian boulders exhibit preferred orientations that point to solar-induced thermal stress

“…Also, the thinner Martian atmosphere will serve to dampen diurnal warming effects compared with those of Earth. Although our calculated values represent an annual maxima 38 , these diurnal temperature ranges calculated for a large slab are also likely lower than those which might arise in an individual, relatively small boulder, which will heat up more than flat ground due to its size and the influence of radiation from surrounding terrain. The thermoelastic stresses induced within the microstructure throughout the solar day are also calculated (Methods).…”

“…Our modelling indicates that the magnitude of stress induced by solar-forcing within the microstructure is primarily controlled by rock surface temperature diurnal range and the Young's modulus and coefficient of thermal expansion of component mineral types 38,39 . This dependence is consistent with other types of models developed for solar-induced thermal stresses in rocks on Earth, see, for example, ref.…”

“…At this time of day, atmospheric radiation causes net heating to continue for longer on Mars than it would on an airless body, and thus this peak occurs slightly later on Mars 38,39 . This result suggests that, on Earth, surfaces should experience this peak state even later in the day, a prediction that is supported by observations of rock surface temperatures that peak in the late afternoon, see, for example, ref.…”

“…Here we use a previously developed 2D finite-element model 38,39 of the thermoelastic behaviour of rock microstructures (small sections of an infinite half-space) subjected to diurnal thermal forcing to simulate, for the first time, solarinduced thermal stresses that might develop on a polycrystalline rock (Supplementary Table 2) on the Martian surface, and determine if such stresses may be sufficient to induce crack propagation (Methods). The microstructures have properties typical of basalt, a common rock type abundant along the MER Spirit traverse.…”

“…Molaro et al 38 found that bodies close to the Sun and bodies that rotate slowly have the largest daily temperature ranges and thus highest stresses. Earth and Mars have nearly the same rotation rates; however, due to its closer proximity to the Sun, Earth is expected to have a larger diurnal temperature range.…”