  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.
