  This work investigates the macroscopic thermomechanical behavior of lunar
boulders by modeling their response to diurnal thermal forcing. Our results
reveal a bimodal, spatiotemporally-complex stress response. During sunrise,
stresses occur in the boulders' interiors that are associated with large-scale
temperature gradients developed due to overnight cooling. During sunset,
stresses occur at the boulders' exteriors due to the cooling and contraction of
the surface. Both kinds of stresses are on the order of 10 MPa in 1 m boulders
and decrease for smaller diameters, suggesting that larger boulders break down
more quickly. Boulders <30 cm exhibit a weak response to thermal forcing,
suggesting a threshold below which crack propagation may not occur. Boulders of
any size buried by regolith are shielded from thermal breakdown. As boulders
increase in size (>1 m), stresses increase to several 10s of MPa as the
behavior of their surfaces approaches that of an infinite halfspace. As the
thermal wave loses contact with the boulder interior, stresses become limited
to the near-surface. This suggests that the survival time of a boulder is not
only controlled by the amplitude of induced stress, but also by its diameter as
compared to the diurnal skin depth. While stresses on the order of 10 MPa are
enough to drive crack propagation in terrestrial environments, crack
propagation rates in vacuum are not well constrained. We explore the
relationship between boulder size, stress, and the direction of crack
propagation, and discuss the implications for the relative breakdown rates and
estimated lifetimes of boulders on airless body surfaces.
