  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.
