On the Response of Chondrites to Diurnal Temperature Change—Experimental Simulation of Asteroidal Surface Conditions

Abstract

Thermal fatigue is a process that degrades rocks and pebbles exposed to space by diurnal cycling. We simulate the diurnal temperature variation at 1 AU of 5 mm-sized sample cubes using an evacuated cryostat, allowing conditions that are closer to those expected in space. Sample cubes of ordinary, CV, CM2, and C2 chondrites were investigated using scanning electron microscopy and microcomputed tomography scans. In contrast to previous work, the ordinary and CV chondrite samples did not show any cracking. The CM2 chondrites respond with the formation and extension of cracks. Pre-existing cracks present in the C2 chondrite Tagish Lake closed in the course of cycling to different degrees, which has not been reported before in the context of thermal fatigue driven rock breakdown. Most changes in crack length occur in the initial ∼20 cycles indicating a rapid buildup of strain followed by a rapid release by the formation and extension of cracks when exposed to diurnal temperature variation. The overall cracking rate is comparable to that of previous work on CM2 chondrites and in 15%–20% of the observed cracking occasions they are associated with fine-grained rims around chondrules implying that the comminution of CM-like rocks and pebbles is controlled by the abundance of chondrules (and other coarse-grained components) and their rims. We propose that thermal fatigue-driven comminution of rocks and especially pebbles on asteroids under the here studied conditions is controlled by the abundance of hydrated minerals. This, in turn, can result in different abundances and natures of finer-grained material on asteroid surfaces.