Temporal variability of thermal-cycling induced fracturing in chondrites

Thermal fatigue is produced by diurnal and/or annual variations of the surface temperature on asteroids and its efficiency depends on the heliocentric distance, the rotation period, and the thermal inertia of the asteroid’s surface. A fundamental assumption of previous studies [1,2], is that thermal fatigue remains effective over thousands, or even millions, of thermal cycles. However, the Kaiser effect, extensively studied in the field of fracture mechanics on terrestrial rocks, states that fracturing on materials ceases when previously exerted load levels are not exceeded [3]. The thermal expansion coefficient of each mineral at specific temperature changes can be translated into different mechanical loads resulting from thermal fatigue.

We aim to observe the time-resolved crack propagation induced by thermal stresses over subsequent thermal load cycles in meteorites – acting as asteroid analogues – in order to understand the role of thermal fatigue in eroding asteroid surfaces using non-destructive methods. We investigate CV, CM, and, for comparison, LL chondrites to examine the behaviour of thermal fatigue on different petrographic types of meteorites. The samples are subjected to a minimum of 100 cycles at ΔT=210K as a typical temperature variation of C-type NEAs is 200K [2]. To identify the spatial occurrence of pre-existing and propagating cracks, the samples have been scanned using X-ray μCT. If the Kaiser effect is applicable, we expect to see the effects of thermal fatigue wane after a few thermal cycles, suggesting that other mechanisms, such as chemical alteration, are contributing to the breakdown process of asteroids.

[1] Delbo M. et al. 2014. Nature 508(7495) 233-236. [2] Molaro J.L. et al. 2015. JGR Planets 120(2) 255-277. [3] Kaiser J. 1950. A study of acoustic phenomena in tensile heat (PhD Thesis).