Mineralogical control of fracturing and micro-flaking due to thermal fatigue

Thermal fatigue driven by diurnal temperature variations can lead to the physical modifications of rocks and boulders that populate airless surfaces [1-2]. These modifications affect regolith evolution, e.g., particle size, porosity and roughness, and thus influence reflected and emitted radiation observed by spacecraft, Earth- and space-based telescopes. In order to study in detail how this process affects rocks of different mineralogy and under different environments (temperature, vacuum) we use a custom-made thermal cycling chamber operating in high vacuum and at cryogenic temperatures. The investigation on achondrite meteorite samples demonstrated moderate cracking after thermal cycling relative to chondritic samples and revealed a new phenomenon, i.e., formation and detachment of micro-flakes for lunar anorthositic samples [3]. The investigation of chondritic samples subjected to thermal cycling revealed i) formation and extension of cracking due to thermal fatigue for Jbilet Winselwan (CM2), Murchison (CM2) and Tagish Lake (C2_ung); ii) absence of newly formed cracks for El Hammami (H5) and Allende (CV3), iii) absence of micro-flaking for all the above-mentioned samples. In addition, we find that in CM chondrites, cracking is often associated with hydrous fine-grained rims that surround chondrules and, in some cases, cracks diverging radially from the chondrules through the rim into the clastic matrix. These results illustrate how the mineralogy and texture, in particular the spatial context with minerals of different coefficient of thermal expansion (hydrous phyllosilicates and olivine/pyroxene), play an important role in crack formation and/or extension.

References : [1] Delbo M. et al. (2014) Nature, 508(7495), 233-236, doi:10.1038/nature13153. [2] Molaro J. L. et al. (2015) JGR: Planets, 120(2), 255-277, doi:10.1002/2014JE004729. [3] Patzek M. and Rüsch O. (2022) JGR: Planets 127.10. doi:10.1029/2022JE007306