Experimentally Induced Thermal Fatigue on Lunar and Eucrite Meteorites—Influence of the Mineralogy on Rock Breakdown

Abstract

Thermal fatigue has been proven to be of fundamental importance for the nature and evolution of surfaces of airless bodies in the solar system. It is a rock erosive process acting in conjunction with meteoroid bombardment. We set up an experiment to simulate the diurnal temperature variation at 1 AU of centimeter sized sample cubes using a liquid nitrogen cooled cryostat, allowing to study unexplored conditions, that is, high vacuum and temperatures of 200 K similar to those occurring on the Moon. The sample cubes are investigated using scanning electron microscopy and micro computed tomography scans before and after 10, 20, 50, 100, and 400 total cycles. Cycling of the lunar anorthosite Northwest Africa (NWA) 11273 and the eucrite NWA 11050 reveal different behaviors: Whereas NWA 11273 responds to the cycling with micro‐flaking of tenth‐of‐µm‐sized grains on its surface and only limited crack growth, the eucrite NWA 11050 is less affected by micro‐flaking but the growth of cracks is observed to occur throughout the whole experiment. The rate of crack formation and growth is lower when compared to previously reported results on ordinary and carbonaceous chondritic samples carried out under nitrogen atmosphere and above 250 K. We propose that the size of particles and their rate of production by thermal fatigue highly depends on the mineralogy of the exposed rock and areas with mature rocks are prone to produce fine‐grained soil, while primary rocks such as basalts are likely to produce blocky regolith in a first step.

Global Mapping of Fragmented Rocks on the Moon with a Neural Network: Implications for the Failure Mode of Rocks on Airless Surfaces

“…Another morphology of Bennu described as disaggregation of a clastsbearing boulder, which leads to highly irregular, hummocky profiles of some Bennu boulders, corresponds to the advectionlike erosion we observed for the "breccia" type. These findings could imply that the morphologies on Bennu are formed by meteoroid bombardment and not thermal stresses, or, perhaps less likely, that very different regimes of thermal stresses present on bodies of different dynamical, orbital, and rheological properties (e.g., Molaro et al 2017;Ravaji et al 2019;Patzek & Rüsch 2022) lead to the development of similar morphologies. It is also very likely that different processes lead to the development of similar morphologies, thus limiting, in some cases, the inference based on geomorphology alone.…”

“…The effects of diurnal temperature variation on the lunar surface are a function of the complex spatiotemporal properties of the stress fields and thus on the rock size, composition, shape, and location (Molaro et al 2017). These effects have been the subject of recent studies (Ravaji et al 2019;Hörz et al 2020;Patzek & Rüsch 2022).…”

Mid‐Infrared Spectroscopy of Feldspars From the Bühl Basalt (Northern Hesse, Germany) Formed Under Reducing Conditions as Terrestrial Analogue of Mercury for MERTIS

“…The same is true for the Moon's regolith, where the plagioclase (”feldspar”—good cleavage) component is enriched in the smallest grain size fraction (Taylor et al., 2001). In addition, the enormous thermal stress through multiple cooling and heating induced by diurnal cycles at planetary objects without atmosphere leads to the formation of flaked off particles especially of anorthositic rocks in the range between 6 and 46 μm (Patzek & Rüsch, 2022). Therefore, if remote sensing spectra of the Moon's regolith can resolve a TF, it will probably be dominated by the plagioclase component.…”

Photometry of LROC NAC resolved rock-rich regions on the Moon

“…This could represent a type of asymmetric shock deformation known to occur during oblique impact process (Pierazzo and Melosh, 2000b;Kenkmann et al, 2014). These differently shocked ejecta boulders in the downrange could also respond differently to space weathering after emplacement (Pieters and Noble., 2016;Patzek and Rüsch, 2022) when compared to boulders in the forbidden/uprange region. Moreover, we note that the impact crater is situated on a ridge within the Hertzsprung S basin, and this kms-scale irregular topography could have contributed to the asymmetric ejecta distribution.…”

“…Due to the effect of abrasion, the rocks develop a debris apron or a 'fillet' around them (Rüsch & Wöhler, 2022). Additionally, depending on the petrology, the boulder surface can undergo enhanced weathering due to diurnal temperature variation (Patzek and Rüsch, 2022). These changes in morphology and surface properties coupled with the decrease in boulder number with time and the associated change in boulder size frequency distribution are key to understanding the evolution of the boulder field and consequentially the regolith forming process (e.g., Basilevsky et al, 2013;Hörz et al, 2020;.…”