Network of thermal cracks in meteorites due to temperature variations: new experimental evidence and implications for asteroid surfaces

Abstract

In recent years several studies have shown the importance of thermal fracturing of rocks due to temperature variations, on Earth and Mars. Rock thermal cracking might also be a process at play on the lunar surface. These temperature variations as well as change rates can reach important amplitude on bodies without an atmosphere, in particular on those that reach small perihelion distances such as near-Earth asteroids. On the other hand, the formation, geometry and extension of cracks on these bodies have not been fully investigated yet. Here we show results of thermal cracking laboratory experiments on chondrite meteorites, which develop networks of cracks when subjected to rapid temperature cycles with amplitudes similar to those experienced by asteroids with low perihelion distances. The depth of the cracks can reach a few hundred of microns in some hundreds of temperature cycles, in agreement with theoretical studies. We find that dehydration of hydrous minerals enhances the cracking process. The formation of crack networks increases the porosity both at the surface and in the sub-surface of our specimens. We propose that this process could help explaining the recent finding of the very highly porous surfaces of most of the boulders on the asteroids Ryugu and Bennu.

Pebbles and sand on asteroid (162173) Ryugu: In situ observation and particles returned to Earth

“…Many returned particles feature curved and straight cracks. Pebbles with a smooth surface could be fragments of particles with straight cracks, possibly formed by shock or thermal fatigue ( 33 ). The common presence of cracks in returned pebbles implies that the small thermal inertia of surface boulders ( 12 , 13 ) is probably due, at least in part, to cracks or fractures in their interior.…”

Thermophysical Investigation of Asteroid Surfaces. II. Factors Influencing Grain Size

“…Internal stresses caused by differential thermal expansion, as a result of cyclic heating and cooling, lead to regolith breakdown and erosion (Eppes et al 2015). The efficiency of this thermal cycling process has been modeled under the thermal environments of airless solar system bodies (e.g., Molaro & Byrne 2012;Molaro et al 2017), and its feasibility has been experimentally demonstrated on meteorite samples (Delbo et al 2014;Libourel et al 2020). Molaro et al (2020) demonstrated that large cracks observed on rocks seen across Bennu's surface were consistent with thermal fatigue model predictions.…”

“…It has been debated whether or not thermal fracturing is a relatively significant weathering mechanism for terrestrial rocks (e.g., Molaro & McKay 2010, and references within), yet Delbo et al (2014) experimentally demonstrated the effectiveness of this process, in vacuo, on a chip of the Murchison (CM2 carbonaceous chondrite) and Sahara 97210 (L/LL3.2 ordinary chondrite) meteorites. Libourel et al (2021) performed a followup study with these two meteorites and Allende (CV3), where they demonstrated that larger temperature gradients, such as those experienced for low perihelion asteroids, result in greater crack growth rates. Images from the encounter of Bennu by OSIRIS-REx provided widespread evidence of thermal cracking across the surface (Molaro et al 2020).…”

Shapes, structures, and evolution of small bodies

“…The efficiency of thermal fatigue should even increase for bodies with low perihelion distances, i.e., experiencing intense temperature variations, and bodies composed of hydrated minerals, which may experience dehydration. Experiments were performed on meteorites using very high temperature variations to demonstrate the increased efficiency of thermal fatigue for low perihelion objects [154]. In these experiments, meteorites composed of hydrated minerals experienced dehydration, enhancing the cracking process.…”