Thermal fatigue as the origin of regolith on small asteroids

Abstract

Space missions and thermal infrared observations have shown that small asteroids (kilometre-sized or smaller) are covered by a layer of centimetre-sized or smaller particles, which constitute the regolith. Regolith generation has traditionally been attributed to the fall back of impact ejecta and by the break-up of boulders by micrometeoroid impact. Laboratory experiments and impact models, however, show that crater ejecta velocities are typically greater than several tens of centimetres per second, which corresponds to the gravitational escape velocity of kilometre-sized asteroids. Therefore, impact debris cannot be the main source of regolith on small asteroids. Here we report that thermal fatigue, a mechanism of rock weathering and fragmentation with no subsequent ejection, is the dominant process governing regolith generation on small asteroids. We find that thermal fragmentation induced by the diurnal temperature variations breaks up rocks larger than a few centimetres more quickly than do micrometeoroid impacts. Because thermal fragmentation is independent of asteroid size, this process can also contribute to regolith production on larger asteroids. Production of fresh regolith originating in thermal fatigue fragmentation may be an important process for the rejuvenation of the surfaces of near-Earth asteroids, and may explain the observed lack of low-perihelion, carbonaceous, near-Earth asteroids.

Size Sorting on the Rubble-Pile Asteroid Itokawa

“…As we have mentioned, granular beds are excellent impact absorbers, and so even high-speed micrometeorites can be expected to be captured by pebble seas, causing them to grow -sandbags are used to stop rifle bullets for this reason. Moreover, in boulderrich regions away from pebble seas, smaller particles produced by fracture, comminution or thermal fatigue 37 can be expected to be ejected skyward by high-speed impacts. Those particles ejected faster than the escape velocity will be cleared from boulder-rich regions, while those traveling slower will undergo ballistic sorting when they return to the asteroid.…”

“…To be sure, sufficiently energetic impacts can destroy landscapes produced by any process; likewise landscapes will be remodeled by many effects 7,37,38 , including tidal disruption, Yarkovsky and YORP effects, rotational losses, gardening, ice sublimation, electrostatics, radiation pressure sweeping, etc.…”

Space-Based Thermal Infrared Studies of Asteroids

“…Delbó et al (2014) posit that thermal cracking plays a key role in regolith production, with an erosion and destruction process that preferentially affects more fragile, low-albedo, carbonaceous objects as they approach the Sun; see alsǒ Capek & Vokrouhlický (2010, 2012.…”

Geomorphology of the Imhotep region on comet 67P/Churyumov-Gerasimenko from OSIRIS observations

“…Their physical nature, composition, distribution, formation, and evolution are fundamental for understanding how planets formed and evolved, and ultimately how water was brought to Earth (e.g., Dones et al 2004;Duncan et al 2004;Hartogh et al 2011). Since the formation of the solar system, comets have been affected by collisions, dynamical perturbations, irradiation processes, and thermal alteration, which have modified their physico-chemical properties and their orbits (e.g., Weissman et al 2004;Bockelée-Morvan et al 2004;Delbó et al 2014). As a result of the sublimation of ices as the comet approaches the Sun, cometary activity is expected to be the main process altering the external layers of the nucleus.…”