A new mechanism for the formation of regolith on asteroids

Abstract:

The soil of asteroids, like that of the Moon, Mars, and other rocky bodies in the Solar System, is made of a layer of pebbles, sand, and dust called regolith.

Previous studies suggested that the regolith of asteroids is made from material ejected from impacts and re-accumulated on the surface and from boulders that are comminuted by micrometeoroid impacts. However, this classical scenario of regolith formation has problems to explain the regolith on km-sized and smaller asteroids: laboratory experiments and impact modelling have shown that the impact fragments can reach escape velocities and breaks free from the gravitational pull of these small asteroids, indicating the impact mechanism is not the dominant process for regolith creation. Other studies also reveal that there is too much regolith on small asteroids’ surfaces to have been deposited there solely through impacts over the millions of years of asteroids’ evolution.

We discovered that another process is capable of gently breaking rocks at the surface of asteroids: thermal fatigue by temperature cycling. As asteroids spin about their rotation axes, their surfaces plunge in and out of shadow resulting in large surface temperature variations. The rapid heating and cooling creates thermal expansion and contraction in the asteroid material, initiating cracking and propagating existing cracks. As the process is repeated over and over, the crack damage increases with time, leading eventually to rock fragmentation (and production of new regolith).

To study this process, in the laboratory, we subjected meteorites, used as asteroid material analogs, to 37 days of thermal cycles similar to those occurring on asteroids. We measured cracks widening at an average rate of 0.5 mm/y. Some fragments were also produced, indicating meteorite fragmentation. To scale our results to asteroid lifetime, we incorporated our measurements into a fracture model and we deduced that thermal cycling is more efficient than micrometeorite bombardment at fragmenting rock over millions of years on asteroids (see Delbo et al. 2014. Nature 508, 233-236).

We describe also some of the implications of this discovery: for instance, that production of fresh regolith originating in thermal fatigue fragmentation may be an important process for the rejuvenation of the surfaces of near-Earth asteroids.