April 4, 2014
Regolith Of Small Asteroids Formed By Thermal Fatigue
John P. Millis, Ph.D. for redOrbit.com - Your Universe Online
Small asteroids contain a layer of loose, unconsolidated rock and dust known as a regolith composed of centimeter-sized fragments and smaller particles. New research from researchers at the Observatoire de la Côte d'Azur, Hopkins Extreme Materials Institute at Johns Hopkins University, Institut Supérieur de l'Aéronautique et de l'Espace and Southwest Research Institute (SwRI), have now determined that this layer is formed by temperature cycling that breaks down rock – a process called thermal fatigue.
Prior to this work, astronomers believed that the regolith of small asteroids – those less than one kilometer in width – were formed from material falling back to the surface of the asteroid after a collision with another object. The process is similar to water crashing back into a pool after someone jumps in, disrupting the surface. However, the team demonstrated that debris from large impacts would accelerate to high enough velocities to escape the gravitational pull of these asteroids, meaning that the material doesn’t fall back to the surface after all.
Instead, the research showed that fragmentation, caused by the heating and cooling caused by the rapid rotation of the asteroid in space – essentially, the asteroid would experience brief, alternating periods of “day” and “night” – was the primary process by which rocks smaller than a few centimeters would be pulverized.
"We took meteorites as the best analog of asteroid surface materials that we have on the Earth," noted Dr. Marco Delbo, from the Observatoire de la Côte d'Azur. "We then submitted these meteorites to temperature cycles similar to those that rocks experience on the surfaces of near-Earth asteroids and we found that microcracks grow inside these meteorites quickly enough to entirely break them on timescales much shorter than the typical lifetime of asteroids."
These results were then extrapolated to demonstrate what would happen to asteroids within about 93 million miles of the Sun (the distance at which the Earth orbits) in the vacuum of space. At such distances, the rate of breakdown on the surface would be 10 times what would be caused by impacts as suggested in previous theories.
While the process slows down as the asteroid moves farther from the Sun, as there would be less temperature variation due to diminished solar energy the greater the orbital distance, the mechanism would still drive thermal fatigue. "Even asteroids significantly farther from the Sun showed thermal fatigue fragmentation to be a more relevant process for rock breakup than micrometeoroid impacts," said Dr. Simone Marchi, a research scientist in the SwRI’s Space Science and Engineering Division.
Results of this research were published Wednesday in the Nature Advance Online Publication.