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An international team of researchers from academic and government institutions, including NASA’s Solar System Exploration Research Virtual Institute (SSERVI) at NASA's Ames Research Center in Moffett Field, Calif., has determined the likely origin for the loose material that covers small asteroids. Researchers found that rock weathering and fragmentation due to temperature changes caused by sunlight is the main process by which debris is generated on small asteroids. The findings will be published in the April 10, 2014 issue of Nature. 

Space missions and ground-based observations have shown that small asteroids, measuring about half a mile (or one kilometer) wide, are covered by a loose layer of dust and debris called regolith. Traditionally, scientists theorized the regolith on asteroids was the result of micrometeoroid impacts that pulverized large boulders or bedrock creating dust that fell back onto the asteroid’s surface. This is the same way craters and regolith form on the moon. However, laboratory experiments and impact models now show that, unlike the moon, these small asteroids do not have enough gravity to keep the debris from escaping into space. Therefore, impact debris cannot be the main source of regolith on small asteroids.

"This insight will help us to interpret astronomical observations of asteroid surfaces in terms of the underlying bedrock, not contaminated by in-falling debris from elsewhere,” said David Morrison, SSERVI chief scientist at Ames. "In other words, we should expect to see the same materials in the regolith that make up the larger boulders and rocks of an asteroid."

While performing experiments in the laboratory, researchers from 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) used an X-ray scanner to measure the growth of cracks – or thermal fatigue – in different types of meteorites before and after a series of temperature cycles.

“We find that rocks larger than a few centimeters break up faster by thermal fragmentation induced by extreme temperature variations between day and night, than by micrometeoroid impacts,” said Marco Delbo from the Observatoire de la Côte d'Azur in Nice, France, and the paper’s lead author.

The production of fresh regolith originating from thermal fatigue fragmentation may be an important process for rejuvenating the surface of near-Earth asteroids, as well as for explaining the observed shortage of fragile carbonaceous-type near-Earth asteroids that pass close to the sun.

"The sun acts like an oven; it heats up space rocks producing internal stresses that, over time, break them apart," said Simone Marchi SSERVI researcher at the Southwest Research Institute and co-author of the paper.

This model predicts that asteroids on the order of several yards in size, such as those that may be targets of future sample return missions, could be covered by coarse regolith and pebbles, and therefore any potential capture mechanism must be able to cope with loose collections of coarse rocks.

Managed from Ames, SSERVI is a virtual institute that brings researchers together in a collaborative virtual setting. The virtual institute model enables cross-team and interdisciplinary research that bridges science and exploration. SSERVI is jointly funded by the Science Mission Directorate and Human Exploration and Operations Mission Directorate at NASA Headquarters in Washington.

For more information about SSERVI and selected member teams, visit: http://sservi.nasa.gov