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Scientists discover how soil is formed on small asteroids

Paul Sutherland, Feature writer
Apr 6, 2014, 7:00 UTC

Sen—Studies of small asteroids that pass close to Earth’s orbit in their journeys around the Sun show that their surfaces are not simply hard rock. Instead, like the Moon, they are covered in layers of a loose soil that is termed regolith. But why is it there?

Until now it has been thought that the main cause of this soil was the impact of countless micrometeoroids that pulverised the surface rock over billions of years, creating dust and debris that has then fallen back to coat it. That is the accepted explanation for the way craters and regolith are formed on the Moon.

Now, however, scientists have gathered evidence that the primary process producing the asteroids’ regolith is rock weathering and fragmentation that is caused by their ever-changing temperatures as they spin around, and in and out of sunlight.

Images from the laboratory show how slices of a meteorite fragmented under the force of temperature changes. Image credit: Simone Marchi/SSERVI

The soil on small Near Earth Asteroids (NEAs) around one kilometre (about half a mile) wide has been detected by observations from space probes as well as observatories on the ground. Experiments in the laboratory have shown that debris caused by impacts will be flung into space because these small asteroids do not have enough gravitational pull to keep hold of it.

The new findings, to be published in the journal Nature this week, tell planetary astronomers that the soil they observe must be from the asteroid itself.

Lead author of the research paper was Marco Delbo from the Observatoire de la Cote d’Azur in Nice, France. Announcing it, he said: “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.”

The experiments were carried out by researchers from the Observatoire de la Cote d’Azur, and the Institut Supérieur de l’Aéronautique et de l’Espace in France, and the Hopkins Extreme Materials Institute at Johns Hopkins University, and Southwest Research Institute (SwRI) in the USA. They used an X-ray scanner to measure how cracks grew due to thermal fatigue in various meteorites exposed to temperature changes.

Asteroid mission

An artist’s impression of a mission to explore a captured small asteroid. Image credit: NASA/Goddard Space Flight Center 

Supporting the research was NASA’s Solar System Exploration Research Virtual Institute (SSERVI), which is based at its Ames Research Center in Moffett Field, California. Its chief scientist David Morrison commented: “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.

“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.”

Summing up the findings, Simone Marchi, SSERVI researcher at SwRI and co-author of the paper, said: “The Sun acts like an oven. It heats up space rocks producing internal stresses that, over time, break them apart.”

Close interest in the new research will be shown by the NASA and commercial operators that plan to intercept asteroids passing Earth to study them, returnn samples or to mine them for their valuable resources.

The research suggests that such asteroids, several metres wide, may be covered with coarse soil or pebbles and the missions designed to capture them must therefore be able to cope with the presence of layers of loose rocky debris.