For nearly as long as astronomers have been able to observe asteroids（小行星）, a question has gone unanswered: Why do the surfaces of most asteroids appear redder than meteorites（陨石） -- the remnants of asteroids that have crashed to Earth? In 2010, Richard Binzel, a professor of planetary sciences at MIT, identified a likely explanation: Asteroids orbiting in our solar system's main asteroid belt, situated between Mars and Jupiter, are exposed to cosmic radiation, changing the chemical nature of their surfaces and reddening them over time. By contrast, Binzel found that asteroids that venture out of the main belt and pass close to Earth feel the effects of Earth's gravity, causing "asteroid quakes" that shift surface grains, exposing fresh grains underneath. When these "refreshed" asteroids get too close to Earth, they break apart and fall to its surface as meteorites.

Since then, scientists have thought that close encounters with Earth play a key role in refreshing asteroids. But now Binzel and colleague Francesca DeMeo have found that Mars can also stir up asteroid surfaces, if in close enough contact. The team calculated the orbits of 60 refreshed asteroids, and found that 10 percent of these never cross Earth's orbit. Instead, these asteroids only come close to Mars, suggesting that the Red Planet can refresh the surfaces of these asteroids.

"We don't think Earth is the only major driver anymore, and it opens our minds to the possibility that there are other things happening in the solar system causing these asteroids to be refreshed," says DeMeo, who did much of the work as a postdoc in MIT's Department of Earth, Atmospheric and Planetary Sciences.

DeMeo and Binzel, along with former MIT research associate Matthew Lockhart, have published their findings in the journal Icarus.