The gravity of the Red Planet causes the asteroid'ssurface material to shift, exposing fresh material, whichexplains the colour variation previously observed.
Richard Binzel, a professor of planetary sciences atMassachusetts Institute of Technology (MIT) and colleagueFrancesca DeMeo calculated the orbits of 60 refreshedasteroids, and found that 10 per cent of these never crossEarth's orbit.
Instead, these asteroids only come close to Mars,suggesting that the Red Planet can refresh the surfaces ofthese asteroids.
"We don't think Earth is the only major driver anymore,and it opens our minds to the possibility that there are otherthings happening in the solar system causing these asteroidsto be refreshed," said DeMeo.
The idea that Mars may shake up the surface of anasteroid is a surprising one: The planet is one-third the sizeof Earth, and one-tenth as massive - and therefore exerts afar weaker gravitational pull on surrounding objects.
But Mars' position in the solar system places the planetin close proximity with the asteroid belt, increasing thechance of close asteroid encounters.
"Mars is right next to the asteroid belt, and in a way itgets more opportunity than the Earth does to refreshasteroids," Binzel said.
"So that may be a balancing factor," said Binzel.
DeMeo, who suspected that Mars may have a hand inaltering asteroid surfaces, looked through an asteroiddatabase created by the International Astronomical Union'sMinor Planet Center.
The researchers looked at 60 asteroids, mapping out theorbit of each and determining which orbits had intersectedwith those of Earth or Mars.
DeMeo then calculated the probability, over the last500,000 years, that an asteroid and either planet would haveintersected, creating a close encounter that could potentiallygenerate asteroid quakes.
"Picture Mars and an asteroid going through anintersection, and sometimes they'll both come through at verynearly the same time," Binzel said.
"If they just barely miss each other, that's close enoughfor Mars' gravity to tug on (the asteroid) and shake it up. Itends up being this random process as to how these thingshappen, and how often," said Binzel.
From their calculations, the researchers found that 10per cent of their sample of asteroids only cross Mars' orbit,and not Earth's.