Asteroids are Shaken up by Mars--Not Just Refreshed by Earth

The surfaces of most asteroids appear redder than meteorites. For years, astronomers have wondered why that is. In 2010, though, the answer was uncovered. It turned out that Earth's gravity could cause "asteroid quakes" that shifted surface grains. Now, scientists have discovered that Earth isn't the only planet where this happens. Mars can also stir up asteroid surfaces and "refresh" them.

When asteroids orbit our solar system's main asteroid belt, they're exposed to cosmic radiation. This changes the chemical nature of their surfaces and reddens them over time. Yet asteroids that venture out of the main belt and pass close to Earth feel the effects of our planet's gravity. This "refreshes" the asteroids as surface grains shift and expose fresh ones beneath. So far, researchers have calculated the orbits of 60 refreshed asteroids, and they've found about 10 percent of these never cross Earth's orbit. Instead, they only come close to Mars.

"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," said Francesca DeMeo, one of the researchers, in a news release.

The findings are somewhat surprising. Mars is one-third the size of Earth and one-tenth as massive. This means that it exerts a far weaker gravitational pull on surrounding objects. That said, Mars' location in the solar system places it in close proximity with the asteroid belt. This increases the chance of close asteroid encounters and make explain the refreshing that the astronomers saw.

"Picture Mars and an asteroid going through an intersection, and sometimes they'll both come through at very nearly the same time," said Richard Binzel, one of the researchers, in a news release. "If they just barely miss each other, that's close enough for Mars' gravity to tug on [the asteroid] and shake it up. It ends up being this random process as to how these things happen, and how often."

The findings reveal a little bit more about the lives of asteroids. More specifically, it shows that Earth isn't the only planet that can refresh them as they pass through the solar system. It turns out that Mars is a little more powerful than astronomers expected.

The findings are published in the journal Icarus.