Many asteroids have a reddish tint as a result of sun-induced weathering and bombardment by micrometeorites, but a select few retain a relatively pristine gray appearance. In the January 21 issue of Nature planetary scientist Richard Binzel of the Massachusetts Institute of Technology and his colleagues report a finding that may clear up the asteroid-coloration mystery once and for all. (Scientific American is part of Nature Publishing Group.)

Binzel and his colleagues found that an asteroid passing through Earth's vicinity is literally shaken by the encounter, turning up fresh material on the asteroid's loosely bound surface.

A great many asteroids show evidence of weathering, a reddening process likely marking degradation of minerals by the charged particles of the solar wind that is "almost like a sunburn," Binzel says. And a 2009 study from a related group had shown that this reddening process can happen in less than a million years, which is fast on an astronomical timescale. So whatever phenomenon is responsible for refreshed asteroidal surfaces, it must be a recurrent or recent process; otherwise the solar wind would have had time to overwrite its effects.

The researchers traced the orbits of 95 known asteroids 500,000 years back in time to test the theory that past encounters with Earth might affect the asteroids' surface properties, an idea proposed five years ago by David Nesvorný of the Southwest Research Institute in Boulder, Colo., and his colleagues. "We found that all of the fresh ones had encountered the Earth within the past few hundred thousand years," Binzel says. Similarly, all of the 20 objects that appear to have remained outside the moon's orbit for the past 500,000 years are ruddy—they have not passed close enough to Earth for a resurfacing seismic shake.

Passing within about 100,000 kilometers, or roughly a quarter of the Earth–moon distance, the researchers propose that asteroids feel a sufficient influence from Earth's gravity to induce what Binzel calls "a little bit of tidal stress, a little tug-of-war with the Earth." Because their surfaces are essentially loose rubble, the shaking caused by that tug-of-war is enough to turn up fresh gray material. "It's kind of like if you dig into a snowbank next to a highway," Binzel says. "You turn a few shovelfuls over and the fresh white snow will replace the dirty snow on top."

The same phenomenon should occur for asteroids whose orbits buzz the comparably sized Venus and, to a lesser extent, the less massive Mars, although the asteroid population near those planets is less well characterized. An observable test of the seismic shaking theory may come in 2029, when the near-Earth asteroid Apophis will pass within about 30,000 kilometers of Earth, well within the proximity limit established by Binzel and his colleagues and near the orbit of some geosynchronous satellites. If Binzel and his colleagues are correct, then Apophis, which currently boasts a grizzled, space-weathered appearance, may leave its 2029 encounter with Earth looking a refreshed shade of gray.