Asteroids “Sunburn” with Age

Main-belt asteroid 951 Gaspra
On October 29, 1991, the Galileo spacecraft flew past 951 Gaspra, an S-type asteroid situated in the inner asteroid belt. Gaspra measures 19 by 11 kilometers, and its faceted shape suggests that it is a fragment from a larger object that was shattered by collision roughly 500 million years ago.
Courtesy NASA, USGS, Cornell University, and the Galileo Project/JPL.
Long-term exposure to the radiation of space seems to affect just about everything — even rocks. Planetary geologists have suspected for some time that "space weathering" — the cumulative effect of cosmic rays, micrometeorite impacts, and high-energy radiation from the Sun — gradually reddens and darkens asteroidal surfaces. Now Robert Jedicke (University of Hawaii), David Nesvorny (Southwest Research Institute), and three colleagues have verified that space weathering really occurs, and they've made the first rough measurement of the rate at which the color change takes place.

The researchers examined 8,416 asteroids, all members of "families" with similar orbits. These orbital relationships imply that all the objects in a particular grouping are the remains of a larger precursor body shattered by a collision. By tracing the orbits back in time, dynamicists can determine when the collision took place and thus the family’s age. The families used in the study range between 3 million and 3 billion years in age. Jedicke’s team combined this information with color data from the Sloan Digital Sky Survey to obtain their results, which were published in the May 20th issue of Nature.

The new information helps solve a long-standing puzzle about the origins of space rocks called ordinary chondrites (OC), which make up about 80 percent of all known meteorites. Astronomers have suspected for decades that these are chips off of the most common class of objects, called S types, found in the inner asteroid belt. The problem is that S-type asteroids are only a rough spectral match to OC meteorites — the asteroids are much redder and their spectral signatures are more muted.

The differences would make sense if space weathering alters the asteroids’ surface layer in ways that disguise their true composition. If that's the case, then the color of a freshly opened OC meteorite should match that of a completely unweathered S-type asteroid. No known asteroid is that young, but Jedicke and his team used their age-color data to extrapolate back to what a freshly exposed S-type should look like — and their model predicts a close color match to an OC meteorite.

"Asteroids get more red with time in exactly the right manner and at the right rate to explain the mystery of the color difference between them and the OC meteorites," says Jedicke.

Most astronomers were already convinced that space weathering takes place, says planetary scientist Richard P. Binzel (MIT). "The outstanding question is how long does it take, and this is the first attempt at getting a handle on the timing." But he cautions that there are other factors to take into account in future studies. For example, the S class of asteroids contains objects of different compositions, which complicates the picture.

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