A comprehensive review of asteroids of varying ages suggeststhe rocks turn color over time, explaining a mystery rooted in space rocksfound on Earth.

Scientists used various methods to estimate 100,000 asteroid'sages from 6 million to 3 billion years. Our Sun is 4.6 billion years old, andasteroids formed shortly after its birth. Many have collided since, sometimescoming together to make larger rocks and sometimes being broken into fresh,smaller pieces.

The new study appears to answer an old question about whysmall meteorites, which are chips off asteroids, are often a different colorthan the typical asteroid.

Ordinary chondrites are the most common type of meteoritefound on Earth. Their surfaces are affected by their fall through Earth'satmosphere, so they are usually studied in laboratories by observing theirfreshly cut and exposed interiors, which are bluish. Asteroids, however, arereddish.

The surfaces of asteroids are reddened over billions by spaceweathering effects, the study concludes.

"Asteroids get more red with time in exactly the rightmanner and at the right rate to explain the mystery of the color differencebetween them and [ordinary chondrite] meteorites," said Robert Jedicke, aUniversity of Hawaii researcher who led the study. "Even though we havefound a link between the two types of objects, we still don't know what causesspace weathering."

Jedicke had a terrestrial analogy:

"If you were given a piece of rock from the Grand Canyon,you might expect that it would be red, like the colorful pictures in travelmagazines. You'd be forgiven for questioning its origin if the rock had abluish color. But if you were then told that the rocks turn from blue to GrandCanyon red because of the effects of weather, then everything might make sense.Your gift is simply a fresh piece of exposed rock, whereas the pictures you'veseen show weathered cliff faces millions of years old."

Once Jedicke's team refines the analysis by obtaining morecolors of the youngest-known asteroid surfaces, it will be possible todetermine the age of any asteroid from its surface color, they say. Theresearchers are currently searching for a space-weathering effect on othertypes of asteroids in the solar system.

The work drew on observations by the Sloan Digital SkySurvey (SDSS) and is detailed in the May 20 issue of the journal Nature.

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