Page last updated at 15:00 GMT, Thursday, 21 January 2010

Meteorite colour mystery 'solved'

By Victoria Gill
Science reporter, BBC News

Near-Earth asteroid (SPL)
Asteroids are shaken up as they pass nearby the Earth

The Earth "changes the colour" of asteroids by shaking them up as they pass, according to scientists.

Researchers report that this solves the mystery of why the meteorites that land on the Earth often do not match the colour of asteroids in space.

Previous research had already shown that the solar radiation in space reddens the the surfaces of asteroids.

The new findings described in Nature journal, explain how they are "re-surfaced" as they pass close to Earth.

Richard Binzel, professor of planetary sciences at the Massachusetts Institute of Technology (MIT), led this research.

He explained to BBC News that his team used an infrared telescope to study the colour of asteroids in space, and compared that data to measurements taken from meteorites - chunks of asteroids that actually landed on Earth - that were examined in the lab.

"Most asteroids have reddish tinge," he said. "The solar wind damages the minerals and turns them red - like sunburn.

"But when we look at the colour of meteorites, it's very odd because [in most cases] it doesn't match."

Near-Earth asteroids are transformed by tidal forces when they come anywhere near our planet
Clark Chapman
Southwest Research Institute

But, according to Professor Binzel, some asteroids that pass close to the Earth do not have this red tinge. These near-Earth asteroids match the meteorites that scientists collect and study here on Earth.

"As they get close to the Earth, it gives them earthquakes," he told BBC News. "The earth just shakes it enough that the the rubble flips over - resurfacing it."

Dr Clark Chapman, an astronomer from the Southwest Research Institute in Boulder, Colorado, in the US explained that these asteroids were "not monolithic, solid bodies", and were more like "rubble piles".

So on the surfaces of these rubble piles, rocks are shaken and turned over, to reveal a fresh, unweathered surface underneath.

Dr Chapman, who was not involved in the study, wrote an accompanying article in Nature explaining its significance.

He wrote that the observations provided "proof that near-Earth asteroids are transformed by tidal forces when they come anywhere near our planet".

Professor Binzel said that this helps scientists to perform a kind of colour-matching, using the shade of an asteroid to find out more about its trajectory and where it came from.

"For each case of a 'fresh' asteroid, we can say it definitely came near the Earth," he said. And the 'old', red ones definitely didn't.

"Meteorites are samples from space and to make the most use of them, we need to know where they're coming from. Now we can match the samples [we have] to where they came from in space."



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