Martian rocks photographed by the robotic Spirit rover in 2006 as it rolled across Mars. Photo: Mars Exploration Rover Mission, Cornell, JPL, NASA
LOS ANGELES - Scientists who study planets dream of sending a geologist to Mars to inspect its rocks by hand. Until then, they have to settle for examining meteorites - chunks of the Red Planet that land on Earth after hurtling through space.

Though a little banged up after their fall through the atmosphere, these meteorites provide a lot of information. ###### And scientists disagree by quite a bit - up to 4 billion years - for certain rocks.

Mars itself isn't much older than 4 billion years. So the question is: Do these rocks tell the beginning, or the end, of Mars' story?

Recently, an international team of scientists sought to sort this out for a particular group of meteorites known as shergottites.

Young, As Martian Rocks Go

###### Most scientists believe they were formed as a result of volcanic activity about 150 to 250 million years ago. This would mean they formed during Earth's Jurassic period, when dinosaurs roamed the land.

If so, shergottites are a sign that Mars might have been geologically active fairly recently. ######

But some scientists have estimated that shergottites are actually about 4 billion years old. If this were true, scientists would have to change the way they think about Mars' history.

"Most of us understand that if there is ... life on Mars, it's not going to be roaming on the surface. ... but it could be thriving underground with the heat energy provided by [volcanoes]," Humayun said. ######

To settle the question, researchers examined a meteorite known as Northwest Africa 5298. They looked at very tiny deposits within it called baddeleyites. These tough, zirconium-rich minerals are "10 times smaller than the diameter of a human hair," said geochemist Axel Schmitt.

Rugged Microscopic Minerals

###### But when the meteorites slam into Earth, that melts the rock down and can erase the evidence.

That's why scientists picked the tiny baddeleyites: Once they form, they are durable little minerals. It's not easy to melt them down. ###### This proved that the baddeleyites were young.

The scientists could also see evidence of the damage done to the meteorite by whatever event sent it shooting off the Martian surface around 22 million years ago. The shock had muddled up the mineral structure inside, though the chemicals themselves were mostly unchanged. ######

"It looks good on the outside, but all the pieces are dislodged on the inside," Schmitt said.

The study further shows that the shergottites weren't old rocks, Humayun said. But that was what he had expected. "If they had found that it was 4 billion years old, I would eat my hat."

###### Their hope is that they will discover new information about our neighbor, the Red Planet.]]>