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 survey its rocks. Until then, they have to settle for examining meteorites from Mars. These rocks are chunks of the Red Planet that land on Earth after hurtling through space.

The meteorites arrive in pretty rough shape. ###### Still, these meteorites provide a lot of information to geologists, whose job it is to study rocks. But it can be hard for geologists to make sense of what they see when they can't agree on how old a rock is. And when it comes to certain rocks, scientists disagree by as much as 4 billion years.

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?

###### These rocks are known as shergottites.

Was There Life On Mars?

Martian space rocks tend to be very old. Shergottites, though, are generally thought to be remarkably young. Most scientists believe they were formed by volcanoes about 150 to 250 million years ago. ######

If so, shergottites are a sign that Mars might have had volcanoes not too long ago. And that means it might have been friendly to some form of life, said planetary geochemist Munir Humayun.

But some people believe that shergottites are actually about 4 billion years old. If this turned out to be true, scientists would have to change the way they think about life on Mars.

###### However, "if there hasn't been [volcanic activity in] 4 billion years, ... it would be very difficult to believe there was life left on Mars."

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.

A Tiny Rock Might Have The Answer

###### But if a big crash or explosion melts the rock down, that evidence gets erased.

That's why scientists picked the tiny baddeleyites: They are hard little minerals. It's not easy to melt them down. By looking at the radioactive uranium and lead in this tough rock, the scientists found that it formed sometime around 187 million years ago. ######

The scientists could also see that the meteorite had been damaged. This had been caused by whatever sent it shooting off the Martian surface around 22 million years ago. The shock had muddled up the minerals inside, though the chemicals themselves were mostly unchanged.

The result was something like shaking up a sealed box of chocolates. "It looks good on the outside, but all the pieces are dislodged on the inside," Schmitt said.

###### But that was what he had expected. "If they had found that it was 4 billion years old, I would eat my hat."

The researchers plan to study other Martian meteorites. Their hope is that they will discover new information about our neighbor, the Red Planet.]]>