January 29, 2007

Space rocks Wadhwa's world at meteorite center

Posted: January 29, 2007

It was a piece of Mars that set ASU geology professor Meenakshi “Mini” Wadhwa on a career investigating meteorites. At the time, Wadhwa, who directs the Center for Meteorite Studies in the School of Earth and Space Exploration, was a graduate student at Washington University in St. Louis and newly arrived from India.

“I had come from a terrestrial geology background, and I wanted to study other planets,” she says. “I knew about the lunar material from the Apollo program, but not about any other extraterrestrial rocks.”

Then one of her professors told her about the meteorites from Mars.

Meenakshi “Mini” WadhwaIncredulous at first that pieces of Mars were here on Earth, Wadhwa knew immediately what direction her career would go. It soon broadened to include all kinds of meteorites.

“Meteorites are rocky pieces from the first solid bodies to form when our solar system was born,” she says.

They include the oldest rocks known, which have an age of more than 4.5 billion years. This age is greater than the oldest rocks native to Earth, and it defines the age of the solar system.

“One of my research programs is to resolve in detail the timeline for the formation of these bodies,” Wadhwa says.

Meteorites arrive on Earth through a haphazard process that begins in a collision between rocky bodies in space called asteroids. These range in size from perhaps a dozen yards across up to hundreds of miles in diameter. As rocky fragments fly after the collision, some escape the gravity of the parent object and sail off into space.

After drifting around the sun for perhaps millions of years, a rock fragment may collide with Earth. If the fragment is larger than an inch or so across – and if it escapes falling into the ocean – it will land on the ground. (Space rocks smaller than an inch mostly burn up as they pass through the atmosphere.) With luck, someone may find the meteorite and let scientists examine it.

Because a meteorite doesn't arrive with a return-address label, scientists first determine what kind of space rock it is. From that, they draw a preliminary conclusion about its birthplace.

Most meteorites are primitive, rocky objects that formed in the same cloud of gas and dust from which the sun and planets formed. Other meteorites are made of mostly nickel-iron: These include the famous Cañon Diablo meteorite that blasted Meteor Crater in northern Arizona some 49,000 years ago.

But scientists found other meteorites that didn't fit into existing categories. One of these oddball space rocks landed in the village of Chassigny, France, in 1815. Another fell on Shergotty, India, in 1865, and a third landed at Nakhla, Egypt, in 1911. Taken together, these meteorites somewhat resembled each other, and scientists took to calling them the SNC meteorites, for Shergotty-Nakhla-Chassigny. The SNCs (or “snicks” for short) are now known to be fragments of the planet Mars.

“If you just look at a Mars meteorite, you'd be hard-pressed to tell it from an ordinary Earth rock,” Wadhwa says.

Scientists finally identified these as coming from Mars by noting that they contain trapped gases. As she explains, these “have a composition matching the Martian atmosphere, as detected by the Viking landers in the mid-1970s. That cinched a Martian origin.”

Likewise, careful analysis showed that other oddball meteorites were pieces of the moon. They were chemically similar to rock samples brought back from the moon by Apollo astronauts. To date, about 40 lunar meteorites and 36 Martian ones are known.

Housed at the Center for Meteorite Studies, ASU's meteorite collection numbers more than 1,500 – including several Mars meteorites and two from the moon. It's the largest meteorite collection at any university in the world.

“We have such an incredible resource to work with,” says Wadhwa, adding that it was part of what drew her to ASU from the Field Museum in Chicago, where she was the curator of meteorites.

“For me, as an analytical geochemist, there are wonderful facilities here,” she says.

Adding to the facilities will be a new, state-of-the-art mass spectrometer, one of the tools for studying chemical elements within meteorites.

Besides meteorites, Wadhwa's research involves developing new methods for investigating tiny amounts of extraterrestrial materials, such as those returned by the recent spacecraft missions Genesis and Stardust. Wadhwa received the prestigious Guggenheim Fellowship in 2005 in support of her work on the solar wind sample returned by NASA's Genesis spacecraft. She also is a science team member of OSIRIS, a sample-return mission to an asteroid; it is one of three Discovery-class missions NASA is considering.

Robert Burnham, robert.burnham@asu.edu
(480) 458-8207