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12/03/1996 08:00:00

Question of the Week: How Do We Know That a Rock Found In The Ice In Antarctica Came From Mars?

Question of the Month Submitted by: Audra Martin, La Puente, Calif., and answered by: Bill Bottke, Postdoctoral Fellow, Division of Geological and Planetary Sciences, Caltech.

Meteorites are rocks that fall to Earth from space. Most are thought to be asteroid fragments that have survived fiery entry through Earth's atmosphere. Twelve of the thousand or so meteorites held in worldwide collections, however, are thought to come from Mars. Eleven of these meteorites, formed roughly 1.3 billion years ago, were named the SNC meteorites after the sites where they were found: Shergotty (India), Nakhla (Egypt), and Chassigny (France).

The 12th Martian meteorite is much older and different from the rest. It is called ALH 84001, named for the year it was discovered, 1984, and the location where it was found, Allen Hills, Antarctica. It was formed 4.5 billion years ago, when Mars had a much thicker atmosphere and liquid water on its surface. Recently, scientists have suggested that ALH 84001 might even contain fossil evidence of ancient Martian life.

How do we know that these meteorites are from Mars, when people have never been there and no rocks have been collected on its surface? In 1976, two NASA spacecraft named Viking 1 and 2 landed on Mars and analyzed its atmosphere and surface. These spacecraft examined soil and air samples using onboard instruments, making careful measurements and radioing their data back to scientists on Earth. After careful study, it was determined that the atmosphere of Mars was very different from Earth's atmosphere or any other combination of gases found in the solar system. Then, by analyzing small traces of gas trapped in the interior of these 12 meteorites, scientists were able to identify the characteristic "fingerprint" of the Martian atmosphere, proof that these rocks were blasted off the surface of Mars at some time in the past.

Even before gas was discovered in these meteorites, scientists were suspicious that they might have originated on Mars. The SNC meteorites have young formation ages, and all 12 Martian meteorites have complex chemical compositions that set them apart from other known meteorite classes. Moreover, the abundance of oxygen isotopes (different kinds of oxygen) in the meteorites are inconsistent with oxygen isotopes found in Earth rocks.