Rock from Mars

How do we know that the rock that the Pittsburgh scientist found was really from Mars?

There are 12 known meteorites that are thought to have come from Mars. All of these meteorites have small pockets of gas in them, and when the gas was analyzed, it was found to have almost exactly the same chemical ratio as gas samples analyzed by the Viking probes which landed on Mars in the late 1970's. This ratio of chemicals is very different than that of the Earth, and matches Mars so well that most scientists agree the meteorites must have come from Mars.

The leading theory on how they get to Earth is that a large asteroid hits Mars. The impact chews up the landscape, throwing pieces of rock from the surface of Mars all over the place. If the impact is energetic enough, the rocks can actually leave Mars completely and start to orbit the Sun. Generally speaking, most debris like this will have have very elliptical orbits around the Sun, bringing them into the Earth's path on occasion. Over long periods of time, the rocks will impact the Earth. This particular meteorite was blown off Mars about 16 million years ago and hit the Earth only 13,000 years ago. Scientists know how long ago the rock was blasted from Mars due to the amount of exposure to outer space it had (in the form of cosmic ray hits; fast moving particles that are for the most part absorbed by a planet's atmosphere), and they know how long it was sitting in Antarctica by looking at the area in which it was found.

A very complete WWW site with all sorts of info, including a transcript of the NASA announcement and related work with Martian meteorites can be found at http://www.fas.org/mars.

Another site with more technical info located here.