A few of the world's known meteorites (~28) are believed to be pieces of Mars that have traveled to Earth following a massive asteroidal or cometary impact with the Martian surface. Because all of these meteorites do not contain any free metal, as many meteorites do, they cannot be found using a metal detector. Therefore, people must observe their fall to Earth, or find them in a conspicuous location, such as the Antarctic ice sheets. The National Science Foundation supports annual expeditions to Antarctica to collect meteorites of all types through the Antarctic Search for Meteorites (ANSMET) Program. This program has collected several Martian meteorites since 1977.

The Martian meteorites traditionally have been referred to as "SNC" meteorites. They were so named because all of the meteorites could be classified in terms of three primary igneous classes based on their composition: (S)hergottites (basalts and lherzolites), (N)akhlites (clinopyroxenites), and the (C)hassignites (dunites). Recently, another meteorite named ALH 84001 (orthopyroxenite), was recognized to be a Martian meteorite that does not fall into one of these three classes. For more information on each of the meteorite samples, see these NASA web pages.

So how do we know that these are pieces of Mars, rather than of an asteroid, as in the case of most meteorites? There are primary characteristics of these meteorites that distinguish them from the asteroid-derived meteorites.

• First, all of these meteorites, with the exception of one, are geologically young, with ages of crystallization ranging from 180 million years (Myr) to 1.3 billion years (Gyr). This may seem old to us, but our solar system is roughly 4.5 billion years old! So most of these rocks were "born" only in the last quarter (or less) of our solar system's existence. Most of the asteroids are small compared to planets, and became fully solid (crystallized) soon after the formation of the solar system, therefore, any rocks much younger than 4.5 Gyr were probably formed on a body that was pretty large and geologically active, like a planet such as Mercury, Venus, Earth, or Mars.

• Second, when a molten rock crystallizes and becomes solid, gases get trapped in the minerals in the rock. All of the terrestrial planets have different abundances of these gases. When scientists looked at the trapped gases in these meteorites, they found that the abundances didn't match the known gas abundances found in rocks on Earth. Luckily, the Viking landers measured the gases of the Martian atmosphere from the planet's surface. When the abundances of gases in these meteorites are compared to the abundances of gases in the Martian atmosphere, it turns out that there is an almost 1:1 correlation (Figure 1), strongly suggesting that these meteorites are rocks that were crystallized on Mars.

Figure 1. Comparison of gas abundances in shergottite EET79001 and the Martian atmosphere. From the NASA Meteorites page.

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