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Identifying Martians In Antarctica

Antarctica is a great place for finding meteorites. Meteorites are easy to spot on the ice. Also, ice floes act as conveyor belts to concentrate the meteorites in certain regions. Ice with a cargo of meteorites, which may have fallen recently or a million years ago, piles up behind mountain barriers. Over time, wind erodes the ice and exposes the meteorites at the surface.

Since the first meteorite find in Antarctica in 1912, about 20,000 meteorites have been recovered. Of these, only a very small number are from Mars. In fact, from finds all over the world, only 34 Martian meteorites have been identified.

Three lines of evidence are used to determine which meteorites came from Mars: age, composition of trapped gases and composition of the rock.

Most meteorites from Mars formed more recently (as little as 200 million years ago) than other meteorites. All Martian meteorites are igneous (formed by volcanic activity), therefore, the rocks had to come from a body that still had volcanic activity at that time, which rules out asteroids or comets.

NASA's many Mars missions, including the Viking probes launched in the mid-1970s, have provided information about the geology and atmospheric composition of the red planet. The proportions of gases trapped in the meteorites – including nitrogen, carbon dioxide and the rare gases neon, argon, krypton and xenon – match those of the Martian atmosphere.

Also, the ratio of hydrogen to deuterium – heavy hydrogen – in the meteorites is lower than that on Earth. Because Mars is only one-tenth the mass of Earth, it cannot gravitationally retain the lighter hydrogen, which escapes and leaves the heavier deuterium behind.

Finally, the chemistry of rocks suggests that they formed in the presence of more oxygen than we would expect to find on small bodies such as asteroids.

Incidentally, the Martian meteorites tend to be shades of gray and black, like the igneous rocks just below the surface of Mars. Mars gets its red color from oxidized iron (rust) produced by the weathering of the surface rocks. None of the meteorites is a weathered surface sample.

One of the most famous rocks in the world is a Martian meteorite labeled ALH 84001, which was found in Allan Hills, Antarctica, in 1984. A decade ago, scientists found strange “fossil-like” structures in the meteorite and claimed that it was evidence for life on Mars. The claim remains highly controversial.

Sherry Seethaler

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