18. ALH 84001

This meteorite, source of the possible traces of ancient martian life, was collected in the Allan Hills area of Antarctica (slide #13) during the 1984 ANSMET field season. As originally found, ALH 84001 was brick-shaped, 17 × 9.5 × 6.5 centimeters, and weighed nearly 2 kilograms. It was quite unusual, and so was the first meteorite of the season processed at the Johnson Space Center. Its name reflects this history: “ALH” for the Allan Hills find site, “84” for the field season, and “001” for being first. It is not much like any of the other martian meteorites and is very similar to one kind of meteorite from the asteroids. So it was classified as a “common” asteroidal meteorite, and its martian origin was not discovered until 1993. It then became the twelfth known martian meteorite.

The other martian (SNC) meteorites are “young” volcanic rocks, less than 1.3 billion years old; ALH 84001 is much older, having solidified from lava about 4.5 billion years ago (slide #19). As the planets (including Mars) formed only about 4.55 billion years ago, ALH 84001 formed very soon (in geologic time) after Mars itself did. Then, about 4.0 billion years ago, ALH 84001 was heated nearly to melting. This probably happened when a large asteroid hit Mars, leaving behind one of the huge impact craters that pock Mars’ surface (see slide #2).

Some time later, carbonate minerals and possible remnants of ancient martian life were deposited in ALH 84001. Many researchers think that the carbonate minerals (slide #22) were deposited when the rock was saturated by martian groundwater. The water was rich in carbon dioxide gas, possibly from Mars’ atmosphere. As the water flowed through ALH 84001, it deposited rounded patches and spheres of carbonate minerals (slide #22). These carbonate spherules contain all the possible microfossils and other possible traces of life described by McKay and his co-workers in Science magazine (Science, August 16, 1996). Then, only 16 million years ago, a comet or asteroid impact on Mars ejected ALH 84001 off Mars and into its own orbit around the Sun. Over time, its orbit changed until it crossed the Earth's orbit; 13,000 years ago, it collided with the Earth and landed as a meteorite in Antarctica.

Martian meteorite, host to the possible microfossils and possible chemical traces of martian life. The cube at the lower right is 1 centimeter on a side. S85-39565, NASA/JSC

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