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6.24 Martian Meteorites

For centuries, people have been looking at Mars in the night sky and speculating about the red planet from afar. Many scientific questions required just one piece of Mars for us to prove or eliminate different theories. What wasn't realized was that needed piece was already sitting right under our noses on planet Earth. Scientists have identified rocks from all over the Earth as meteorites. Among these, they’ve found some unusual samples that don't seem to fit among other meteorites. Scientists made a surprising discovery during the 1980s: these rocks are from Mars!

One of the Martian meteorites, Chassigny, was actually observed falling. In 1815, villagers in France heard a loud explosion, and then watched as a large rock (4 kilograms, or about 9 pounds) fell from the sky. Not until 1981 was Chassigny, along with a group of similar meteorites, suspected to have a Martian origin. Before it was determined that these meteorites are from Mars, they were called SNCs, after the three characteristic meteorites in the group: Shergotty, Nakhla, and Chassigny. Most Martian meteorites are in the Shergotty group, named after a meteorite that fell in India in 1865. They are made of igneous rock and most appear to be surprisingly young, around 200 million years old. The next largest group are also igneous rocks, named after the Nakhla meteorite, which fell in an Egyptian village in 1911 (A widely told but apocryphal story says that it killed a dog in the village). Only one other meteorite like Chassigny has been recovered, an olivine rock that fell in Morocco called "Black Beauty." 

Their unusual origins was revealed by the gases trapped in the minerals. Within these rocks, the trapped gases exactly match the Martian atmospheric composition measured by landers on Mars. This is a very robust measurement, with both the mixture of chemicals and the proportions of different isotopes of each element matching exactly.

These rocks were blasted off Mars when asteroids hit the surface and excavated craters. The fragments drifted through space for hundreds of millions of years before landing on the Earth. Martian meteorites are rare. Of more than fifty thousand meteorites found on Earth, only about 120 are believed to come from Mars. Many Martian meteorites were discovered on the Antarctic ice pack, where extraterrestrial material is easy to identify against the bright ice. More and more meteorites are also being found in the Saharan desert of Northwest Africa, where dark meteorites are conspicuous on the surrounding sand. Mars meteorites are expensive. On the open market you could expect to pay a lot for a small chip. Depending on the type of meteorite, it might cost up to $30,000 per ounce, thirty times the price of gold!

The meteoric Martian rocks are samples of basaltic lava. Using the technique of radioactive dating, we can tell that a group of them formed about 1.3 billion years ago. Scientists believe they came from the lava plains that cover much of Mars. This evidence of relatively young lava confirms that Mars has had major volcanic eruptions in the last third of its history. At least one Martian meteorite, ALH84001, is 4.5 billion years old, dating from the oldest parts of Mars’ surface. The “ALH” in its name stands for Allan Hills, the area of Antarctica where the meteorite was found. This old rock is rich in carbonates deposited by evaporating moisture (in the same way that evaporating water in your tub leaves a carbonate “bathtub ring”). The Allan Hills meteorite also contains what some scientists claim are fossil remnants of bacterial life, although this has not been proven and most planetary scientists think the evidence is not convincing.

As tools for understanding the red planet, Martian rocks have one big advantage and one big disadvantage. The advantage is that they are essentially free! Compared to the cost of a sample return mission to Mars, the cost of sending expeditions to the Antarctic or the Sahara to look for more meteorites is tiny. On the other hand, we don’t know exactly what geological locations they came from on Mars. So we can’t use them to date or describe specific geological units. For this reason, scientists would still like to land probes on Mars that could return rocks from known features, such as riverbeds, volcanoes, and the polar regions.

Thin section in cross polarized light of Chassigny meteorite. Click here for original source URL.