Rock Doc: Meteorite makes impression on geologists

I think the most memorable single day of all my years as a student was the afternoon I got to examine moon rocks in graduate school.

Rocks on Earth are exposed to water throughout their existence, and water breaks down mineral grains on a tiny scale. If you look at thin slices of rock under a microscope – a normal activity for geologists like me – you see this tiny breakdown at work. To use a technical term, the mineral grains appear “cruddy” because they are breaking down to minerals like clay due to the presence of water.

What impressed me so much about the moon rocks we looked at that memorable afternoon was that they were pristine – they had not reacted with water over time because there isn’t water on the surface of the moon. So looking at the thin sections of moon rocks under the microscope was close to breathtakingly beautiful – the little mineral grains were perfectly formed and preserved.

We had to send people to the moon to bring back the samples I looked at as a student. But sometimes Mother Nature does the hard work of bringing to Earth pieces of the moon and even more distant planets in the solar system. They arrive as meteorites, raining down from the skies.

Recently there’s been some news about a meteorite breaking up over Russia. There’s also been news about a highly unusual and interesting meteorite that landed in Northwest Africa. The glad tidings were published in Science, and a piece about them appeared in Science News. The meteorite was given a name based on where it was found, so it’s called Northwest Africa 7034. It’s a piece of Mars; we know that from its chemical composition. NWA 7034 looks like the rocks examined by NASA’s Spirit rover in Gusev Crater.

Meteorites can be dated in the same way that rocks here on Earth can be assigned dates. From “parent/daughter” calculations we can say that NWA 7034 is about 2.1 billion years old. On Earth we geologists would call that time part of the Proterozoic Eon. “Proto” implies earlier, and “zoic” comes from the Greek for life. Life on Earth 2.1 billion years ago was quite simple, made up largely of single-celled organisms in the oceans.

On Mars, a different set of names describes time. NWA 7034 formed during what’s known on the Red Planet as the Amazonian. Some 2.1 billion years ago stands near the time of linkage between an earlier wet and warm Mars and the dry and cold planet we know today.

One of the really interesting things about NWA 7034 is that it contains a lot of water. Scientists believe the water inside NWA 7034 came from Mars and is not contamination from the Earth, because of the different ratios of hydrogen types found in the sample.

As further analysis is done on the little meteorite, more information may be teased out of it. That’s good for those of us interested in Mars beyond tales of little green men.