Wednesday, July 30, 2014

Rock doc: Rocks from the Moon and Mars

March 20, 2013 |

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 here on Earth are exposed to water throughout their existence, and water acts to break 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 liquid 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.

Meteorites can land anywhere on our planet. One recently broke up over Russia. But it’s generally most fruitful to hunt for them in deserts or places like Antarctica. In both those environments, meteorites that land tend to stay at the surface, not being covered by soil or breaking down due to liquid water.

Recently there’s been some scientific 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 or NWA 7034 for short. It’s a piece of Mars – we know that from it’s chemical composition which is highly similar to certain parts of the Red Planet.

NWA 7034 looks like the rocks examined by NASA’s Spirit rover in Gusev Crater. That’s interesting because most Martian meteorites we find here on Earth are relatively poor matches for the parts of the planet examined by rovers and orbiters. We think that such meteorites may come from elsewhere on the planet or deeper than the surface we now see.

Meteorites can be dated in the same way that rocks here on Earth can be assigned dates. The techniques used depend on radioactive decay of certain elements found in rocks. Knowing how long it takes a “parent” atom to decay to its “daughter” atom counterpart is something determined in the laboratory. Knowing the number of parent and daughter atoms in a natural sample like a mineral allows us to apply a little math and calculate how old the mineral – and therefore the rock enclosing it – is.

From parent/daughter calculations we can say that NWA 7034 is about 2.1 billion years old. Clearly, that’s a long ways back. On Earth us 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, being 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. The water may have come from liquid water that existed on Mars when a volcanic eruption formed rock, or it may have come from permafrost melted by that eruption. 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 are 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.

Dr. E. Kirsten Peters, a native of the rural Northwest, was trained as a geologist at Princeton and Harvard. This column is a service of the College of Agricultural, Human and Natural Resource Sciences at Washington State University.



E. Kirsten Peters



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