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Monday, October 13, 2014

Meteorite Basics

From space.com - thank goodness it's only a painting!
Meteors. Rocks that fall from space. Who doesn't love a space rock? Okay, I'm weird. I admit that. But I do find meteors fascinating and I'd love to share some of my fascination with you.

Where do they come from?
The easy answer is: Outer space. But where in outer space?
Meteorites come in three basic types: iron, stony iron, and stony. There are more technical terms for them and they are further classified, but for now, these three will work. (If you want to know more, check out this website.)

Iron meteorites are by far the easiest to identify. They are an iron and nickel mix, but mostly iron. Anytime geologists or meteorite hunters find a blob of metal, especially one that shows crystalline structure, they know they've found a meteorite.
Stony iron meteorites are a little more tricky to spot. They're a mix of iron-nickel and silicate minerals, like clay. Stony meteorites are very difficult to find unless you can track them falling. They're very similar to rocks you'd find on the surface of Earth. Surprisingly, a lot of stony and stony-iron meteorites contain quite a bit of water and carbon compounds. If you remember news stories from quite a few years ago, astronomers were surprised to find amino acids, basic building blocks of life, present in meteors and comets. These compounds are sometimes called "organics", not because they were created by life processes, but because they are carbon-oxygen-hydrogen long-chain molecules that are the same as those produced by life processes. Things like methane and ammonia.

So what would they look like if you analyzed them in a spectrometer? You'd get a readout of the basic chemical concentrations of different elements. Take a look at the graph halfway down this page. See how the concentrations of silicates (SiO2) are different for meteors from Mars, the moon, or other known asteroids? That's one way we can tell they are meteorites and not Earth rocks.

Back to the question of where they come from. Chemical composition will give us a clue to the original body. Most meteorites come from asteroids, although we do have some from the moon and Mars. Different asteroids have different compositions but they are still pretty similar to each other. All of these have minerals in different proportions than rocks from Earth.

From space.com - a Martian meteorite
The other way we find out where meteors come from is to find our how old they are. Age is usually determined by using radio-isotope dating techniques. If you've heard of carbon-14 dating, that's the process we're talking about, except with space rocks, scientists usually use rubidium-strontium because it has a much longer half-life. (This site has a great experiment to illustrate what a half-life is. Basically, it is the amount of time needed for half of the radioactive atoms to decay to non-radioactive atoms.) When a rock is melted, it resets this clock. So when a volcano erupts and the lava cools, if we found the age of those rocks, it would be essentially zero.

Our solar system formed about 4.56 billion years ago. Meteorites from the asteroid belt are all about that age. Rocks from Mars are about 1.5-4.5 billion years old. Rocks from the moon are about 3.3-4.4 billion years old. The oldest rock on Earth is about 4.37 billion years old. Most of the rocks on Earth are much younger because Earth is so dynamic and is constantly recycling rocks. Mars and the moon are pretty much dead, geologically speaking. Not much is happening with erosion or renewal of the surface on those bodies.

What would you think if we found a meteor that was dated at 4.89 billion years old? Or what if it gave an age of 6 or 7 billion years? It most likely came from outside our solar system, or it was a chunk of pre-solar system space dust that survived the formation of the sun and planets and other bodies. Whatever it might be, a rock that old would be an incredible find.