Wednesday, July 05, 2006

The meteorite primer

It seems Kansas has been back in the blogsphere again recently (1, 2) . And not surprisingly, it’s for an infiltration of creationism, yet again.

This time, young earth creationists are making an appearance at the Haviland Meteorite Festival in the form of such videos as “The Privileged Planet” and one that comes from a company that publishes "biblically based, family oriented video."

But while many other blogs out there are doing a fantastic job covering the inanity of the situation, Joshua Rosenau of Thoughts From Kansas recommended I actually present some of that niggling little detail the religious right hates: Science.

So I’m going to take a brief deviation from my more frequent topics and strike while the iron’s hot about meteors and the like.

Before I begin, let’s first address an underlying issue that comes up with all astronomy: “What are we trying to learn?”

That’s really the fancy way of saying, “Who cares?” but phrase it however suits you best.

The reason astronomers study meteorites is because they give us insight into the formation of the solar system. They’re not the only things though. Comets are also suspected to be leftover building blocks that haven’t been altered significantly since the formation of the solar system ~5 billion years ago.

But unfortunately for astronomers, and fortunately for life on Earth, comets don’t generally come to us. In one instance we went to a comet and picked up some scraps which were returned home. In another instance we crashed a giant chunk of metal into a comet and analyzed it by remote.

This means that the best opportunity for astronomers to pick up a piece of the solar system, is to let it come to us. I’m not going to go too much into the specifics beyond this, mainly because it’s rather technical and additionally because the astro-geology isn’t really my field.

So now let’s quickly define a few terms to avoid confusion:

Before a piece of space debris enters the atmosphere, it’s known as a meteoroid. If it’s quite large, it’s called an asteroid.

If a meteoroid enters the atmosphere, it’s then called a meteorite. This is what’s generally known as a “shooting star.”

If/When it a meteoroid strikes the ground, it’s a meteorite.

Sometimes, a bunch of meteoroids show up all at once. This is known as a meteor shower and there are several annually. If it’s especially heavy, it gets upgraded to a meteor storm.

So now you’ve got some fancy terminology to impress your friends.

But where do they come from?

While a meteor can come from just about anywhere, the most common variety are grains of dust and dirt left by comets that cross Earth’s orbit. Comets have those nice pretty tails which are composed of this dust. But as the comet moves, the dust doesn’t just disappear. It disperses, but ends up sitting around.

If that dust happens to end up in Earth’s orbit, the Earth crashes into it and it falls through our atmosphere making a pretty little glowy streak. However, these grains are so small, they never reach the ground. Instead, they burn up as they streak through the atmosphere.

Not much good for getting in a lab.

Instead, the ones we really care about are the larger chunks that reach the ground intact. These ones are far less frequent and can come from many sources.

The most common type amongst these is known as the chondrites. Chondrites are meteors containing small particles known as chondrules. These chondrules formed long ago when the solar system was first forming.

As the cloud that formed our solar system collapsed, it became extremely hot. Small rocks and chunks of other metals melted and formed into spherical droplets which are the chondrules.

Eventually these chondrules collided with one another and other material to build larger chunks. The largest of these chunks kept on accumulating material and eventually formed planets. However, as the planets coalesced, they again became hot and any chondrules would have been destroyed. Thus, only the smaller chunks were able to maintain their chondrules.

In studying these chondrites, astronomers discovered that they come many groups, each of which have distinctly different properties and were formed in separate ways.

What this tells astronomers is that the nebula that formed the solar system was not homogeneous. In other words, different materials ended up different places when things began to really take shape. This conforms to modern day observations of the solar system given that we see small rocky bodies closer in, followed by gas giants and then some other stuff way out there.

Another typical form of meteorites that come from the early solar system is known as the iron meteorite. As the name suggests, these meteorites are metallic and are very similar in nature to the naturally occurring on Earth, magnetite. One unique property that distinguishes them is a crystalline pattern in crisscrossing plates that can be brought out when properly treated in the lab.

Additionally, magnetite has a black or purplish-brown hue as do most other terrestrial rocks, while iron meteorites have a shiny silverish interiors. Their surfaces will also often be scalloped (like a scoop taken out) due to the stresses of passing through Earth’s atmosphere.

Pallasites are another type of meteorite (the sort in question in Kansas). They are a mixture of metal and silicate materials. Due to the metal (frequently olivine) in them, they are heavier than typical terrestrial rocks.

The last sort of meteorite is known as an achondrite. These ones are especially valuable because they come from other bodies in the solar system. However, they are extremely hard to tell apart from terrestrial rocks because they have frequently undergone the same processes.

To determine where such objects are from, scientists will frequently analyze small amounts of gas trapped in the rock. The composition for such meteorites does not match the atmosphere of Earth at any known time, and thus an extraterrestrial origin is suspected.

Thus, astronomers look around to see what it does match. The main candidate is Mars and a number of meteorites have been found from there, including the infamous ALH84001, which some consider to hold fossilized early life forms.

Other meteorites of this type have come from the moon. To demonstrate that such objects are truly lunar in origin, astronomers compare isotope ratios, chemical compositions, and other properties since the moon has no atmosphere (for a more detailed treatise go here).

While it’s possible that meteorites could also come from Venus, Mercury, or any of the other rocky bodies in the solar system (or even beyond), none have been positively identified to my knowledge.

So now you know about the basic types of meteorites. But how to go about finding them?

The best place to go is Antarctica. Since this continent is completely covered by nice white snow, the dark colored meteors generally stick out like a sore thumb. While not every hunk of rock is a meteorite, there are far fewer other terrestrial rocks to complicate the search.

In the past few decades, American and Japanese astronomers have collected over 15,000 meteorite samples this way alone. To compare, less than 3,000 confirmed meteorites had been discovered prior to that point.

So there’s a brief introduction to meteorites and what they’re all about. As I said before, I’m not a huge expert in this field which is why I stuck to the basics. However, looking at NASA’s Astrophysics Data System, there’s a ton of research out there on them. Doing a search for “meteorite” turned up 11,850 hits with two from this month already.

Some of the journal articles look rather interesting. One from last month discusses properties of meteorites from Mars that give insight into the presence of water during its formation. Another from March discusses a probable period of irradiation during the early history of the solar system based on various isotopes found in meteorites. There’s also one discussing the history of the Martian atmosphere from them as well. I’ve also seen references to a meteorite named Murchison CM2 which was found to contain amino acids.

As you can tell, the meteorite field is quite exciting and there’s a lot of good science to be done. So if you happen to be in Haviland for their meteorite festival, spend some time thinking about the good science being done and don’t mind that creationist propaganda.

And if anyone happens to be going and picks me up a sample, I’d love them forever. If not, I’m considering buying a Mars meteorite from The Meteorite Market.


Jeffrey said...

There are some really good books about meteorites available. I recommend Rocks from Space by O. Richard Norton as a good popular introduction, and the Cambridge Encyclopedia of Meteorites by the same author, if you want a more encyclopedic treatment.

By the way, iron meteorites are now believed to be the cores of asteroids that have subsequently undergone impacts.

Jon Voisey said...

Thanks for the recommendations.

I'm not a huge fan on meteors and the solar system (I'm primarly interested in extrasolar and especially extragalactic astronomy), but it's always good to know where to look should I need some info.

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