Kitchen Science - Is it a Meteorite?

18 November 2007

Colin - "I was walking back home and I heard this clatter about ten yards away from where I was standing. I thought nothing of it. I walked back home, got in bed and I thought to myself I know what that must be because it can't be anything else. A meteorite."

Meera - That was Colin from Ipswich who sent in a sample of his meteorite to the Naked Scientists. To find out if it really was from out of this world I took it along to the Natural History Museum in London and analysed it with Metoeritics Researcher, Dr Sarah Russell. To start with Sarah told me what the definition of meteorite is.

Sarah - A meteorite is a natural object that originates from beyond the Earth but comes through the atmosphere to land on the ground. Before it lands on the ground it's actually a meteor. When it hits the ground it turns into a meteorite.

Meera - We've got the sample just here now. What do we do first?

Sarah - Well, first I'm going to take it out of this little packet. It looks like an igneous rock. An igneous rock is a rock that's at some point in its history been melted. This is very common for terrestrial rock. But there are some meteorite samples that can also be igneous in texture. It doesn't exclude the possibility it's a meteorite.

Meera - What should we do next?

Sarah - I've got a magnet here. Most meteorites contain some free iron in them and that means they're attracted to a magnet. The first thing I'm going to do is to see if the sample's attracted to the magnet.

Meera - It is actually sticking.

Sarah - yes, it actually is - which surprises me because most igneous rocks are not magnetic.

Meera - There's hope yet?

Sarah - There is hope yet. For this kind of rock I think it's more likely that instead of having iron in it it's got a magnetic mineral in it like magnetite.

Meera - What's magnetite, is that just in a lot of rocks?

Sarah - It's in quite a lot of igneous rocks. It's an oxide of iron.

Meera - We've now found out there is some magnetism in it. Is the next stage to look at it under the microscope?

Sarah - Yes! Exactly. I've got a microscope here so I'll put it underneath.

Meera - the microscope's actually hooked up to a camera here so I can see the sample on the computer screen. What can we see here Sarah?

Sarah - It's not looking good. It's got some dark minerals which might be magnetite. The texture of this looks slightly sugary. This confirms my suspicion that it's an igneous rock. It's got white crystal there. That'sprobably quartz and that's quite an unfortunate result for a meteorite.

Quartz is a very common rock on Earth but it's essentially absent in meteorites. To make quartz you actually have to have a rock that's come from a big planet that's spent billions of years evolving and producing different kinds of rocks. It's not found in asteroids and asteroids are the most common source of meteorites.

Meera - Could the presence of quartz have burst Colin's bubble? Here's Sarah's verdict:

Sarah - I think this isn't a meteorite. I hate to be the bearer of bad news but I don't think this is a meteorite.

Meera - Even though the sample didn't turn out to be a meteorite I asked Sarah what she would have done next if the rock had looked promising under the microscope.

Sarah - If we looked at the rock and we saw that it was a mineralogy that was compatible with a type of meteorite then the first thing that I would do is to get a very thing slice of it called a thin section. We'd use the thin section to look in the electron microscope to look in detail at the composition of the minerals. There are some indicators, for example in meteorites if there is free iron it nearly always has nickel associated with it and that's not true of terrestrial rocks. That's one simple test you can do once you've got this thin section of material. The other diagnostic test is oxygen isotopes because the oxygen isotope composition of most extraterrestrial material is quite different to that of Earth. We can do an oxygen isotope analysis on that and it can tell us for sure whether it originated on Earth or not.

Meera - To show me what real meteorites look like, Sarah took out some samples from the Museum's collection.

Sarah - I wanted to show you this meteorite here which is a meteorite called a chondrite which is by far the most common meteorite that is seen to fall. The first thing to notice when you pick it up it's actually denser than you would expect a terrestrial rock to be. That's because it contains a lot of iron metal in it. Iron metals tend to be denser than rock. Also if you look on the outside you can see it's covered in this very thin dark black layer.

That's called a fusion crust and that formed as the meteorite came through the atmosphere, the outside melted. The fusion crust is really the best diagnostic thing to say if it's a meteorite.

Meera - What are the main types of meteorite that you get here at the Natural History Museum?

Sarah - This meteorite, called a chondrite, is so-called because it contains - I don't know if you can see - it's got some rounded blobs in it. A round whitish blob there and a dark blob there. These are called chondrules.

They're made of silicates that actually formed as individual bits of red hot molten rock that were floating round in the solar system. They solidified and it eventually came into the meteorite. There are two other types: iron meteorites which are made mostly of iron and the other one is stony iron which are complex mixtures of rocky material and iron material.

Meera - So there are three types of meteorite out there to be discovered. Unfortunately Colin's sample showed the presence of minerals formed only on Earth. That doesn't need to stop us from keeping an eye out for meteorites in the future - especially if the source of these space rocks can range from debris dating back to before a solar system was formed to fragments from the surfaces of other planets when those planets were hit by even bigger meteorites. The information they can provide us about our solar system is endless. That's it for Kitchen Science this week. If you want to know more about meteorites then go along to the Natural History Museum at the end of the month as they're launching their new permanent mineral gallery, The Vault, on the 28th of November.

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