[Emily] Hey I'm here with Philipp Heck, who you guys will probably remember from the Starstuff and Nanodiamonds episode. Today we're going to talk about this fossilized meteorite. But before then, what is this machine?

[Philipp] Ya, this is an electron-scanning electron microscope. This allows us to get super high resolution images of any kinds of solid samples like meteorites and also determine their chemical compositions.

[Emily] That's great, we're going to sample one of these meteorites, not this one a different one, but.

(0:36)

[Emily] These were found, or excavated, um, extracted, from a rock quarry in Sweden. When did that happen?

[Philipp] So these meteorites, the first one was found in 1952, but hasn't been recognized as a meteorite until about 30 years later.

[Emily] These are really rare though, this is the only quarry in the world, the only area these fossilized meteorites have ever been found.

[Philipp] It started in 1992 - the systematic search and by today, 2014, they have found 101 fossil meteorites in that quarry.

[Emily] And those are the only ones in the world?

[Philipp] And there was another quarry where there was a chance find, and uh, but that was about, it was not very far away about 20 miles further out. And I got a sample of this meteorite as well, analyzed it, and found it's exactly the same type of meteorite as all these other 100 meteorites, uh, from that quarry.

[Emily] Why haven't we found more fossilized meteorites on earth today?

[Philipp] We found that this quarry was actually at the right time, they sampled rock at the right time.

[Emily] The right time in, in geologic period,
[Philipp] In geologic...
[Emily] This was about 500 million years ago, like at the very beginning of the Ordovician.

[Philipp] There were many more meteorites coming down at that time than today.

[Emily] It was like a whole shower.

[Philipp] Ya It's like a meteorite shower, but they're, interestingly, not only found in one bed, in one sediment bed. But across, I would say, 6 feet or more fossilized meteorites were found.

[Emily] So we know from our trip to Wyoming, doing the fossil fish excavation, that even a couple of inches can indicate thousands of years difference. So what does it mean if you find one at 6 feet then another at like 3 feet? Like how much space is in-between the landing of those meteorites.

[Philipp] This can be several hundred thousand years.

[Emily] So the same meteorites were falling on earth over a period of several hundred thousand years?

[Philipp] Exactly, yes.

(2:35)

[Emily] How were we able to age this meteorite?

[Philipp] We do that by cosmic ray exposure age dating. So when the meteorite flies through space, it gets hit by cosmic rays. Cosmic rays are particles which move through space at extremely high speeds, and they hit the rock. They not only get implanted in the rock, but they actually change the composition of the rock.

[Emily] They're hitting it so fast that other elements or things or atoms get knocked off of it.

[Philipp] Yeah, so they basically hit the rock and they can fracture atoms. They can fracture atoms and these fragments of atoms, these are the products of those collisions with these cosmic rays. And we call them cosmogenic nuclides, because it sounds cool. So we just measure how many of those cosmogenic nuclides we have in the rock per mass. And we know how many are produced per time because this can be tested in the lab or recreated in the lab. And then we can just calculate an age.

[Emily] So this was part of an asteroid that was living pretty happily in that asteroid belt between Mars and Jupiter.  And it was going around and around and then something happened.

[Phillip] Yeah. 

[Emily] It was hit by another asteroid.

[Phillip] Exactly.

[Emily] And then, it was knocked off course, so instead of going around in a circle, it started - As I understand it - it started getting pulled by Jupiter into more of an ellipse.  And then when it turns into an ellipse, it crosses Earth's orbital planet - er - orbital rotation.  And then falls to Earth.  

[Philipp] Mhmm. Yeah.

[Emily] So that's also how we're able to determine how long these have been falling on the planet, because you can measure the composition of this meteorite against meteorites that are still falling today, millions - hundreds of millions of years later.  And they're from that same original collision.

[Philipp] Yeah

[Emily] That's amazing.

(4:37)

[Emily] What is a fossil meteorite?

[Phillip] A fossil meteorite is a meteorite that fell into the - into the sea and got embedded in the sea floor, and got preserved to the present day.

[Emily] When you say you have a fossilized meteorite, I found that kind of confusing because I associate fossilization with only happening to organic materials.  So can something that's inorganic become fossilized?

[Phillip] Yes.  In this case, the same process happens: the original minerals, they have been replaced, and that's during the fossilization, and so we can call it a fossil, like a fossil of a, um, cephalopod.  Everyone knows what a cephalopod is.

[Emily] Yeah.

[Both laugh]

[Emily] Knowing that, how do you know it's still a meteorite?

[Phillip] I will show you how a typical analysis looks like, to determine that it's, uh, from a meteorite, and not a terrestrial rock, and you will see a, uh, mineral that we extracted from a meteorite.

[Emily] And we're gonna get its, like, identifying markers so we can compare that to a known, and then identify what is actually in the machine, pretending that we don't know what it is.

[Phillip] Exactly.  Exactly, yes.  We are comparing an unknown to a known and doing pattern recognition.

[Emily] Cool.

[Phillip] Yep.

(6:00)

[Emily] We're going to be processing a sample of, of a meteorite, but it doesn't really - like this doesn't look like a meteorite.  So what is it that we're actually doing with this.

[Phillip] Yeah, so we are actually - we extracted those from a meteorite, and these are mineral grains, tiny mineral grains.  You cannot see them without a microscope.  They are mounted in the center of that plastic holder, and we cannot just put it in, we need to put it in this brass holder.  There's a little spring here, that pushes it up.  So put it in the center here, and then load it into the chamber.  So this is our sample now.

And we'll put the - our holder onto the stage, and there's a little, um, structure that we can slide it onto the stage.  We can now close the door and pump out the air.

[Emily] That's awesome.  Why do you need to pump out the air from the chamber?

[Phillip] Um, with the air, you wouldn't be able to get a good image.  The electrons would collide with air molecules, and -

[Emily] Yeah.  It'd be like, "This chamber is filled with hydrogen and oxygen!"

(7:11)

[Phillip] The mineral that we are talking about is called chromite.  It's mainly chromium and iron and oxygen, but it also has some other elements in it, and it has, for example, titanium in it.

Chromites on Earth, they have much less titanium than the chromites in meteorites.  And that's one way of, uh, identifying if it's meteoric or not.

[Emily] So you have this, like - when I think of it, I think of, like, it in terms of biology, where if you're trying to match one species to another, you match up their genetic identities.  Is it the same thing with meteorites? Like, you can take a sample of that and match it to a known meteorite and try and figure out what kind of "species" it is, in a way?

[Phillip] Exactly.  That's exactly how we work.  We measure the chemical composition and try to match it with known meteorites.

(8:04)

[Emily] We're looking at some brains.  I mean, everything looks pretty brainy from my perspective over here.  But what are we looking at?

[Phillip] So we zoom in to one of those chromite grains from a fossil meteorite from that same quarry as this fossil meteorite that I showed you.

So we still don't know what it is, right?

[Emily] Yeah.

[Phillip] But then, it's very simple.  It needs to be calibrated and everything, but once it's calibrated, you just chose your target, shake, and just click here.  It takes a spectrum.

[Emily] So there is a lot of oxygen in there?  I see a lot of green.

[Phillip] There is lots of oxygen because chromite is an oxide.  So it's iron, chromium, and oxygen, mainly.  But we don't want to see - I don't care about the oxygen in this chromite, so I just click here, so I just see iron and chromium, and it's pretty uniform.

[Emily] Yeah.

[Phillip] I mean this guy has been cooked pretty well.  It comes from an asteroid that experienced thermal metamorphism.  That's what it tells me.

[Emily] Yeah. That's awesome.

(9:10)

[Phillip] These are the, um, the few pieces of extraterrestrial matter that was found in Earth's record, in the archive.  All, almost all of the other 50,000 meteorites that we know - that are known to science - they were found on the surface of Earth.

[Emily] Well that brings up a really interesting point too, because we have this one that was found at the bottom of the ocean, but recently, there was one that is from the same parent body, from the same meteoric impact, that was found in Chicago, on the other side of the planet, and 500 million years later.

[Phillip] Yeah.  That was a, actually, a meteorite that came down in 2003 in the south Chicago suburb of Park Forest, and it turned out to be the same type of meteorite than this one.  So that these pieces find them - they have the same origin, they came from the same parent body, parent asteroid - and they find themselves, again, back on Earth.  They get reunited on Earth. Exactly.

[Emily] Yeah.  Yeah, and here in the Field Museum.  They're together again, 500 million years later.  That's great.

(10:28)

[Emily] It still has brains on it.