Where Meteorites Come From
Chris - Tell us a bit about your field. What do you actually do?
Ian - I'm a geologist and we study rocks. Meteorites are rocks and that's one of the things we study.
Chris - So where do you go to find them, because I've heard that the South Pole is a really good place.
Ian - Yes, remarkably about half the meteorites in the world's collections are from Antarctica.
Chris - is it just because they're easy to see against the snow?
Ian - Yes, but the story's a little bit more subtle than that. They land on the ice but immediately get covered by more snow and disappear. The ice in Antarctica is about four kilometres thick, and it's creeping out under its own weight. Icebergs are breaking off around the edge of Antarctica, and many of the meteorites in them would just end up on the bottom of the sea. But there are one or two places where that ice rides against a buried mountain range under the ice. That forces the ice up and the wind blowing over the surface evaporates the ice. So deeply buried ice ends up coming back to the surface.
Chris - Bringing meteorites with it.
Ian - Yes exactly. And once they get to the surface, they're stuck and can't go anywhere. So they accumulate in these places where the ice is evaporating. It's possible to identify these places because they're blue, as viewed from space. We send people to these blue ice fields and they collect everything they see.
Chris - So even Maggie can help you here by spotting them from space! So you can use cameras mounted on satellites in space in order to spot a meteorite?
Ian - Well no, from space you spot the blue ice field and know where to go.
Chris - But what about spy cameras, because they can read the text on a newspaper.
Maggie - Well there's lots of hearsay and lots of science from the movies. It's trying to draw the line between what's reality and what isn't. What the military is capable of is very hard to define, but we can get to better than one metre resolution with standard space cameras these days, and we're working on improving that as well. So in the one metre square we can see from space, we wouldn't quite be able to resolve the print on a newspaper. But you'd be able to see building densities and things like that.
Chris - So how long have the meteorites that we find in Antarctica been in the ice?
Ian - That's a good question, and remarkably scientists have a handle on that. They think they go back tens or hundreds of thousand of years. Maybe even as much as a million years. The reason they know that is that while they're in space, they're picking up radioactivity from cosmic ray bombardment. The longer that they're sitting in space, the more these new atoms are made. It's possible to measure how much has accumulated in the material, and that tells how long it's been sitting on Earth.
Chris - Now in the context of the meteorite that Phil was talking about earlier that came from Mars and people thought that it might show signs of life, an obvious question to ask is how did something from the surface of Mars end up on the surface of Earth? Did something slam into Mars and release that piece of rock, which then ended up drifting around in space for a long time, and then we picked it up like a giant hoover?
Ian - That's pretty well it, yes. For along time it was a bit like the story that bumblebees can't fly. Scientists said it wouldn't be possible for an impact into the surface of Mars to launch a piece of Mars at more than Mars' escape velocity. But indeed, as you said, these meteorites that we think come from Mars were proven to come from Mars from this trapped gas which is identical to the gas that was measured on Mars in 1977 by the Viking lander. So we know it comes from Mars, and so Mars must have been hit by a big meteorite, which caused rocks to fly into orbit until they got picked up by Earth.
Chris - Ok, so we're happy that these things are landing on Earth, but what actually are they? Apart from stuff being ejected from planets, there's other debris out there too.
Ian - Yes, in fact only a very small fraction are thought to be from Mars. The great majority are almost certainly pieces from the asteroid belt, which lies a good bit beyond Mars but not as far as Jupiter. Astronomers have known for a long time that there are a huge number of small planet-like objects in orbit around the sun in that zone.
Chris - Is this kind of debris left over from when planets were forming in the early days?
Ian - Yes. At one time it was speculated that there was a planet out there that exploded into lots of bits. But the evidence we have now suggests a very different mechanism. The idea is that when the solar system first formed, there were no planets, but there was newly formed sun surrounded by a disk of dust and gas. Gravitational instabilities within that disk led to small planets forming that were just a few kilometres across, and then they grew gradually bigger like snowballs. Over a period of perhaps ten million years, these small bodies aggregated and aggregated and ended up as the planets as we know them.
Chris - So studying them does actually tell us quite a lot about how everything formed in the early days.
Ian - Absolutely. The asteroid belt is part of this early disk that never made it to make a big planet. So we're looking at things that formed in the first ten million years in the solar system's history.
Chris - Now you've bought some bits and pieces for us to look at, so Phil, why don't you talk us through it and we can ask Ian what they are.
Phil - Well there are some quite interesting samples here. We've got different types. One is quite dark and grainy; another is light and grainy and there's another that doesn't even look like rock at all. It looks like a sheet of polished aluminium or something like that.
Ian - It's steel, and it's not far off stainless steel.
Chris - So it actually arrived in the form of unrefined steel?
Ian - Yes.
Chris - So tell us about these samples. What actually are they and where did they come from?
Ian - I mentioned this disk of dust that was going round the early sun. Well the dark grainy sample is primitive material in which dust has aggregated but nothing much has happened to it since those first couple of million years.
Phil - So all these little bits of grain inside this rock are actually the original bits of grain and rock that formed the original meteorite?
Ian - Yes. It turns out that the story is never quite as simple as one might like it. Those little grains themselves have a history. A lot of things were going on in that first two million years. They're not the very first bits of dust, as they've actually been processed through heating and melting before they were made into that aggregate of grainy rock.