One of the Natural History Museum’s experts in meteoritics, Dr Caroline Smith says the meteorites that land on earth predate our planet by about 150 million years and it’s not surprising that they land here
Your first book is Meteorites by Robert Hutchison.
This is the core textbook for someone who is wanting to get into meteorite research professionally, so that could be an undergraduate student who’s got a project, or a post-graduate who is looking at meteorites to study. It’s a fantastic book. It’s quite recent – published in 2004 – but it’s a textbook that covers all different kinds of meteorites, all different aspects of meteorite research. This is the most up to date and the most comprehensive. It’s got lots of pictures in it, lots of tables and graphs; it’s a really good reference work. I’ve got a very well-thumbed copy on my desk.
Being a complete moron, I don’t know anything about meteorites. Could you define what a meteorite is and what the areas of research are?
In their most basic form, a meteorite is a rock from space. They can be made out of stone or iron or a mixture of stone and iron. There are three main types. The stony ones are made out of rocky minerals, very similar to terrestrial ones. You get iron meteorites which are made mostly of iron but there’s nickel in there as well and other bits and bobs. And then the stone and iron meteorites, which are mixtures of rocky minerals and metal. Most meteorites we have on Earth come from asteroids, although there are a few from Mars and a few from the Moon and then a very few, only nine out of over 40,000 known meteorites, called the CI chondrites, which may derive from comets, but the jury’s still out.
I thought an asteroid was a comet.
No, no, no! The asteroids live in the asteroid belt between Mars and Jupiter. These are ‘small Solar System bodies’. Asteroids are the builders’ rubble left over when the Solar System formed. These are the materials that never got into making a planet.
Why do they move from where they are?
Well, there are asteroids that cross or are near Earth’s orbit anyway, but you can get instability in the gravitational field of a Solar System object – it’s all to do with this thing called orbital resonance. When an asteroid’s orbit interacts with Jupiter’s gravitational field you can get a wobble which can cause the asteroid to be thrown out of its stable orbit, which then can result in collisions in the asteroid belt. Some of these fragments can make their way into the inner Solar System and then hit the Earth – this is the stuff we know as meteorites. The Solar System is a very, very complex place, so it’s not surprising at all that meteorites land on earth.
And what is meteorite research? Looking at where they come from and what they are?
All sorts of things. Although it’s quite a small field, there are probably about 1,000 people around the world who have meteorite research as their main research focus, and it covers a multitude of sins. Talking about the research that we do at the museum, just to give you an example of the diversity, I have colleagues who are looking at meteorites for the very first materials formed in our Solar System, and they are looking at and dating those materials and seeing what minerals they are made out of to try to get a real handle on the chemical and physical environment of the Solar System over four and a half billion years ago, prior to planets forming. These are the primordial building blocks of the Solar System. I have colleagues who are looking at meteorites from Mars to try and better understand the geology of Mars. I look at meteorites which have undergone certain amounts of geological processing but not to the extent of Earth or Mars or any of the other terrestrial planets. They are the things that were trying to become planets but didn’t; they stopped in their tracks. So looking at meteorites like that gives us an idea of the processes the Earth was undergoing at that time as well. We are also very interested in organic matter in meteorites, so some meteorites, the carbonation chondrites, contain quite considerable amounts of organic matter, and it is quite feasible that the early Earth was seeded with…well, we know that the early Earth was bombarded by meteorites and comets…
Oh yes. You can see it on the surface of the Moon as well. If you think of the Earth and the Moon as one thing, as the Earth-Moon system…
But why did we stop getting bombarded then?
Well, it’s do with the Solar System sorting itself out.
Is it sorted out now?
Well, there are still things going on. We have been whacked by large asteroids and comets in the past and undoubtedly will be in the future as well. Obviously things are a bit calmer that they were, but the Solar System is still an active place. Four billion years ago it was extremely active. So, in the first few hundred million years of the Earth’s life, material was bombarding the Earth and much of this material would have had organic matter in it. It is quite possible that the chemical building blocks you need for life, and I’m not talking about there being little bacteria on the meteorites or anything, but the chemical building blocks, all of those elements you need for life to start, could have been delivered from space by meteorites.
That’s so exciting. And is that all in this book?
That book, Meteorites, is actually giving you the information about the different types of meteorite – how old they are, the things that they contain – so, yes, there is information about the organic components within carbonation chondrites, for example. So if you were student trying to get into meteorites or you were starting a PhD, this book would be a very, very good starting point to give you an overview. It is a very good primary reference source.
And what’s your next book?
My next book is called Solar System Evolution, A New Perspective by Stuart Ross Taylor. This is a great book for anybody interested in our Solar System: how it formed, why it formed as it did and how we know these sorts of things. The first chapter looks at the historical views of how our Solar System formed, things like Copernican theory that the Sun is at the centre of the Solar System. Prior to that people thought the Earth was at the centre, so you often get references to pre-Copernican and post-Copernican. And then you’ve got Galileo. So you get a brief introduction to that, which is nice because I don’t know how much of that is taught nowadays. Then it goes into the structure of the universe and the Solar System’s place in the universe.
Also, very importantly, it also talks about the stellar nucleosynthesis of elements. So all the elements that we are made of, the calcium in your teeth and the iron in your blood, are produced in stellar explosions. That material gets recycled and recycled and recycled in the cosmos, and some of it accumulates in the area of space where our Solar System formed, and that’s why we have the elements that we have. So he discusses that and explains how that works.
How does it work?
It’s quite complicated. It’s all to do with stars behaving like nuclear reactors. You start off with hydrogen, which was ubiquitous and formed in the Big Bang, but everything heavier than hydrogen was formed in these stellar nuclear furnaces. So you get hydrogen – you know about nuclear fusion? Where you’re fusing atoms together rather than separating them? – you get hydrogen and you fuse those atoms together and get helium and you fuse more helium atoms together and you get elements like lithium, boron and beryllium. More reactions form elements like carbon, nitrogen and oxygen… See what I mean? It goes on and on and on and you get all the way up the periodic table to iron. To get elements heavier than iron you need these very large supernovae explosions or they can be formed in red giant stars.
Yes. You get hours of lectures on this and there are different processes that can happen – it’s all quite complicated and difficult to explain!
But the elements that make up the human body…
Were made in the stars. Yes.
That’s fantastic. It sounds almost religious.
Yes. I’m not going to get into that one way or the other, but it’s certainly a mind-bending concept.
Are you religious yourself?
Not really. I’d best describe myself as a ‘submarine Catholic.’ We surface when we’re in trouble. It’s quite an amazing concept and a lot of people don’t realise this. When you think about it – all the stuff in my blood that makes it red comes from the stars.
This one is out of print. Out of the Sky: an Introduction to Meteoritics, by H H Nininger, and this one – it isn’t actually mine, it’s the department’s, but I love it – was printed in 1952. The first time meteorites were actually seen as objects that were not from this Earth and worthy of scientific study was in the late 1700s, early 1800s, so we’ve only been doing it for a couple of hundred years. Many of the techniques that were applied to meteorites were the same techniques that were applied to Earth rocks. That’s still the same today. We use the same labs and the same microscopes. But in the 1940s a lot of scientific techniques were developed, mostly because of WWII, like radiochemistry. So this was when people started saying: ‘Let’s have a look at meteorites and see how old they are.’ You can date them with radioactive clocks. And then people said: ‘My goodness, look at these. These are four and a half billion years old. They’re older than rocks we’re finding on Earth.’
Are they really?
Yes. Meteorites pre-date the earth by about 150 million years.
That’s the thing. In my day job I can be holding the oldest object known to man in the morning and a piece of Mars in the afternoon. Not many people can say that.
They certainly can’t.
But this book by Nininger is brilliant because it was published just at that time when meteoritics was coming up with very interesting results, so this book includes some of the new data that was coming out, using all these very innovative chemical techniques. That’s the science bit. But Nininger was a great guy. He really tried to involve the public in meteorites. He went round various places in the US where there’s lots of farming land and he said to the farmers: ‘If you’re ploughing in your field and you hit a big rock with these characteristics, let me know and I’ll come and have a look at it.’
Well, does it feel heavier than it should do? Is it a big lump of metal that looks out of place? Is a magnet attracted to it? There are certain characteristics that meteorites have that we don’t commonly see in Earth rocks. The stony meteorites are mostly made out of rocky minerals, so they’re similar to earth rocks but they have got metal in them so they will be a bit heavier. That’s what he was saying to farmers. It’s the same way we look for meteorites today. He engaged people and asked people to look out for them. He gave public lectures and set up a museum next to the big crater in Northern Arizona, so not only was he a fantastic scientist but he was a brilliant engager of the public imagination. There are pictures and a lot about the history of meteorites and reports about meteorites that have been seen to fall. When Nininger retired from meteorites he sold his collection, and we bought part of it and the Smithsonian bought the other part, so I actually curate samples that he collected. I feel I have that connection with him even though he died in 1986.
Fourth book. Meteorites: A Journey Through Space and Time by Alex Bevan and John de Laeter.
This is a book I would recommend if you don’t have any geological background, if you’re like my mum. Very much like the book I’ve co-written, actually. Pitched at the same sort of level. It’s a fantastic book. It’s hardback and the illustrations are fantastic, the photography is fantastic and the way it is written makes it a pleasure to read. I’m very good friends with the lead author Alex Bevan, but I’d plug it even if I didn’t know him. I had to review it for The New Scientist when it came out in 2002 and I read it from cover to cover. What I like about it is that it will explain that meteorites are four and a half billion years old and we know this from doing radiometric dating, but instead of just saying that, there will be a whole page on radiometric dating and how it works and how you do it, which is why it’s a great book.
How do you do it?
Well, you know you have radioactive elements? You have your starting radioactive element and this is unstable so it will decay to form either another element or maybe the same element but with a different atomic mass. We call the starting element the parent isotope and the decayed one the daughter isotope. You can measure the rate of decay, and when half of your parent isotopes have decayed to the daughter isotopes we call this the half-life. So you know the half-life and you can look at the ratio of parent isotopes to daughter isotopes and you can do some maths and work out when the material formed. That’s the most basic way of describing a complicated technique that you can do in slightly different ways depending on what elements you are measuring and what you are trying to date.
This book’s full of background information like that and a lot of interesting stories about early people recognising meteorite falls. In the Aboriginal communities in Australia there are stories about things falling from the sky and making a hole, and in fact there is research going on now which is tracking back meteorite craters which can be linked up to Aboriginal stories handed down for thousands of years. People would have seen this event and passed the stories down.
It’s quite amazing. So it talks about the whole science of meteorites and why they’re interesting to study, why they’re important to study, what they tell us about our Solar System, but also you get the more personal aspects of it as well and the cultural importance.
So have you got meteorites in the museum that are mentioned in Aboriginal stories?
The Henbury meteorites, yes. We’ve got some of those. There are a number of stories and legends in Aboriginal cultures that describe things falling from heaven and landing on Earth. Anthropologists, scientists and historians have been able to work out that, yes, this tribe was in that area and there is an impact crater 4,000 years old and there is archaeological evidence that this tribe was in that area then so it’s very likely that people did see this event happen.
Have you got a favourite meteorite?
Oh God. They’re all my favourites! It depends on the day of the week.
Well, one of my favourite meteorites is the Wold Cottage meteorite which is sitting outside my office. It’s a very important meteorite that fell in Yorkshire in 1795. There are not very many British meteorites and that’s one of them. Meteorite studies only started getting going in the late 1700s early 1800s and prior to that they’d been considered not worthy of study by the scientific intelligentsia. The reason for that is socio-cultural, because meteorite falls would be witnessed by people who were working outdoors, like peasants and farmers. So the landowners just dismissed them.
But then there were a number of falls which were witnessed by people who would be listened to. There was a fall in 1794, a shower of stones over Siena, and many British aristocrats were there doing the Grand Tour and they witnessed the shower and brought some of the stones back to the UK. We have some in the collection here. So these were obviously people who you could trust. And then in 1795 this stone fell in Yorkshire and was witnessed by a farmhand who was believed by his landowner, who brought the specimen down to London. Joseph Banks, the President of the Royal Society, had a look at it and thought yes, we should be studying this. And so this group of stones were analysed by a British chemist who recognised that the stones had metals in them, which was unusual for a terrestrial rock and, importantly, the metal contained nickel. The decision was that they are all similar and they are not from Earth. That was really what got it going, and Wold Cottage was pivotal to those studies.
What does it look like?
It looks like a big lump of rock about the size of a football. On one side it’s very dark greyish black – that’s the fusion crust where it’s burnt as it’s come through the atmosphere – and on the other side is a sawn surface.
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