Science Interviews


Thu, 11th Aug 2016

Spot the difference: Meteor, meteroid...

Dr Carolin Crawford, University of Cambridge

Listen Now    Download as mp3 from the show Connie won a meteorite

Do you know your meteors from your meteorites? Neither did Graihagh Comet Hale-Bopp as it flies over the sky of Pazin in Istria, CroatiaComet Hale-Bopp as it flies over the sky of Pazin in Istria, CroatiaJackson, so she met Carolin Crawford to find out...

Carolin - I should have brought some of my Mars and Moon ones in - I keep them at home because theyíre too precious.

Graihagh - Yes, she is talking about meteorites from the Moon and Mars that just happened to fall on Earth and she is Dr Carolin Crawford - lover of space rock, naturally, but also an astronomer at Cambridge University. We met to compare our rocks but first things first - whatís the difference between a meteor, meteorite and meteoroid but a comet and an asteroid?!

Carolin - Yes, it is complicated and astronomers do love our terminology. So, youíve got lots of lumps of rock; the debris that you said floating around in the solar system.

Now if it is made of ice - like a frozen iceberg 5-10 kilometres across or smaller, we tend to call it a comet even though it might have bits of dirt and dust within it. So comets are the icy bodies.

Asteroids are the rocky debris. And we wonít talk about things called Centaurs which are half ice half rock. Theyíre out beyond Neptune - we wonít worry about them so much. But, basically youíve got your asteroids which are rocky and youíve got your comets which are icy.

Now a meteoroid is like a tiny asteroid. Youíre talking about a few microns across to a few metres across in size. So itís just debris, like the asteroids, left over or fragments from a collisions or protoplanets, thatís been hanging around for billions of years in the solar system and floating around between the planets, around the Sun following their own orbits.

Now as soon as that meteoroid starts to enter Earthís atmosphere it becomes a meteor. Thatís what you know as you're shooting star and thatís when you see it burning up in the atmosphere. It sort of excites the air molecules and you get this trail of ionised particles which shine, and you also get the lump of rock, even if itís tiny, it will still give of an amazing amount of light because itís travelling quite fast. Most of them disintegrate in the atmosphere completely but, if they survive to hit the surface of the Earth, then they become a meteorite.

So thatís the distinction: Meteoroid when itís in space, meteor when itís in air, and meteorite when itís sitting on the ground.

Graihagh - Simplez.

Carolin - Did you bring this meteorite?

Graihagh - I did.

Caroline - OK let's have a look - see which one you got.

Graihagh - How much do you know about meteorites?

Carolin - Iíd say itís a nickel/iron one.

Graihagh - Gosh - thatís impressive! I was going to bring out the label because I couldnít even remember what it...

Carolin - Sikhote Alin?

Graihagh - No.

Carolin - So itís not Sikhote Alin. Itís not Campo del Cielo because itís metal rather thanÖ

Graihagh - Oh - controversial!

Carolin - Oh it is! Itís Campo de Cielo.

Graihagh - Iím going to ask you how much you can tell me about this because this is Connieís meteorite that she won and, other than giving it a quick look up on the internet, we know nothing.

Carolin - This is one of the more common falls.

Graihagh - Ah, OK.

Carolin - Youíve got two very famous falls - youíve got the Campo del Cielo. And this is part of the Sikhote Alin fall.  I mean I like this one - you can see how itís just sort of melted as itís fallen through the air. Itís all kind of deformed and ruffled up - I like it particularly.

Graihagh - It reminds me ofÖ do you remember The Futurist (the painters) - The Futurists. You know how they had these very choppy, jagged types of paintings - thatís what it reminds me of.

Carolin - Or it just looks like a parrot!  It is deformed. I mean you look at that and you see a mangled bit of metal and you think gosh, that metals gone through a lot to get to be that shape. So I think this is part of the Sikhote Alin fall, so this fell in the Sikhote Alin mountains in 1947.

Graihagh - So youíre one's much more recent?

Carolin - Yes, this one they actually saw the fireball through the sky. They saw the meteorite coming in and later found the pieces. Whereas the Campo del Cielo, we think fell thousands of years ago but it was only more recently found.

Graihagh - Youíve mentioned two types of different falls there. Is it quite rare for this sort of stuff to fall to Earth or is it quite common?

Carolin - You have stuff coming into the atmosphere the whole time. Youíve got millions of meteors happening daily, itís just that most of them are tiny. A shooting star you can get from a thing about the size of grain of sand. Most disintegrate in the atmosphere but, even so, you get a lot that land on the Earth or, in fact, fall into the ocean probably at a rate of about 15 million kilogrammes per year. Thereís a huge amount of stuff that is just continually piling onto the Earth

You donít look like you believe me.

Graihagh - Well, I was really pleased that weíd won there so now youíre telling me (I say we), Connie has won this and itís not even that rare.

Carolin - They are rare. I mean it sounds like a lot but, in terms of volume and mass of the Earth, itís miniscule. Also, what youíve got there is one of the metal ones - they're the rarer ones.

Graihagh - The reason why I ask if itís rare, and Iím glad you say itís rare, Iíve got this meteorite for the weekend now. If I was to go away and sell it, what sort of price does this thing fetch?

Carolin - For the size youíve got there I would guess about £60 or something.

Graihagh - Oh, thatís pretty reasonable!

Carolin - I think itís pretty reasonable.

Graihagh - Dinner out! Because now Iím going to go everywhere I walk Iím going to have my eyes on the ground looking for these things.

Carolin - You get three basic types: The most common ones are the ones that are rocky. Then you get the iron ones, so thatís iron and nickel alloy. And then you get stoney iron which are the mix of the two.

This is a rocky one - well we call it stoney and you see itís got a sort of black crust on the outside. Now that would have been the outside of a meteorite as it fell through the air so this is where it had been burnt. We call this the fusion crust because itís where the rock has fused, effectively, under the extreme conditions as it falls through the air. It gives you a bit more of a character.

And then this is just a segment, a fragment of the meteorite and you can see, when we open it up, itís much more rocky on the inside of the meteorite again. Thereís a little tiny bubble of metal thereÖ

Graihagh - Oh yeah. Tiny, like a pinhead.

Carolin - Yes, but just a little inclusion of the metal alloys within it.

This is an example of a stoney one that is very nice because you see it's got a mottled appearance.

Graihagh - Dare I say, a bit like sometimes when Iím walking down the street and you see all the little bits of rock within, whatís it called - pebbledash is what Iím thinking but in a miniature polished form. Is that a terrible thing to say?

Carolin - No. Youíre getting the idea across beautifully there. It is just speckled and youíve got these almost like spherical - we call them inclusions - little bubbles of dust and carbon materials thatís held within the silicate base.

Graihagh - So where do these things come from? I mean I know theyíre space rock but why do they end up falling here on Earth for people like us to scurry around and collect them?

Carolin - They come from a number of different places, but youíre quite right, theyíre lumps of space rock or, if you like, space metal that are floating around there orbiting round the Sun. Now some of them, especially those rocky/stoney ones, are left over from the original solar system formation. They are bits that just didnít get incorporated into being planets.

Or they could be fragments of asteroids. You know, you can imagine protoplanets in the early solar system, asteroids now colliding and scattering fragments off.

And then you have some that are really exciting and theyíre where a meteorite has impacted another planetary body. So itís something like the Moon or Mars or say Vesta, the largest asteroid in the asteroid belt. And when the meteorite impacts that body, it send an ejector out but because things like Moon, Mars, Vesta donít have so much gravity, not all of it falls back down to the planet. It then goes out into space and starts orbiting the Sun and, maybe millions of years later, it happens to fall on Earth.

So some of these meteorites are bits of Mars, theyíre bits of the Moon, or bits of Vesta.

Graihagh - Oh, are you about to show me a rock from Mars?

Carolin - No, this oneís a bit of Vesta.

Graihagh - Wow!

Carolin - Which is surprising common. If you look at Vesta, this is the object thatís about 500 kilometers across in the asteroid belt, it looks a bit more more like a punctured football. Itís got a huge sort of gouge out of the southern pole and thatís where we think it underwent a huge collision sometime ago in its past. So there are lots of fragments of Vesta around and this is one here.

Graihagh - How do you know that?

Carolin - You know because of the mineral makeup. I was telling you youíve got all these different origins for different kinds of meteorites and, looking at the chemical composition of these rocks, you can tell a lot about whether theyíre pristine parts of the solar nebula or theyíve come from bodies that have undergone what we call geological differentiation. So youíve started to have that geological processing, the separation out of the metals and the sort of silicate crust.

So, when you hold something like your Campo del Cielo thatís completely metal, that would have come from the inside of one of these differentiated bodies. So like a small asteroid that was forming or a protoplanet that was forming before it got smashed so youíve got a bit of the core part of the asteroid there.

A lot of these stony meteorites are much more of the pristine debris left over from the early solar system.

Graihagh - So why is it them that weíre interested in things like 67P, and weíve sent out Roseta and weíve seen Philae land on the comet? Why though if all this stuff is falling down on Earth all of the time?

Carolin - thereís a lot of science you can only do when youíre in contact with the astronomical body, like the comet Nucleus, and that was what was important about the Philae lander. And, of course, youíve got to remember that comets are mostly ice, not just water ice, but also methane ice, and ammonia, carbon monoxide, carbon dioxide. If you want t understand the icy part of the comet, youíve basically got to go out there.


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