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Lecture 3.11: Types of meteorites: Achondrites
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Del curso dictado por Caltech
The Science of the Solar System
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Unit 3: Big questions from small bodies (week 1)
Conoce a los instructores
Mike BrownProfessor
Planetary Astronomy
In the last lecture, I told you about chondritic meteorites.
And now, I'm going to tell you about the other two types which are achondrites,
A in this case meaning not chondrites.
But they're still stony,
they're still made out of those silicate rock-like materials for the most part.
Achondrites then down to irons, and I'm talking about these together because in
many ways, these are related things, so let's get back to the irons.
I have again my favorite little chunks of iron meteorites sitting here in
front of me.
And let's ask ourselves the question, where would we get a chunk of pure
iron falling out of the sky in the inner solar system?
Chondrites, okay so we don't really understand chondrites or
why the chondrites are what they are?
But we can understand them as these are the materials that the solar system was
built out of.
If we look at the composition of chondrites, they are the overall
composition of the sun minus some of the materials that would
have been lost from the solid chondites, but they are chunks of the solar system.
When you look at iron meteorites, they're not the composition of the sun,
they're the composition of, well, iron and nickel,
is the other main component that's inside there.
And how do we isolate these things out?
Well, if you think through the solar system that we have today.
Where do we isolate iron today?
There's one really easy answer to that question, which is, you take a planet.
We can take the Earth if you'd like.
And you start forming that planet, and
it forms out of, let's say, chondrites, a big collection of chondrites.
But as the planet gets bigger and bigger, and it gets warmer and
warmer, eventually, it melts.
When the planet melts, the heavy materials fall to the bottom,
the light materials float to the top, and you get an iron core.
This is the process of differentiation, and
it led to what we talked about is that iron and other components core on Mars.
It's the same sort of process as leading to the helium raining out on Jupiter and
Saturn, and it's what leads us to have iron in the interior of the Earth.
So that's nice, but then how do I get this piece of iron
in my hand without having to break apart the Earth or break apart Mars?
The answer is, you have to break apart something.
Presumably, these iron meteorites came from the destruction
of what was once a little mini-planet.
Something that was much, much, much smaller than the Earth potentially.
But it had gotten warm enough and melted enough that the iron sunk to the inside,
and we had a nice iron core on the inside.
What's the size?
Well, people talk about things like 20 kilometers up to maybe 100 kilometers.
And you can figure out the sizes in a lot of different clever ways.
Again, that you have these sitting in front of you,
lets you do all sorts of experiments.
One of the things that you can do is use some of the properties of the iron
meteorites to figure out how long it took them to cool.
You can imagine if you had a huge planet in a molten iron core,
that core is insulted so it would cool very slowly.
If you had a very small planet and a small iron core, it could cool very quickly.
And so by figuring out those cooling times from the properties of the iron, you could
actually figure out how big it was, and that's where these numbers come from.
Although occasionally, you hear people talking about things as big as 1,000
kilometers which will be not actually more like this size compared to Earth.
But the general idea is that these things became like little mini-planets.
They differentiated, they had iron cores.
They would have had a mantle and a crust like the Earth has.
Earth has a mantle and a crust which is different from the composition
of chondrites, because all the iron, for example, has been taken out of the mantle.
And the crust is different from the mantle because the crust is the stuff that's
distilled through the process of plate tectonics, for example.
All of the granitic material on the crust is different from the stuff on the inside.
Do we see anything like that in the asteroid belt?
We see one.
We see the Asteroid Vesta, which is about 500 kilometers in diameter.
And it is the only asteroid that we know,
it's actually been visited by a spacecraft recently.
So we know this for sure, but we were pretty sure of this before.
It's nearly circular, it's got a big sort of impact crater on the south,
makes it look a little bit funny, but we're very certain it is differentiated.
It has an iron core on the inside, and we are dead certain that it was melted
because we see molten material that has flown on the outside.
We see the salt.
The salt is that left over product from a magmatic flow.
The salt is the stuff that makes the Hawaiian Islands that dark rock.
It's the stuff that's on the mare of the moon that lets us know that the moon had
volcanoes that was spewing out on a surface.
That's to has basaltic material that is flown and covered much of the surface.
We know that its interior was molten.
We know that it has an iron core.
So are the iron meteorites from Vesta?
Well, no, because you would have to crack Vesta open to find that iron core.
Vesta's clearly never been cracked up and
although, when impact tried very hard to do it.
In fact, you could see that really big impact based on at the south pole of Vesta
from this beautiful dawn image.
You see sort of these thing here on the middle, but
that's actually the mountain in the middle of the crater.
The crater takes up almost the entire South Pole, and it's from this crater
where all these meteorites have been delivered to the Earth.
You can see another consequence of this really massive impact.
If you look at Vesta from the side, now, the South Pole is down here.
And that impact was so large and so close to almost breaking the entire body apart,
that you get these striations along the equator where you had compression as this
thing slammed into the bottom down there.
There are some asteroids, they're called M class asteroids.
We'll talk about the classes of asteroids a little bit later.
M class asteroids look like they are just remnants of iron core.
They are just total chunks of iron.
And you can actually tell they're iron by using radar to reflect off of them.
As you can imagine, a big chunk of iron is very reflective in radar.
And sure enough, they're just chunks of iron.
And they presumably started out as larger mini-planets.
Not as big as I'm drawing relative to the Earth here.
So I should draw these all really small, like, to expect to,
I want to give you the idea.
That impact stripped the mantle material away from them, and
left just this raw iron core leftover.
What does a metallic asteroid look like?
I don't know, but we're going to find out.
Maybe it looks like this.
NASA is sending a probe to an asteroid called Psyche.
And Psyche is also the name of the spacecraft just to make things confusing.
But it might look like this, it might have strange craters,
it might have weird land forms.
I can tell you, it will be nothing we've ever seen before, I can't wait.
You can imagine this process also happened that would have shattered the iron core.
When the iron cores get shattered, chunks of iron flow off into space,
some of which eventually land on the earth.
I love holding this little piece of iron meteorite in my hand, and showing it to
people, and explaining to them that this is the core of a tiny, mini-planet.
That was forming back at the very beginning of the solar system, that sadly,
had an impact which catastrophically shattered it into pieces, but
then let parts of it fall onto the Earth.
It's a cute story.
Is it really true?
Well, there are just so many aspects of it that make the story so incredibly true.
Let me show you my favorite picture,
this time though, of my favorite type of meteorite.
I have to show you a picture because these are so cool that they're very expensive,
and I don't have one.
I would love to have one.
If anybody has one in their collection, they'd like to send it to me, please do.
Here's what the pictures look like.
This is called a Pallasite, and this one is a beautiful, polished up one.
And what do you see? Well, this stuff,
it's hard to tell because it's reflecting, this is iron.
This is essentially an iron meteorite, this material in through here.
And this, these are crystals of olivine, pyroxene.
These are the crystals that we find inside the mantle of the Earth.
Where would you get iron and olivine next to each other?
Well, you would find it in one place.
It's actually, you wouldn't quite find it on the Earth.
But you would find it in, if you had an iron core and
you had a kind of small planet,
you would have at the core mantle boundary right there, you would have olivine.
This would be olivine in here, maybe pyroxene too.
And you would have that right on the core mantle boundary.
If you shattered that little planet to pieces, there would be some of these
places where the iron had where the iron and the olivine were still mixing.
This is, this represents an incredibly special spot in the solar system.
In the, right at the core mantle boundary of some mini-planet that was
just forming at the beginning of the solar system before it got smashed apart.
We will never see the earth's core mantle boundary or any other planet.
But here we have, this person is holding,
in his hands, the core mantle boundary of this little mini-planet.
It's just an astounding thing.
This is officially an achondrite, because it's not chondritic, but it's not iron.
Or maybe this one is called a stone in the iron because it has stone and iron.
But you can find things that are pure achondrites which are from these parts,
from the mantel parts, or maybe even the crust parts, those would be achondrites.
They don't have chondrites anymore because the melting that happened
would have melted the chondrites.
And they would have just then turned into whatever the materials here, well,
olivine, pyroxenes, whatever is on the surface, these things.
We find achondrites that came from Vesta, the asteroid that I just showed you
of those basaltic parts that come from the top.
We find other achondrites,
some of them have now been associated with coming from the surface of Mars.
And we can tell, by their compositions,
that they match precisely the composition of Mars, they're at the surface of Mars.
We find achondrites on the surface of the Moon, quite an astounding thing.
We can get lunar samples by just sitting here waiting for
them to fall down, pretty cool.
All of these things tell you, again, of this incredible and
dynamic period very early in the solar system,
where things are melting, things are forming into little mini-planets.
Things are being shattered to pieces, reaccumulating.
Some of them were melting, some of them are not.
The fact that these are maybe 10 kilometers across or
100 kilometers across.
And yet, we also have asteroids that are those sizes is sort of a surprising
things, chondrites.
Why did some things melt?
Why did some things not melt?
Presumably, it has do with how fast things form.
If things form really fast then they have some extra radioactive materials
left over, in particular, aluminum 26 is the important one.
There's a lot of aluminum 26 right at the beginning of the solar system.
It has a half life of something like 700,000 years.
And so if you form quickly and
you still have active aluminum 26, you get a lot of heat from that aluminum 26.
Maybe that's why these melted very quickly even though when they were small.
The things in the chondrites maybe they formed much more slowly,
maybe that's why they didn't melt.
There's a rich dynamical history being told here.
Again, as I said in the last lecture,
it's a little bit difficult to connect this rich and dynamical history
with what exactly we're learning from other aspects of planetary science.
But finally, after decades of meteoritic people not talking
to really planetary science people, not talking to really astronomical people.
Everybody's finally getting together, and
these stories are all starting to make sense.
We're starting to be able to understand how these mini planets formed.
Why they broke apart?
Why there aren't any left?
Maybe Vesta is the only one left that's differentiated.
Maybe that's not true.
Maybe there are differentiated things that are leftover, but
they have iron cores, and they have mantles.
But maybe on top of them, they sort of had chondrites that landed later, and
never made it through the differentiation process.
That could explain a lot of the fact that we see so many iron meteorites, and yet,
we don't see that many leftover mini-planets.
There are many mysteries about meteorites.
It's a rich and fascinating field.
And the thing I want to leave you with is that these things are in all of your local
museums.
The most beautiful pallasite I've ever seen.
I'm sure there are more beautiful ones, and
I'm sure you guys will tell me about it.
But the most beautiful ones I've ever seen was at the American Museum
of Natural History in New York at the Haden Planetarium.
And at the time I was there, maybe they moved it, but you walk in the front door.
And it is the size of a person propped up fairly thin, so
you can see through these crystals and you see the metal.
And you look at that and just have to stop yourself, and think that you're that
looking at the core mantle boundary of a mini-planet which is just fascinating.
You can find iron meteorites, it's actually not that expensive to buy.
You can find them if you know where to look.
And these are the little cores of mini-planets, and chondrites.
Chondrites, if you can ever hold a chondrite, I recommend it highly.
But even if you can't, again, museums are full of these.
They often don't get much visiting because nobody really
knows why something with funny little circles in it is very exciting.
A black piece of carbon with funny little rocks in it.
Who cares?
You care.
This is the first stuff that formed in the solar system just a few
million years after the sun started to form.
These are the building blocks of planets.
These are where we all came from.
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