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Lecture 3.13: Asteroid compositions
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來自 Caltech 的課程
太阳系科学
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從本節課中
Unit 3: Big questions from small bodies (week 2)
與講師見面
Mike BrownProfessor
Planetary Astronomy
We've talked about how asteroids formed, where they are,
what happens when they land on the Earth, what we can see about them.
We haven't actually really talked much about the asteroid itself.
What is it made out of, what does it look like?
Let's start with what we can learn by looking at them from the Earth, looking at
them with telescopes and trying to learn something about what they're made of.
For the most part, with telescopes on the Earth,
we actually can't see the asteroid itself.
We can't see the shape, we can't see the roughness.
It's not entirely true.
For the biggest asteroids, we can actually get vague images of them from the ground.
And for very close asteroids, we can get some really interestingly detailed images
of them by using radar observations.
We’re not going to talk as much about that.
When we want to know what an asteroid really looks like,
I’ll show you real pictures from spacecraft that have gone by,
which really give you some detailed ideas of what these things are.
First though, I want to show you what we’ve learned by using telescopes to
look at the overall picture of what is on the surface of asteroids.
And this is a really critical point, it’s what’s on the surface.
If you use a telescope and you look at the spectrum, the light coming back from
the telescope, to make a fingerprint of the chemicals, just like we talked about
on Mars, you're looking at a tiny, tiny, tiny region of that asteroid.
You don't know what’s on the inside, you only know what’s on the outside.
And what do these spectra look like when you look at them?
I love this figure, I love this paper, this is a data dump of spectra.
Each one of these little squiggly lines, which looks like, I don’t even know what
it looks like, is a spectrum going from about 0.5 to about 2.5 microns.
Each one of these things is an individual 0.5 to 2.5 microns, and
it shows you how much light is being reflected at that location.
And even without having to do too much,
you can start to see that there are some sort of differences.
There are some sort of similarities.
There are a lot of objects that look like this, you see with the little dip down and
going up, dip down and going up.
There's another, there's another, another, another.
Let's find some more, dip down, going up, dip down, going up.
You don't have to know anything about what these spectra mean to be able to start at
least putting those in the same category.
What else do you see?
Well, flat, not very interesting but
there are some flat ones that you might want to classify together.
Generally increasing in spectrum, that means generally red,
that means they reflect more red light than blue light.
So, we have some just sort of generically red objects.
We have, well, if you look, we have not really much else.
We have these sorts of things.
That dip down and dip back up again.
We have the things that are kind of red, like that, like that, like that.
We have things like this that are a little flat and then going up red.
And you can see more like that here.
Maybe a little bit like that here.
Maybe like that here.
And we have some that are very flat across like that.
There are other variations, more complexities,
that you'll see in through here, but you have already looked at and found the most
important categories of asteroids when we look at what they're made out of.
I'm not even going to bother telling you what you're actually seeing on these
because in the end,
it doesn't matter as much as it means that you can group these things together.
And when you group these things together, you can look at the asteroids,
group them together, and see where in space the individual asteroids are, and
you get something that looks sort of like this.
Okay, before I show you where all the asteroids are, what their composition is,
let me just remind you again where the asteroids are.
And I'm going to give you now a broader view than in the last lecture,
where we only saw a smaller portion of it.
Here's that main belt again, and the colors that you see,
the hot yellow colors are high density, lower density in red, and
then blue are individual scattered things.
And again, you see the main belt in through here with all of its families
going in through here.
And you see other little clusters up in here and up in here, families, there's
that Koronis family that we were looking at, and other families in through here.
Big outer edge, strong outer edge, but there are objects outside here.
These are called the Cybels, they just get names based on,
who knows what they get names based on.
These are called the Hildas, this one little clump.
And an important population we'll talk about later are the Trojan asteroids.
The Trojan asteroids are asteroids that have the same semi-major axis as Jupiter.
But the Sun is here, Jupiter is here, the Trojan asteroids orbit in front of or
behind Jupiter by about 60 degrees.
They're a big cloud of object in through here.
I spend a lot of my time looking at these asteroids in particular as an interesting
transition from the inner solar system to the outer solar system and
what it tells us for where all these objects came from.
Okay, so when you see these names you'll know what they mean.
Hungaria are these really inner things here.
The inner belt is this stuff.
Mid and outer is kind of the main belt.
Cybels and Hildas are a minor population, and Trojans are out through here.
So let's see what these things are made out of.
Okay, let's look at what we're looking at here.
Semi-major axis, your favorite things,
here's the Trojans out at 5.2 semi-major axis.
Earth would be inside here, as would Mars.
And total mass in different parts of the asteroid belt
is an estimate of how much mass is out there.
And then, those categories that we saw before, Hungarias, inner, middle, outer,
Cybele, Hildas.
And then a classification.
So here are the classification that we are talking about.
There are things, the major ones are Cs, I'm just giving out letters,
Ss and then these are some sort of minor ones in through here.
Cs and Ss are generally associated with, S-type asteroids are thought to be
stony asteroids, are thought to be the same sorts of things as stony meteorites.
C-type asteroids are thought to be
asteroids made out of carbonaceous chondrites.
You'll often hear them talked about as sort of stony and also primitive.
You understand why primitive, primitive is because they have all
of this carbon that only condenses at low temperatures.
Some of the subclasses that we will care about are these Ps and
Ds that are underneath the Cs.
Ps and Ds are like Cs.
You'll notice that they tend to be in the very outer part of the solar system.
I'll show you in a minute.
And they are thought to be the most primitive,
the least modified from when the asteroids were first formed.
Perhaps things like that Allende carbonaceous chondrite is what is a P or
D, or maybe it's just a C.
We don't really know how to connect the meteorites to these specific classes.
But let's look where they are.
Let's look at the main things, let's look at the top of these curves.
because the top of these curves tells you what the main component is at
any one spot.
So in this inner belt, you are mostly this red,
most of the masses in whatever these red things are.
What are these red things?
S, stoney-type asteroids dominate the inner part of the asteroid belt.
Let's move out to the middle part of the asteroid belt and notice now,
blue, what is blue?
They are the Cs.
The Cs were down here before, and
now they are becoming approximately equal with the Ss.
So the carbonaceous chondrites, by the time you get to the middle of the asteroid
belt, the chondrites, the carbonaceous chondrites are the major asteroid type.
But the stony types are quite common too.
As we move even further out, you can see what happens.
Now the blue is the biggest, the C-type.
The primitive meteorites are the most common.
Stones are coming down and
then some of these minor classes are starting to come up.
What happens when you get to these outer ones?
The Cybels and the Hildas are kind of minor populations as you recall.
You can see from their masses,
that they are an order of magnitude below these other things over here.
And what are they?
Well, there's these blues again, these are the Cs.
But what's coming up, this dashed line, what's the dashed line?
Well, those are the Ps.
These are the things that are perhaps even more primitive.
Here they are there, here they are there, the Ps coming up in through here.
This other dashed line, the Ds here, maybe a couple of Cs, and Ps and
Ds dominating out in the Trojans.
There are a lot of interesting different ways to interpret these sorts of data.
First off, there are a lot of asteroids down here we didn't even talk about.
There are things like Ps and Ds all the way down in this area in through here.
And Ss, let's see if we can find the furthest out Ss.
There's some Ss all the way out through here.
There's a lot of mixing within the asteroid belt.
Is that surprising?
No, I don't think that's surprising.
I think we saw when we were looking at, for example,
the formation of terrestrial planets, that those things that made up the asteroid,
things that made up the terrestrial planets, got strewn all over the place and
went in different places.
And one thing that you might actually find surprising is that there is actual
structure.
There are mostly Ss in this region here.
There are mostly Cs in this region out through here.
And as you get further and further out, there are mostly Ps and Ds.
We like to think of that as showing us both the mixing that goes on, and
some of the limitations of mixing.
The mixing doesn’t look like it extends all the way across these regions in
through here for the most part, although there is quite a bit in through there.
The other thing that we like to think that this tells us is something about
the formation temperature.
If they haven't been thoroughly mixed, then the things that are in close,
formed in closer when it was warmer.
The things that are further away, formed further away.
And this makes sense with our overall picture of the meteoritic record.
These are the chondrites, the ordinary chondrites.
These are in fact the same sorts of things that we think went into
the formation of the Earth.
These are those little stones that got heated up.
They're regular rocks, rocky material.
Out through here are the carbonaceous chondrite regions.
Carbonaceous chondrites, more primitive, things that did not heat up as much.
And this is, of course, the domain of Ceres, the largest asteroid, and
one that we think actually has a good bit of ice on it.
But these objects were certainly heated some and one of the things of the many
things we didn't talk about meteorites is that there's evidence from the meteorites,
even the primitive chondrites, that those primitive chondrites had a lot of
processing due to liquid water running through them.
So you can imagine that these things had ice incorporated in them.
Those ices got heated up and melted and the minerals that formed were the sorts of
minerals that we talked about on Mars, as forming in sort of water rich condition.
Finally, we get to the outer part of the asteroids where there are these Ps and Ds.
It's not clear if we really know what P and D-type asteroids are.
We've never visited any of those type of asteroids.
We may or may not have samples on the ground for
what those sorts of asteroids are.
And it's very possible that these are very water rich object, and
that they still have water ice on them.
That at these distances away from the Sun,
they could have water ice on their interiors, that are still very stable, and
has never melted over the history of the solar system.
On the other hand, they could very well have gotten warmer,
they could have formed closer in, gotten warmed up and had that ice melt.
This is an active area of research, and one of the reasons why I'm spending
a lot of my time looking at these Trojan asteroids trying to see if they
have evidence for water on them at these extreme distances.
That's the conceptual overview of the compositions that they are,
the distribution of these compositions.
In the next lecture, let's take a look at some of the pictures that have
been returned from asteroids by spacecraft.
