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.
