Again, you've seen these pictures before, but you only saw the little version that I
showed you for Jupiter, and I showed you this part of Jupiter.
And now let's put the other planets on there for comparison.
First off, something that you don't necessarily realize because you often see
drawings of the solar system that are totally off on scale, Uranus and
Neptune are quite a bit smaller than Jupiter and Saturn.
In fact, if you put Earth inside one of these, Earth would be about this big.
Uranus and Neptune are sort of midway between the size of the Earth and
the size of one of these other giant planets.
Where the structure of Jupiter was something like this molecular hydrogen,
a little bit of helium on top, and metallic hydrogen down at the bottom,
with the helium having rained out, so more helium down here.
A little bit of a core.
Saturn, where it's cooler,
is able to keep even less of its helium up in the upper atmosphere.
So that rain that was, we talked about for
Jupiter, has really occurred down here, and look at that.
30% helium down at the bottom compared to 14% at the top.
There's a question mark here because we don't really know that this is true.
We've never gone inside of Saturn and measured the amount of helium like
we have in the upper little itsy-bitsy amount from the Galileo spacecraft.
Overall, though, I would say that these two planets are exceedingly similar.
They both have these envelopes, metallic hydrogen interiors and potentially a core.
And the core on Saturn.
We still assume there really is a core on Jupiter because all the best measurements
and best experiments suggest it's true.
On Saturn there's no getting away from the fact that there's a core in the interior.
You can see it from the gravitational field of Saturn.
You can see it from the, the flatness of Saturn.
I don't know if you can tell just from here, but
the drawing that they've made here of Saturn itself is flattened,
it's pulled in here and sticks out here in ways that Jupiter's not.
Jupiter looks pretty round.
That's really true.
If you just look at pictures of Saturn you can see that flattening.
And that flattening is caused by that extra
amount of material in the very center changing the shape of the planet.
You can't not put a core inside of Saturn.
What about Uranus and Neptune?
Uranus and Neptune,
we already saw that they had a lot of things besides just hydrogen and helium.
We actually don't know very much about the interiors of Uranus and
Neptune very well, for the simple sake that Uranus and
Neptune have had one single encounter that flew by both of them.
That was the Voyager space craft, and just flew by and was gone.
Whereas Jupiter had the Galileo spacecraft in orbit around it.
It had many spacecraft flying by, so,
many opportunities to measure magnetic field's gravity.
Saturn has had the Cassini spacecraft in orbit for a decade measuring gravity,
measuring magnetic fields.
We do know more details there.
Here we just don't know as much.
But here's what we think is going on on these other planets.
There is molecular hydrogen and helium in this outer envelope.
We see that.
When we look at the atmosphere, we can see that the atmosphere out here
is made of those materials plus a lot of other trace things.
But there needs to be so much more material in Uranus and Neptune that
the best way to do it is to have a massive chunk of the interior be essentially ice.
This part, through here, is consistent with being compressed ice,
and maybe some rock down in the bottom.
By ice, I mean ice, like an ice cube that you would go have that.
If this is, these planets are predominantly big chunks of ice with
little envelopes of hydrogen and helium on the top of them.
And if they're big chunks of ice, high-pressured ice, maybe, in some regions
up near the top there's enough convection of something some sort of salty material,
who knows what exactly, that's convecting, that's causing a dynamo.
This would help to explain why the dynamo on Uranus and
Neptune seems to make a field that doesn't seem so dipolar.
Here the, the magnetic field is being generated deep in the interior.
When we look at it we're far away.
Here if it's sort of in the outer fringes where that magnetic field is being
generated, then it makes sense that when we look at it,
it looks still very complicated.
Is this all true for Uranus and Neptune?
