Learn about the science behind the current exploration of the solar system in this free class. Use principles from physics, chemistry, biology, and geology to understand the latest from Mars, comprehend the outer solar system, ponder planets outside our solar system, and search for habitability in our neighborhood and beyond. This course is generally taught at an advanced level assuming a prior knowledge of undergraduate math and physics, but the majority of the concepts and lectures can be understood without these prerequisites. The quizzes and final exam are designed to make you think critically about the material you have learned rather than to simply make you memorize facts. The class is expected to be challenging but rewarding.
Lecture 2.15: Saturn and the ice giants
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From the course by Caltech
The Science of the Solar System
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From the lesson
Unit 2: The insides of giant planets (week 2)
Meet the Instructors
Mike BrownProfessor
Planetary Astronomy
We've spent all this time talking about Jupiter, the interior of Jupiter,
how Jupiter formed.
We do, of course, have three other giant planets in our solar system, and
it's worth considering the differences, the similarities, and what else we can
learn about these interiors of giant planets by looking at these other three.
We're going to look at these planets the same way we initially looked at Jupiter.
If you remember, when we, we looked at Jupiter,
we realized that a planet of the mass of Jupiter and the radius of Jupiter,
that is, given the density of Jupiter, we could figure out how much
material other than hydrogen and helium it had, from these theoretical calculations.
If you were made out of just hydrogen and helium, you should follow along this line,
and Jupiter falls a little bit below that line in radius.
It's a little smaller than it should be.
How does it get a little smaller than it should be?
It has extra heavy material that contracts that radius a little bit more.
Here's a model, for example, that it fits that includes a 10 Earth
mass core in addition to the hydrogen and helium, and it fits pretty nicely.
Let's look at Saturn, Uranus and Neptune.
This is the symbol for Saturn.
It's even lower.
It doesn't fit on the same line for just a 10 Earth mass core.
It requires even more solid material.
And as we said before, there, the models which suggests that Jupiter has something
like a 10 Earth mass core, those are uncertain enough to make us
not 100% clear that there needs to be a core on Jupiter.
Saturn, though, because you can see, it's even more dense for its size
than it should be than, than Jupiter is, Saturn almost certainly has a core.
We'll talk about that core in a minute.
Let's look at Uranus and Neptune.
Uranus and Neptune are quite a bit less massive than Jupiter and Saturn.
They're way down through here and because there's so little mass, the density,
the density makes it hard to distinguish between different amounts of, of core.
But you can really see that purely hydrogen and
helium would be way up here, and they're well below that.
So Uranus and Neptune, even more so that Jupiter and Saturn,
look like they're composed of a lot more material than just hydrogen and helium.
You might remember this figure too.
This is where we figured out that Jupiter, as you go up in pressure,
meaning go down in the interi, into the interior, you go up in temperature.
That's, here's what happens with Jupiter.
And at some point you reach the metallic hydrogen threshold.
And that's, in here is the interior of Jupiter at the very highest
pressures are indeed metallic hydrogen.
What happens on Saturn?
Well, Saturn is, is not as massive, so the pressures are a little bit lower for
a given temperature, but it does cross that threshold.
And indeed, we see that Saturn, like Jupiter, has a vigorous magnetic field,
presumably caused by what's going on down here in the metallic zone.
Uranus and Neptune, on the other hand, are so
much less massive that they are well below the lines of Jupiter and Saturn.
You can see they're increasing in temperature, increasing in pressure.
And they may or may not cross into that metallic hydrogen zone.
The question you should ask then do they have magnetic fields like Jupiter and
Saturn do?
And the answer is yes, but they're weird.
The magnetic fields of Jupiter and
Saturn are pretty close to being what you would call [COUGH] a dipole.
Dipole means like the Earth they have a, a north pole and a south pole, and
they are more or less aligned with their rotational axis.
It's not exactly the case.
Jupiter's is, is tilted by about 10 degrees.
Saturn's is nearly straight up and down.
Part of that is a consequence of the fact that the conducting zone is
deep in the interior, and by the time we see the magnetic fields,
we are far enough away that we really only see that dipole hard left.
Uranus and Neptune, they do have magnetic fields, quite strong magnetic fields, but
they don't really look dipolar much at all.
They aren't aligned with a rotation axis.
They have a much more complicated structure
even further away from the planets.
What's going on?
Well, part of the reason might well be that the magnetic fields
are not being formed down in a deep metallic layer, but that there's something
else that's conducting through there that's causing magnetic fields.
We'll continue to explore to see if we can figure out what those other
things might be.
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?
We think so.
We don't know. It would be nice to have an orbiter around
one or around both to really understand the magnetic field better,
understand the gravitational field better, get these measurements.
And it'd nice to send a probe down inside one of them, even just the upper parts,
to understand if they're even more enriched than we thought before.
The important thing is that these two planets
are very much unlike these two planets.
Lumping these four together as giant planets,
gas giant planets, really is a mistake.
These are gas giants.
They're predominantly gas in through here.
These, astronomers often call, call these two ice giants.
They are mostly giant ice balls with a little bit of atmosphere on their tops.
Okay, now that we know this about the structure of these, or at least we think
we know this, let's think back one more time about the formation.
We talked about the core collapse versus disk instability,
and the question of how Jupiter itself formed.
And one of the questions was whether or not there's really a core.
On Saturn, there really is quite a massive core that needs to be there.
It's very difficult to think of a way that Saturn could have formed
other than this core instability.
And yet, if you continue that idea out to Uranus and Neptune,
it gets sort of interesting.
Uranus and Neptune are mostly solid material, much more solid material
probably than Jupiter and Saturn, and only a little bit of gas.
Why didn't Uranus and
Neptune grab all the gas in their feeding zones like Jupiter and Saturn did?
One potential explanation is that you're looking at a history
of gas in the outer solar system, that things that are closer to the sun,
there's more mass around, they form faster.
So Jupiter formed first.
There was a lot of gas around.
It grabbed it.
Saturn formed pretty soon there after.
A lot gas around, grabbed it.
By the time Uranus and Neptune formed their cores,
it takes longer where they're so far away, by the time they form their cores,
the gas that was there with the rest of the solar system
was slowly being blown out of the system and being lost to space.
So Uranus and Neptune just had the last gasp of gas that they could grab onto,
and they didn't get very much.
Had they formed earlier, had it taken longer for
the gas to dissipate, we might have had four gas giants instead of these.
Is that story true?
Maybe.
Answering questions like these in the solar system is a very difficult thing.
We have four big planets.
I'll call them two gas giants, two ice giants.
And we don't know what happened 4.5 billion years ago, and so we're trying to
reconstruct an entire history just from looking at these four objects.
One of things that would be nice would be to find other planetary systems and
ask questions like, do they have
gas giants in the middle of the planetary system, ice giants on the outer edge?
Are there sometimes four ice giants?
Are there sometimes four gas giants?
What are the range of possibilities?
The whole process of planet forming is presumably so
stochastic that understanding one iteration that we have here,
using only four instances, is potentially impossible.
We'll consider those possibilities and what else we might do
when we start looking at other planetary systems in the next lecture.
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