Quite an amazing amount of detail,
you can learn about the magnetic field of a planet
just by looking at things like this aurora,
by flying spacecraft through it,
measuring these magnetic fields,
and by looking at many different types of radio mission that you can see from it.
So, why does Jupiter have a magnetic field?
We talked a little bit about magnetic fields when
we were talking about Mars and its interior.
But we're going to talk in a little bit more detail about it this time.
Magnetic fields in astrophysical objects are created by a dynamo.
And there are three crucial ingredients to get a dynamo to generate a magnetic field.
First, you have to have a conducting liquid.
In the case of the earth,
that is liquid iron.
Liquid iron is conducting,
it's electrically conducting, and it's liquid.
So, that's good. In the case of Jupiter,
what are you going to guess in the case of Jupiter?
It's that liquid metallic hydrogen.
That strange state that hydrogen attains at high pressures is
critical to the fact that Jupiter has a magnetic field and a strong one at that.
What else do you need for dynamo?
You need convection.
You need to have this liquid not just sitting around doing nothing,
but you need to have it rising and falling within the planet,
making these looping motions.
How do we get convection? Well, we talked about that.
You have to have heat on the inside,
and you're trying to get rid of the heat on the outside,
and the fastest way to do that is through this convection.
Yet have one other really important thing which is rotation.
Even if you have a conducting liquid and you have all the convection in the world,
if you don't have rotation, no dynamo.
No magnetic field. Why is that the case?
The answer is really complicated.
The answer is so complicated that scientists have not actually really
solved the entire problem of how dynamos are generated,
how they stay alive.
But the basic idea you can sort of handwave at with a picture that looks like this.
This is a picture that's drawn for the Earth's magnetic field,
but you'll get the general idea.
On the earth, there is a solid inner core of iron that you see here,
and this liquid outer core from here to here,
and then the mantle. This is the mantle.
Their is the surface of the earth would be out here somewhere.
So, all this magnetic field on the Earth is generated in
this liquid outer core. How does it happen?
Well, material in the outer core rises
up because of convection and falls back down again.
But, because of the rotation,
there is a Coriolis force.
Coriolis force of course is the thing that leads
to hurricanes all spinning in nice directions.
It's a sufficiently strong force that when
ships are shooting shells many kilometers away,
they have to calculate the Coriolis force.
So, instead of pointing directly at the thing they're aiming at,
they have to point a little bit to the side because as they shoot,
it curves duo to Coriolis force.
Anything moving in a rotating frame will be subject to this Coriolis force,
and will be deflected.
So, what happens is, these rising fluids are
deflected and form these loops that look like this.
These liquid metallic, these liquid conducting loops of
material spinning around are what generate and sustain these magnetic fields,
and you get these big north-south magnetic fields moving through here like this.
The same process happens on Jupiter,
where this might be that solid core on the inside of 15 Earth masses.
This is the region of liquid metallic hydrogen,
and above here is the region of molecular hydrogen. Why does this matter?
Well, for one, it tells us there really is this liquid metallic hydrogen,
this crazy phase of hydrogen that's inside here that's causing this giant magnetic field.
Two, if we can figure out where in the planet that magnetic field is being generated,
we understand a lot more about this transition from
hydrogen molecules to this liquid metallic hydrogen.
How do we figure out where it's being generated?
Well, from far enough away,
you can't tell the difference between a field that's
just generated directly in the center as
a point and a field that's generated in an annulus somewhere further away.
This is exactly the same thing as we were
talking about with gravitational field, when I said,
if you have a spacecraft really far away,
it can't tell the difference between a point source and an oblong field here.
If you have a spacecraft measuring the magnetic field here,
it can't tell the difference between a simple dipole from the center looking like this,
and maybe something more complicated where the magnetic field is generated here,
and is a little more complicated out through here.
But by the time it gets up to here,
It's exactly the same.
What's the solution? There's always a solution.
Take your spacecraft, put it in as close as you can.
The closer you get to where these regions of the magnetic field are being generated,
the more deviation there is from this simple dipolar field of just a magnet,
and the more complicated things you see.
We already know that this is going on.
We saw that from the Aurora.
We're getting some clear indications of
a very complex field in some indication of where the magnetic field is being generated.
How did you better? Notice I drew once again the Juno spacecraft.
Juno spacecraft in orbit around Jupiter.
One of the things that's going to do is fly in very close,
measure what's going on with that magnetic field,
and understand not just what's on the very inside or if not,
there is a core of material like we talked about before.
But also understand that transition from the liquid metallic hydrogen out to
the more normal state of matter that we understand this hydrogen molecule.
So, we had two types of measurements that we could do from a spacecraft.
We could measure the gravitational field particularly as you go close,
we can measure the magnetic field again as you go close.
Both of those help us to understand the details of what's going on in the inside.
We have one more measurement that we can
make that will help us to make sense of the interior of Jupiter,
and we'll talk about that next.