We figured out from the density of Jupiter that

it's probably not made out of rock, in fact

it's probably not even made out of ice, even

though it's density is higher than that of ice.

But it would be compressed on the inside by

so much that it's density would be even higher.

We suspected in fact that it's made out of gas.

What gas?

Well we're going to start, by just taking

an educated guess, and then work from there.

An educated guess might be that Jupiter is made out of the

same materials and the same amounts of materials that the sun is.

The sun is approximately 75% Hydrogen and 25% Helium.

Close enough,but not exactly right.

But it's worth exploring what would Jupiter

be like if it had this same composition.

Can we explain.

The size of Jupiter.

The density of Jupiter and other properties that we'll

talk about later, by assuming a composition like this.

The answer is going to turn out to be no.

It will be close to this but there will be some very important details that

we figure out, but this will be a good start to help us on our way.

Okay, so the first thing that we keep talking about is that.

Inside of Jupiter there is so much pressure from all the material

on top of it that everything is going to be compressed so much,

and get higher density so the first question we might ask ourselves,

is how can we figure out what the pressures are, inside of Jupiter?

Here we''re going to make an assumption, and that

assumption is that Jupiter is an hydrostatic [SOUND] equilibrium.

Lets look at that for a minute, hydrostatic equilibrium.

Hydro means liquid.

Static means not moving.

We're going to make the assumption that Jupiter, the

interior of Jupiter is a liquid that's not moving.

An equilibrium in this case means that

there's some sort of balance between something.

And something else.

We'll figure out what that is in a minute.

But what we're saying when we say that something's in

hydrostatic equilibrium, we're saying that the support for Jupiter, the

reason Jupiter doesn't collapse back on itself is not because

there is roiling motion inside of there that's keeping everything supported.

Or huge temperatures that making, making everything boiled

because Jupiter is simply behaving like a stationary fluid.

Now, seems crazy I said that, that's it's mostly Hydrogen

and Helium, why could we call it a fluid hydrostatic?

In fact, the hydrostatic equilibrium was a pretty good assumption.

For all of the planets, even the Earth.

The interior of the Earth is in something close to hydrostatic equilibrium

even though it's actually more like solid than it is like a liquid.

And we use hydrostatic equilibrium to discuss things like the

Earth's atmosphere which is again a gas and not a liquid.

So when we say hydrostatic equilibrium we can mean gases, we can mean liquids.

We can even mean solids.

And what we really mean is something very specific [SOUND] and that is, that the

weight of all the stuff on top of you is balanced by the pressure of you.

And by you, I mean a little parcel of, in

the case of the earth, a little parcel of air,

in the case of Jupiter, a little parcel of Jupiter's

interior, in the case of Earth, a parcel of rock.

If I look at, let's look at the Earth case, if I look at

this little piece of air, in my hands right this minute, well this piece of

air in my hands right this minute has a lot of air above it

which is pushing down on it and the only reason it doesn't collapse on itself.

Is because as it pushes down on it my [INAUDIBLE] also

has pressure pushing out and those two things balance, they're in equilibrium.

We can use that idea to figure out what the pressure

is as the function of altitude in something like the Earth's atmosphere.

Let's do it this way.

What we'd like to figure out is the pressure.

As a function of height, let's say, above the earth.

Here's the, here's the surface of the earth.

Here's height going up.

And we'd like to know what the pressure is as you go

up above the surface of the earth, p of z, we'll call it.

Well, we know the pressure at the surface of the earth, because we can measure it.

You get out your barometer or something else and see what the pressure is.

And we can actually figure out pretty easily what the pressure

is just a little bit above the surface of the Earth.

Let's say that we go up by one meter above

the surface of the Earth, not very much, we actually know

that the pressure one meter above the surface, surface of the

Earth is about the same as the pressure at the surface.

But let's say, pressure at zero.

Is equal to we'll call it p not pressure

at one meter, [SOUND] well it's almost the same as

pressure down here at zero, but because it's all due

to the weight of the air on top pushing down.

But there's less weight.

Why is there less weight?

Because this little parcel of air is no longer sitting

on top of us because we're up a little bit higher.

So we subtract the pressure due to this little parcel of air.

What is the pressure of that?

Well, it's the density of this material, whatever that is, we have the pressure

at zero minus the pressure that would have been caused by this little bit

of this one metre high slab so that's equal to the density of the gas row,

the gravitational pole of the gas and now our one metre size.

Let's make sure this makes sense, pressure as

you remember as a force per unit area force

per meter squared The gravitational force of the

earth times mass, will be equal to a force.

Which you have instead of a mass, you have a density which is mass divided by volume.

We'll multiplying that by the height here to get the mass per unit area.

So we have force per unit area is minus row g times one meter.

What's the pressure at two meters?

Well, the pressure at two meters, [SOUND] is

pressure at one meter, [SOUND] minus row g.

Times one meter.

We can do this forever.

I think you see the pattern.

And I'm going to write it down in a suggestive way.

P(zdeltaz) = P (z)- pg x Delta Z.

Now I'm not just talking about one meter steps, I'm

talking about any kind of small step that we could do.

And I'm going to rewrite this in an even more suggestive way.

And I'm going to then take you back to

high school calculus [SOUND] and remind you that

this thing that I just wrote down, is

the definition that you've seen before of the derivative.

This is dP.

Dz, the derivative of p with respect to z is equal to minus row g.

This is a differential equation, but its

about the simplest differential equation in the world.

And in our simplest possible case, we can write the solution to this

differential equation as p of z is equal to p not minus row gz.

Lets make sure that this differential equation works, if I take dp dz.

This is a constant that goes away, the PDZ is minus

row g, that's exactly what our differential equation was over here.

It's okay if you haven't seen differential equations in 30 years or perhaps

have never seen them all this is saying is that the change in pressure.

As a function of height is proportional to row times g.

And all this is saying is that the pressure is

a linear function of z in a very special case.

The very special case is that row and g are not functions of z.

They do not change with height.

In fact, this is only true if, if row, if the density of the

material does not change with compression and

this is true for an incompressible fluid.

Now, we haven't been talking about incompressible fluid.

We've been talking about how the high pressures inside

of Jupiter cause the gas to get higher density and

that's certainly the case too, true, but there is a

good example of an incompressible fluid or nearly incompressible fluid.

Where this equation works very well, and that's water.

Water, as you know, if you take a piston full of water

and you try to compress that water, it's really hard to do.