When did they discover the Gas Giants were fluid?

mopc

The other day I was watching this old Disney movie from the 1950s called Man in Space, specifically a part when they speculate what life would be like on the other planets of our Solar System.

The speculation of life on Jupiter showed a cartoon of a rocky planet with a normal surface and atmosphere.

Did they not know in the 50s those planets were devoid of a solid surface?

When did astronomers establish that fact?

qraal

Early models of the giants had them all as primordial objects that had yet to cool down - the cloud activity and Red Spot looked like volcanism of some kind. Then in the 1930s a bunch of early models came out which indicated that the giants had to be mostly hydrogen, due to their low density, but probably overlain by ammonia & methane gases and water ice, because those were the gases that could be observed spectroscopically. Thus the "rocky planet" in those pictures, which came from artwork by Chesley Bonestell, was actually made of ice sitting on top of metallic hydrogen, which was believed to form at about 100,000 bars of pressure due to early estimates of the pressure needed for it to form.

Only in the 1960s did the excess heat from the giants get observed directly, thus leading to the molecular fluid hydrogen on top of metallic hydrogen models that we have today. But that's only really applicable to Jupiter and Saturn, as Uranus and Neptune are too low mass and too dense to be mostly hydrogen/helium. Instead they're most probably a mix of fluid (not liquid as we know it) water/methane/ammonia on top of post-perovskite silicates and a metallic core. But there's a lot of uncertainties in such models due to our poor understanding of extreme pressure conditions acting on a mix of molecules. A recently discussed possibility is for a layer of carbon from dissociated methane. The pressures are so extreme that the carbon layer is probably mostly diamond, possibly in a very exotic liquid form as well as with floating crystalline chunks.

mikeph

It's very strange to imagine an entire planet (or most of it) just gas- why doesn't it all blow away, like Mars' atmosphere?

I remember reading (LONG time ago, like 1993) that Uranus had the highest wind speed of any planet -but if it's gas... where is the surface and how can there be wind (therefore gas) moving above the surface?

qraal

For the same reason planets form in the first place: gravity.

The Gas Giants are mostly at a higher density than gas as we usually experience it. For example most of the hydrogen & helium inside Jupiter & Saturn exist as molecular fluids which are almost as dense as water, steadily getting denser as the pressure increases.

Plus the Gas Giants all have hefty magnetospheres, unlike Mars, so the Solar Wind didn't blow them away.

That's a really good question and kind of tricky to work out. A planet's rotation speed used to be estimated by how fast the clouds moved, but of course that means you can't work out the wind speeds. But the Gas Giants have lots of different bands of cloud, which means you can cross-compare and work out the relative speeds of all the bands, giving a rough idea of the windspeeds.

But then the Voyager probes flew past the big planets and measured the speed at which their magnetic fields were rotating - leading many planetary scientists to declare they now knew the rotation speed of the planets and they could measure the windspeeds at last. Neptune, not Uranus, was declared the fastest as a result, which is what you would've read in 1993.

However since then the assumption that the magnetic field producing layer within the planets will rotate with the "solid" body of the planet has been challenged. As a result new rotational periods have been worked out which give somewhat lower windspeeds, but confirming the new rotation periods will require more work.

"Science in Progress" signs should go up I guess.

mopc

Well two years too late but thank you for the reply qraal