As if Titan's methane rain weren't weird enough, Jupiter's now thought to have helium-neon rain. (I suppose neither of these is quite as weird as the snowfall of diamonds that's supposed to be happening within Neptune.)
Anyway, to explain: a press release this afternoon describing a new paper published in Physical Review Letters by Hugh Wilson and Burkhard Militzer explains that helium-neon rain can account for a puzzling discovery from the Galileo Probe mission: that Jupiter's atmosphere doesn't have nearly as much neon as it should, given that it formed from the same nebula that the Sun did. In fact, there's only a tenth as much neon as the solar abundance. It was already known, before the Galileo Probe mission, that there was less helium than expected too.
Here's what happens. The part of Jupiter that we see is just its uppermost atmosphere, where the elements form gases and ices and behave in ways that we're pretty familiar with. But as you go deeper within Jupiter, the planet's gravity and the pressure of all the material above it pack those substances that we ordinarily think of as gases into less familiar forms. Both hydrogen and helium eventually behave as metallic fluids deep within Jupiter.
But the transition from gaseous behavior to metallic fluid behavior doesn't happen at the same depth within Jupiter for hydrogen and helium. Hydrogen becomes metallic at lower temperature and pressure than helium does. So there's an elevation within Jupiter, from about 10,000 to 13,000 kilometers below the cloud tops, where the hydrogen is a metallic fluid but the helium, although fluid, is not metallic. The metallic hydrogen and fluid helium are immiscible -- like oil and water, they don't mix. The helium is denser than the hydrogen, so the helium forms droplets that fall through the hydrogen to deeper levels, depleting the upper layers of Jupiter of helium.
So, that explains the depletion of helium, but what does that have to do with neon? At that crucial elevation, 10,000 to 13,000 kilometers below the cloud tops, neon and helium behave very similarly. Neither dissolves in the metallic hydrogen, but neon is perfectly happy to dissolve into the helium droplets. So when those helium droplets fall, they take the dissolved neon with them.
Eventually, the falling helium droplets reach a temperature and pressure where they, too, go through a phase transition and become metallic, at which point the helium and hydrogen happily mix.
The study was done to explain the lack of neon seen near Jupiter's cloud tops, but in the end, it provides evidence that there exists this layer within Jupiter where hydrogen and helium don't mix. All of this is, of course, theoretical; nobody's sent a probe to see helium rain falling through metallic hydrogen at a depth within Jupiter equivalent to the diameter of Earth!
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