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.I'm not sure that "atmosphere" and "rain" are meaningful words under these conditions, either. Let's face it: when you're dealing with a depth of 1 earth diameter, and a gravitational field 2.5 times that of earth, there's just not a whole lot of words that are going to be meaningful to frail earth protoplasm such as that of which we're composed.
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.
This Week's Geo-Quiz: Other Planets
3 months ago