Jupiter’s rain is made of helium

The planet Jupiter

Jupiter's interior is so strange that helium gas forms droplets and falls like rain.

On Earth, helium is a gas used to float balloons, as in the Hollywood movie “Up.”

In the interior of the planet Jupiter, however, conditions are so strange that, according to predictions by University of California, Berkeley, scientists, helium condenses into droplets and falls like rain.

Helium rain was earlier proposed to explain the excessive brightness of Saturn, a gas giant like Jupiter, but one-third the mass.

On Jupiter, however, UC Berkeley scientists claim that helium rain is the best way to explain the scarcity of neon in the outer layers of the planet, the Solar System’s largest.

Diagram showing layers in Jupiter's atmosphere

Jupiter's top layers are missing helium and neon. Below, helium drops condense and fall. Deeper, helium and neon mix with fluid hydrogen.

Neon dissolves in the helium raindrops and falls towards the deeper interior where it re-dissolves, depleting the upper layers of both elements, consistent with observations.

“Helium condenses initially as a mist in the upper layer, like a cloud, and as the droplets get larger, they fall toward the deeper interior,” said UC Berkeley post-doctoral fellow Hugh Wilson, co-author of a report appearing this week in the journal Physical Review Letters.

“Neon dissolves in the helium and falls with it. So our study links the observed missing neon in the atmosphere to another proposed process, helium rain.”

Wilson’s co-author, Burkhard Militzer, UC Berkeley assistant professor of earth and planetary science and of astronomy, notes that “rain”—the water droplets that fall on Earth—is an imperfect analogy to what happens in Jupiter’s atmosphere.

The helium droplets form about 10,000 to 13,000 kilometres below the tops of Jupiter’s hydrogen clouds, under pressures and temperatures so high that “you can’t tell if hydrogen and helium are a gas or a liquid,” he said. They’re all fluids, so the rain is really droplets of fluid helium mixed with neon falling through a fluid of metallic hydrogen.

The researchers’ prediction will help refine computer models of Jupiter’s interior and the interiors of other planets, according to Wilson.

Modelling of planetary interiors has become a hot research area since the discovery of hundreds of extrasolar planets living in extreme environments around other stars.

The study will also be relevant for NASA’s Juno mission to Jupiter, which is scheduled to be launched next year.

Adapted from information issued by UCB / NASA-JPL / Burkhard Militzer.

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