Helium Rain on Jupiter Explains Lack of Neon in Atmosphere

On Earth, helium is a gas used to float balloons, as in the movie "Up." In the interior of 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. 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."

Militzer and Wilson are among the modelers, using "density functional theory" to predict the properties of Jupiter's interior, specifically what happens to the dominant constituents -- hydrogen and helium -- as temperatures and pressures increase toward the center of the planet. These conditions are yet too extreme to be reproduced in the laboratory. In 2008, Militzer's computer simulations led to the conclusion that Jupiter's rocky core is surrounded by a thick layer of methane, water and ammonia ices that make it twice as large as earlier predictions.

Their simulations showed that the only way neon could be removed from the upper atmosphere is to have it fall out with helium, since neon and helium mix easily, like alcohol and water. Militzer and Wilson's calculations suggest that at about 10,000 to 13,000 kilometers into the planet, where the temperature about 5,000 degrees Celsius and the pressure is 1 to 2 million times the atmospheric pressure on Earth, hydrogen turns into a conductive metal. Helium, not yet a metal, does not mix with metallic hydrogen, so it forms drops, like drops of oil in water.

This provided an explanation for the removal of neon from the upper atmosphere.

"As the helium and neon fall deeper into the planet, the remaining hydrogen-rich envelope is slowly depleted of both neon and helium," Militzer said. "The measured concentrations of both elements agree quantitatively with our calculations."

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