Jupiter Forecast: Cloudy, with Helium Rain

THE GIST:

• New research shows it is raining droplets of helium on Jupiter.
• The phenomenon may also be occurring on Jupiter-class worlds beyond the solar system.
• This finding may help explain why Jupiter is so short of neon.

It's raining helium inside Jupiter, say scientists who believe that's the best explanation for why the giant planet is so lacking in neon.

When the planets formed out of the original solar nebula, they should have started with an identical mix of ingredients. But a NASA probe that descended through Jupiter's atmosphere in 1995 showed a significant dearth of the trace element neon.

"It was crying out for an explanation," University of California, Berkeley astronomer Burkhard Militzer told Discovery News.

New computer models show the most likely scenario is that neon is being removed from the upper atmosphere inside droplets of helium, says Militzer.

It's not "rain" in the sense of how rain falls on Earth. Pressures and temperatures on Jupiter are so high that the droplets of liquid helium are falling through a fluid of metallic hydrogen.

David Stevenson of the California Institute of Technology predicted Jupiter's helium rain even before the atmospheric probe released by the Galileo spacecraft returned the data on Jupiter's atmosphere.

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"Because neon is insoluble in metals, (hydrogen is metallic on Jupiter) it occurred to me that it might be good to check if this was happening," Stevenson told Discovery News. "I'm delighted that they did this work."

Additional insights about Jupiter's atmosphere should come from a new probe, called Juno, which is scheduled to be launched next year for a five-year trek to Jupiter.

Understanding the dynamics of Jupiter also is of interest to scientists studying planets beyond the solar system, most of which are Jupiter-sized or larger.

"This process could also be going on in the exoplanets," Militzer said.

Based on the computer models, the helium droplets should be forming about 6,000 to 8,000 miles below the tops of Jupiter's hydrogen clouds.

"It's happening in a region much more dense than Earth's atmosphere. It's like the density of water," Stevenson said.

Unlike rain, fog and other weather systems on Earth, helium droplets on Jupiter don't cycle through the atmosphere, but instead are being deposited deep into the planet.

Militzer's research appears in this week's Physical Review Letters.