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Exotic rain drenches Jupiter
Posted: 23 March

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Next time you visit Jupiter remember to take an umbrella with you. Researchers at the University of California, Berkeley have discovered that drops of helium rain, laced with neon, could be falling from the clouds.

Somewhere deep inside Jupiter it is raining helium. Image: NASA/JPL/University of Arizona.

Conditions on the gas giant planet are vastly different to on Earth. Where the droplets are forming, 10,000–13,000 kilometres below the upper cloud deck of hydrogen gas, the temperature rises to 5,000 degrees Celsius, and the pressure reaches two million times Earth’s surface pressure. Such an environment sees hydrogen become an electrically conducting liquid, called metallic hydrogen. The helium rain, which begins in misty clouds of fine drops that grow larger with depth, fall through this ocean of liquid hydrogen the same way oil refuses to mix with water.

A slice of the interior of Jupiter. Image: Burkhard Militzer.

Neon gas comes into the story because in 1995, when NASA’s Galileo spacecraft launched a probe into Jupiter’s atmosphere, all elements were found to be slightly enriched compared to the Sun, except for helium and neon. On the Sun neon atoms are one part in 600 by mass, but on Jupiter they are only one-tenth as abundant, i.e. one part in 6,000. Since Jupiter formed from the same gases that formed the Sun, there should be a closer match that that. Instead, Dr Hugh Wilson and Assistant Professor Burkhard Militzer of the University of California, Berkeley, propose in a paper in this week’s edition of Physical Review Letters that Jupiter’s neon is dissolving in the helium raindrops.

“As the helium and neon fall deeper into the planet, the remaining hydrogen-rich envelope is slowly depleted of both neon and helium,” says Militzer. The neon the settles into the mighty core of Jupiter, which supercomputer simulations (the only way of modelling the interior of Jupiter accurately, for no physical laboratory on Earth can replicate those extreme conditions) suggest is made of a solid rocky core surrounded by layers of water, ammonia and methane.

Jupiter isn’t the only gas giant to have showers of helium. Saturn, too, has been proposed to have helium rain; as the helium drops condense, they release latent heat that explains why the ringed planet is hotter than it should be.