NASA’s Juno spacecraft has spotted a strange new type of lightning on Jupiter, dancing high above its clouds, and it may be caused by ammonia “mushballs”. Lightning has been seen on the gas giant before, but never so high up in its atmosphere, which means here it can’t form how we thought it did.
Ever since Voyager first clocked lightning flashes on Jupiter in 1979, planetary scientists have thought lightning forms there just like on Earth, in water clouds during thunderstorms. Voyager saw this lightning as bright spots across Jupiter’s cloud tops that suggested it was indeed originating from deep within water clouds. However, the “shallow lightning” now seen by Juno on Jupiter’s dark side occurred way too high – where it’s far too cold – for water clouds.
Instead, researchers led by Heidi Becker, Juno's Radiation Monitoring Investigation lead at NASA's Jet Propulsion Laboratory, think this lightning is caused by liquid ammonia acting as an ”antifreeze”.
"Juno's close flybys of the cloud tops allowed us to see something surprising – smaller, shallower flashes – originating at much higher altitudes in Jupiter's atmosphere than previously assumed possible," said Becker.
In Nature, Becker and colleagues suggest that the violent thunderstorms Jupiter is famous for fling water-ice crystals high up into its atmosphere, where atmospheric ammonia melts the ice and forms a new water-ammonia solution that scientists have called “mushballs”. Where the lightning was seen, at around 25 kilometers (16 miles) higher than Jupiter's water clouds, temperatures average -88°C (-126°F), which is far too cold for liquid water to exist.
"At these altitudes, the ammonia acts like an antifreeze, lowering the melting point of water ice and allowing the formation of a cloud with ammonia-water liquid," said Becker. "In this new state, falling droplets of ammonia-water liquid can collide with the upgoing water-ice crystals and electrify the clouds. This was a big surprise, as ammonia-water clouds do not exist on Earth."
A second paper in the Journal of Geophysical Research: Planets has described how these mushballs – essentially slushy ammonia-rich hailstones – form. On Jupiter, at very low temperatures, ammonia vapor dissolves into water ice to make a liquid phase that is about one-third ammonia, two-thirds water ice. During Jupiter’s violent storms, hailstones form from this liquid in the same way they do on Earth; a slush like substance forms surrounded by a layer of ice, which gathers more ice as it gets bounced around by updrafts and downdrafts in the atmosphere.
It could be these mushballs colliding with ice particles at high altitudes while being flung about by Jupiter’s raging storms that build up an electrical charge and spark this shallow lightning.
If you fancy it, take a simulated trip inside one of Jupiter's exotic high-altitude electrical storms and see the shallow lightning, navigate the towering thunderstorms, and dodge ammonia-water rain for yourself.