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The Juno spacecraft has spotted a new kind of lightning on Jupiter unlike anything we’ve seen before, and it may be caused by strange slushy balls of ice and ammonia.

Planetary scientists have long thought that Jupiter’s lightning happens in much the same way as Earth’s: through liquid water and ice interacting within clouds and building up electric charge. That was supported by the fact that we’d only seen lightning coming from a layer of water clouds deep beneath the cloud tops that we see as Jupiter’s “surface”.

But now, NASA’s Juno spacecraft has spotted lightning flashes coming from an area much higher in the planet’s atmosphere, where it is far too cold for liquid water. “It’s very different from anything that happens on Earth, and it was a big surprise – completely different from the assumption of where the lightning happens on Jupiter and how it works,” says Heidi Becker at NASA’s Jet Propulsion Laboratory in California, a member of the Juno team.

Instead of pure liquid water, the researchers found that this strange lightning could be caused by liquid ammonia acting as an antifreeze. That would create what they have named “mushballs” of a slushy ammonia-water mixture surrounded by water ice. The strange, high-altitude lightning could occur when these mushballs collide with ice particles and build up electric charge.

“It would be kind of like a dirty snowball with a crispy crunchy crust and a chewy centre,” says Becker. “As it’s thrown around in updrafts and downdrafts it would get more ice around it, kind of like rolling a snowball around to make it bigger.”

These slushy snowballs could then drop deep into Jupiter’s interior, solving another mystery – the question of why the interior doesn’t appear to have as much ammonia gas as we expected. The ammonia may be hiding in mushballs, bringing the thunder.