Superficial lightnings and remarkable hailstones discovered on Jupiter

The hailstones are very different from Earth, for example they consist largely of ammonia.

This is evident from new observations by Juno. The probe has been circling Jupiter since 2016, where it studies the turbulent atmosphere of the gas giant. Everything is going on in that atmosphere, as it turns out once again. This not only produces very remarkable hailstones, but also lightning flashes high up in the atmosphere.

Lightning bolts
It has been known for some time that lightning strikes on Jupiter. In 1979, the space probe Voyager already saw flashes of light on the gas giant. They were believed to originate in a similar way to the lightning bolts we know here on Earth. According to that assumption, the flashes occurred in thunderstorms that contained liquid water, water ice, and water vapor. It means very concretely that they would originate about 45 to 65 kilometers below the cloud tops visible to us, where the temperature is around 0 degrees Celsius.

But observations from space probe Juno now reveal that there is also a completely different type of lightning on the gas giant: Juno has spotted superficial lightning bolts. “Shaving flights along the tops of the clouds enabled us to observe something very surprising,” says researcher Heidi Becker. “Smaller, more superficial flashes of light, which were created at a much higher altitude in Jupiter’s atmosphere than previously thought possible.”

Originate
These superficial lightnings arise from the grace of deeper thunderstorms, Becker and colleagues suspect. Those deeper, powerful thunderstorms would whip up water ice crystals, which then rise higher into the atmosphere – about 15 miles above Jupiter’s water clouds – where they encounter ammonia vapor that melts the ice, creating a mixture of ammonia and water. “At this altitude (where the temperature is around -88 degrees Celsius, ed.), The ammonia acts as antifreeze and lowers the melting point of water ice,” Becker explains. “This can create clouds consisting of liquid water and ammonia.” Drops falling from these clouds – and thus also consisting of ammonia and water – collide with upward-traveling ice crystals. “And so electrify the clouds. This was a big surprise, as clouds of water and ammonia do not exist on Earth, ”said Becker.

Hailstones
This theory not only explains the origin of the superficial lightning bolts, but can also help explain why surprisingly little ammonia can be found in some parts of Jupiter’s atmosphere. “When Heidi discovered superficial lightning bolts, we realized we now had evidence that ammonia mixed high with water in the atmosphere,” said researcher Scott Bolton. Drops of that mixture drop and collide with ice crystals, causing not only flashes of lightning, but also developing into exotic hailstones, the core of which consists of that mixture of liquid water and ammonia and around which an ice crust is formed. The result is a mushy ammonia-rich hailstone that the researchers refer to as a ‘mushball’. “Ultimately, the mushballs grow so big that (…) they fall deeper into the atmosphere, where they hit higher temperatures and eventually evaporate completely,” says researcher Tristan Guillot. For example, the hailstones transport ammonia from the upper layers of the atmosphere. “Combining these two results was critical to solving the mystery of Jupiter’s missing ammonia,” said Bolton. “As it turns out, the ammonia is not really lost, but because it mixes with water it is transported in disguise, as it were. … When the water and ammonia are liquid, they are invisible to us.”