NASA's Juno spacecraft has uncovered a new form of lightning high in Jupiter's atmosphere, where ammonia hailstones also form, driving the gas giant's dynamic weather patterns.
Since Voyager 1 flew past the Jovian system in 1979, scientists have observed lightning in Jupiter's swirling clouds. Previously, these were thought to resemble Earth's, forming in water clouds existing as solid, liquid, and gas between 45 and 65 km below visible cloud tops.
Recently, Juno detected shallow lightning—occurring at much higher altitudes—about 25 km above the water clouds, where temperatures drop to around -88°C, far too cold for liquid water. This challenges our understanding and points to a unique mechanism.
Following Earth's rules, such lightning shouldn't form here. Instead, powerful storms loft ice crystals into the upper atmosphere, where they mix with ammonia vapor. This melts the ice, forming a cloud of mixed ammonia and water vapor.
“At these altitudes, ammonia acts as an antifreeze, lowering water ice's melting point to around -100°C and enabling liquid ammonia-water clouds,” explains Heidi Becker of the Juno mission. “Falling droplets collide with rising ice crystals, charging the clouds—a process absent on Earth.”
This also produces "mushballs": semi-molten hailstones with an ice crust around ammonia droplets. As they grow heavy, they fall into warmer layers, evaporate into vapor, and restart the cycle.
These Juno discoveries, ahead of the probe's departure from the Jovian system next year, also explain variations in atmospheric ammonia levels. "Mushballs" transport ammonia deeper into the planet.
“Linking shallow lightning and mushballs solves Jupiter's missing ammonia puzzle,” says Scott Bolton, co-author of the study. “It's not missing—it's transported covertly in water-ice shells, becoming detectable only after evaporation at depth.”
An animation of this shallow lightning is shown in the video below.
