Juno Discovers ‘Shallow Lightning’ on Jupiter
An unexpected form of electrical discharge, ‘shallow lightning’ originates from Jovian clouds containing an ammonia-water solution, according to a new analysis of data collected by the Stellar Reference Unit instrument onboard NASA’s Juno spacecraft.
Since NASA’s Voyager mission first saw Jovian lightning flashes in 1979, it has been thought that the planet’s lightning is similar to Earth’s, occurring only in thunderstorms where water exists in all its phases — ice, liquid, and gas.
At Jupiter this would place the storms around 45 to 65 km (28-40 miles) below the visible clouds, with temperatures that hover around 0 degrees Celsius (32 degrees Fahrenheit).
Voyager, and all other missions to the gas giant prior to Juno, saw lightning as bright spots on Jupiter’s cloud tops, suggesting that the flashes originated in deep water clouds.
But lightning flashes observed on Jupiter’s dark side by Juno’s Stellar Reference Unit tell a different story.
“On the night side of Jupiter, you see fairly frequent flashes — as if you were above an active thunderstorm on Earth,” said Cornell University’s Professor Jonathan Lunine, co-author on the study published in the journal Nature.
“You get these tall columns and anvils of clouds, and the lightning is going continuously. We can get some pretty substantial lightning here on Earth, and the same is true for Jupiter.”
“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 Dr. Heidi Becker, Juno’s Radiation Monitoring Investigation lead at NASA’s Jet Propulsion Laboratory and lead author of the study.
Ammonia is the key. While there is water and other chemical elements such as molecular hydrogen and helium in Jupiter’s clouds, ammonia is the antifreeze that keeps water in those upper atmospheric clouds from freezing entirely.
The collision of the falling droplets of mixed ammonia and water with suspended water-ice particles constitutes a way to separate charge and produce cloud electrification — resulting in lightning storms in the upper atmosphere.
“The shallow lightning really points to the role of ammonia, and the team’s models are starting to confirm this. This would be unlike any process that occurs on Earth,” Professor Lunine said.
“Giant planets in general are a fundamentally different kind of world from Earth and other terrestrial planets,” said Cornell University researcher Dr. Yury Aglyamov, co-author of the study.
“There are hydrogen seas transitioning gradually into skies stacked with cloud decks, weather systems the size of the Earth and who-knows-what in the interior.”
“The discovery of shallow lightning on Jupiter shifts our understanding of the planet.”
“Shallow lightning hadn’t really been expected and indicates that there’s an unexpected process causing it.”
“It’s one more way in which Juno’s observations show a much more complex atmosphere of Jupiter than had been predicted. We know enough now to ask the right questions about processes going on there, but as Juno shows, we’re in a stage where every answer also tends to multiply the questions.”
H.N. Becker et al. 2020. Small lightning flashes from shallow electrical storms on Jupiter. Nature 584, 55-58; doi: 10.1038/s41586-020-2532-1