Ever since NASA’s Voyager 1 spacecraft flew past Jupiter in March, 1979, scientists have wondered about the origin of Jupiter’s lightning, which had been theorized for centuries. Now, new results from NASA’s Juno mission, which has been orbiting Jupiter since July 4, 2016, suggest our solar system’s largest planet–an immense spinning colorful sphere of methane and ammonia so large it could easily swallow all the other planets– is home to what’s called “shallow lightning” and slushy hailstones Juno’s science team calls “mushballs” that capture ammonia and water in the upper atmosphere and carry them into the dark depths of Jupiter’s atmosphere.
Lightning Bolts Act Like Radio Transmitters
“No matter what planet you’re on, lightning bolts act like radio transmitters—sending out radio waves when they flash across a sky,” said Shannon Brown of NASA’s Jet Propulsion Laboratory a Juno scientist in 2018. “Jupiter lightning distribution is inside out relative to Earth. There is a lot of activity near Jupiter’s poles but none near the equator. You can ask anybody who lives in the tropics—this doesn’t hold true for our planet.”
Since NASA’s Voyager mission first saw Jovian lightning flashes (image below), it was 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 28 to 40 miles (45 to 65 kilometers) below the visible clouds, with temperatures that hover around 32 degrees Fahrenheit (0 degrees Celsius). 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.
An unexpected form of electrical discharge, shallow lightning originates from clouds containing an ammonia-water solution, whereas lightning on Earth originates from water clouds.
The image at the top of the page obtained by NASA’s Juno mission to depict high-altitude electrical storms on Jupiter. Juno’s sensitive Stellar Reference Unit camera detected unusual lightning flashes on Jupiter’s dark side during the spacecraft’s close flybys of the planet. ( NASA/JPL-Caltech/SwRI/MSSS/Gerald Eichstädt)
Juno’s Unique Orbit
“Our unique orbit allows our spacecraft to fly closer to Jupiter than any other spacecraft in history, so the signal strength of what the planet is radiating out is a thousand times stronger,” says Scott Bolton, principal investigator of Juno at the Southwest Research Institute. “Also, our microwave and plasma wave instruments are state-of-the-art, allowing us to pick out even weak lightning signals from the cacophony of radio emissions from Jupiter.”
Pop Up Clouds
The detailed, color-enhanced JunoCam image below reveals a complex topography in the cloud tops of Jupiter’s northern mid-latitude region. Small, bright “pop-up” clouds in the center of the image rise above the surrounding features, standing out at the tops and edges of the swirling patterns; the darker areas nearby reveal greater depth.
“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 Heidi Becker, Juno’s Radiation Monitoring Investigation lead at NASA’s Jet Propulsion Laboratory in Southern California and the lead author of the Nature paper.
Powerful Thunderstorms Fling Water-ice Crystals Up into the Atmosphere
Becker and her team suggest that Jupiter’s powerful thunderstorms fling water-ice crystals high up into the planet’s atmosphere, over 16 miles (25 kilometers) above Jupiter’s water clouds, where they encounter atmospheric ammonia vapor that melts the ice, forming a new ammonia-water solution. At such lofty altitude, temperatures are below minus 126 degrees Fahrenheit (minus 88 degrees Celsius) – too cold for pure liquid water to exist.
This animation takes you on a simulated journey into Jupiter’s exotic high-altitude electrical storms. Get an up-close view of Mission Juno’s newly discovered “shallow lighting” flashes and dive into the violent atmospheric jet of the Nautilus cloud. (NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill)
“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.”
Another Puzzle –The Missing Ammonia
The shallow lightning factors into another puzzle about the inner workings of Jupiter’s atmosphere: Juno’s Microwave Radiometer instrument discovered that ammonia was depleted – which is to say, missing – from most of Jupiter’s atmosphere. Even more puzzling was that the amount of ammonia changes as one moves within Jupiter’s atmosphere.
“Previously, scientists realized there were small pockets of missing ammonia, but no one realized how deep these pockets went or that they covered most of Jupiter,”said Bolton. “We were struggling to explain the ammonia depletion with ammonia-water rain alone, but the rain couldn’t go deep enough to match the observations. I realized a solid, like a hailstone, might go deeper and take up more ammonia. When Heidi discovered shallow lightning, we realized we had evidence that ammonia mixes with water high in the atmosphere, and thus the lightning was a key piece of the puzzle.”
A second paper, released yesterday in the Journal of Geophysical Research: Planets,envisions the strange brew of 2/3 water and 1/3 ammonia gas that becomes the seed for Jovian hailstones, the mushballs. Consisting of layers of water-ammonia slush and ice covered by a thicker water-ice crust, mushballs are generated in a similar manner as hail is on Earth – by growing larger as they move up and down through the atmosphere.
“Eventually, the mushballs get so big, even the updrafts can’t hold them, and they fall deeper into the atmosphere, encountering even warmer temperatures, where they eventually evaporate completely,” said Tristan Guillot, a Juno co-investigator from the Université Côte d’Azur in Nice, France, and lead author of the second paper. “Their action drags ammonia and water down to deep levels in the planet’s atmosphere. That explains why we don’t see much of it in these places with Juno’s Microwave Radiometer.”
“Combining these two results was critical to solving the mystery of Jupiter’s missing ammonia,” said Bolton. “As it turned out, the ammonia isn’t actually missing; it is just transported down while in disguise, having cloaked itself by mixing with water. The solution is very simple and elegant with this theory: When the water and ammonia are in a liquid state, they are invisible to us until they reach a depth where they evaporate – and that is quite deep.”
Understanding the meteorology of Jupiter enables us to develop theories of atmospheric dynamics for all the planets in our solar system as well as for the exoplanets being discovered outside our solar system. Comparing how violent storms and atmospheric physics work across the solar system allows planetary scientists to test theories under different conditions.
The solar-powered Jupiter explorer launched nine years ago today, on Aug. 5, 2011. And last month marked the fourth anniversary of its arrival at Jupiter. Since entering the gas giant’s orbit, Juno has performed 27 science flybys and logged over 300 million miles (483 million kilometers).
Image at top of page: depicts the evolutionary process of “shallow lightning” and “mushballs” on Jupiter. NASA/JPL-Caltech/SwRI/CNRS