Recent findings from NASA’s Juno mission, in orbit around Jupiter since July 4, 2016, may have solved an ongoing mystery about the composition of the giant planet’s upper atmosphere; namely, the case of the missing ammonia. (Jupiter is composed mostly of hydrogen and helium but also contains trace amounts of ammonia, methane, and water vapor.) North and south of Jupiter’s equator, where the effects of the planet’s rapid 10-hour rotation create less mixing within the atmosphere, Juno had previously detected a curious lack of ammonia concentrations. As it turns out the ammonia is there, but it’s being trapped inside partially-frozen, hailstone-like structures nicknamed “mushballs”— and super-high-altitude lightning flashes are the telltale signs of their existence.
“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.”
— Scott Bolton, Juno’s principal investigator
News from NASA on Aug. 5, 2020:
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 28 to 40 miles (45 to 65 kilometers) below the visible clouds, with temperatures that hover around 32 degrees Fahrenheit (0 degrees Celsius, the temperature at which water freezes). 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.
“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.
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.
“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.”
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, known as 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.
“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 Scott Bolton, Juno’s principal investigator at the Southwest Research Institute in San Antonio. “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.”
“Combining these two results was critical to solving the mystery of Jupiter’s missing ammonia, 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.”
— Scott Bolton, Juno PI