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Before the Juno spacecraft arrived in Jupiter, astronomers had noted lower than expected ammonia levels in the upper atmosphere of the giant world. Following Juno’s arrival at Jupiter in July 2016, astronomers found higher ammonia concentrations near the center of the planet than those observed closer to the poles. These findings have remained a mystery – until now.
“While most latitudes show low abundance, the equatorial zone of Jupiter has high ammonia abundance that is almost uniform with depth. At the same time, the equatorial zone is special for its absence of lightning, which is moreover widespread almost everywhere else on the planet ”, report the investigators in the Journal of Geophysical Research: Planets.
Lightning was first seen on Jupiter, by the Voyager spacecraft in 1979. On Earth, thunderstorms form in regions where water exists in the three common states of matter – solid, liquid, and gas.
On Jupiter, as on our own planet, water moves through the atmosphere. About 50 kilometers (just over 30 miles) below the visible surface of Jupiter’s cloud tops, where temperatures hover around zero Celsius (32 Fahrenheit), storms are taking shape. When they become powerful enough, water ice crystals are lifted high into the atmosphere.
Once there, the water crystals interact with the ammonia, which acts as an antifreeze, melting the ice. This process forms thick, ammonia-rich mushrooms that fall deep into the Jovian atmosphere. As they warm deeper in the atmosphere, they melt, releasing ammonia and water, trapping material under the tops of Jupiter’s clouds.
“At these altitudes, ammonia acts as an antifreeze, lowering the melting point of water ice and allowing a cloud to form with ammonia liquid. In this new state, falling droplets of ammonia-water liquid can collide with rising ice-water crystals and electrify clouds. This was a big surprise, because ammonia-water clouds do not exist on Earth, ”says Heidi Becker, head of the radiation monitoring investigation at Juno at NASA’s Jet Propulsion Laboratory.
This process dries up Jupiter’s atmosphere, resulting in relatively low ammonia levels in Jupiter’s upper cloud layers.
“[A]mmonia vapor can dissolve in water ice to form a low temperature liquid phase containing about one third ammonia and two thirds water. We believe that as a result of the process that creates hailstorms on Earth, this liquid phase enhances the growth of hailstorms we call mushballs, ”the researchers describe in the Journal of Geophysical Research.
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