Before the arrival of the Juno spacecraft at Jupiter, astronomers had noticed lower ammonia levels than expected in the upper atmosphere of the giant world. After Juno’s arrival at Jupiter in July 2016, astronomers found higher concentrations of ammonia at the center of the planet than were seen closer to the pulse. These findings have remained a mystery – until now.
‘While most latitudes have a low abundance, Jupiter’s Equatorial Zone has an abundance of ammonia that is high and almost uniform in depth. At the same time, the equatorial zone is peculiar to its absence of lightning, which otherwise occurs everywhere on the planet, ”report researchers in the Journal of Geophysical Research: Planets.
Lightning strikes were first seen on Jupiter by the spacecraft Voyager in 1979. On Earth, thunderstorms occur in regions where water exists in all three common states of matter – solid, liquid and gas.
On Jupiter, as on our own planet, water moves through the atmosphere. Roughly 50 kilometers (just over 30 miles) below the visible surface of Jupiter’s cloud tops, where temperatures above zero Celsius (32 Fahrenheit) storms form. When they become powerful enough, crystals of water ice are lifted high into the atmosphere.
Once there, the water crystals interact with ammonia, which acts as an antifreeze, and the ice melts. This process forms thick, ammonia-rich mushrooms that fall deep into the Jovian atmosphere. As they heat deeper into the atmosphere, they melt, releasing ammonia and water, trapping the materials beneath Jupiter’s cloud heads.
“At these altitudes, the ammonia acts as an antifreeze, lowering the melting point of water ice and allowing the formation of a cloud of ammonia-water liquid. In this new state, falling drops of ammonia-water liquid can collide with the rising crystals of water-ice and electrify the clouds. This was a big surprise, because ammonia water clouds do not exist on Earth, ”explains Heidi Becker, Juno’s radiation monitoring research leader at NASA’s Jet Propulsion Laboratory.
This process dries out Jupiter’s atmosphere, resulting in relatively low levels of ammonia 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 estimate that this liquid phase, following the process of creating hailstorms on Earth, increases the growth of hail-like particles we call dishes, ”describe researchers in the Journal of Geophysical Research.
