May Form Ammonia-Water Hailstones
A new study suggests that during Jupiter’s violent storms, hailstones form from a cooled mixture of water and ammonia gas, similar to the process in Earth’s storms where hail forms in the presence of supercooled liquid water; growth of these Jovian hailstones, dubbed ‘mushballs,’ creates a slush-like substance surrounded by a layer of ice, and mushballs fall, evaporate, and continue sinking further in the gas giant’s deep atmosphere.
NASA’s Juno mission revealed that Jupiter’s atmosphere is much more complex and intriguing than previously anticipated.
Most of the planet’s atmosphere was shown to be depleted — which is to say, missing — in ammonia.
While ammonia was expected to be well mixed, large scale variability of ammonia was detected at least 100 km (62 miles) below the cloud level where condensation occurs.
Dr. Scott Bolton, Juno’s principal investigator at the Southwest Research Institute, Dr. Tristan Guillot, a Juno co-investigator from the Université Côte d’Azur, and their colleagues propose a new mechanism to explain this depletion and variability.
“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,” Dr. Bolton said.
“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.”
“Jupiter’s 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,” Dr. Guillot added.
“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,” Dr. Bolton said.
“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.”
The study was published in the Journal of Geophysical Research: Planets.
Tristan Guillot et al. Storms and the Depletion of Ammonia in Jupiter: I. Microphysics of ‘Mushballs.’ Journal of Geophysical Research: Planets, published online August 5, 2020; doi: 10.1029/2020JE006403