Violent thunderstorms in Jupiter’s atmosphere may form “mushballs” – ammonia-rich hail –that play a key role in the planet’s atmospheric dynamics.
This theory, developed using new data from NASA’s Juno mission, sheds light on some puzzling aspects of the meteorology of Jupiter, researchers say, and has implications for how giant planet atmospheres work in general.
The work was led by France’s Centre National de la Recherche Scientifique (CNRS) and the findings are described in three papers in the journals Nature and JGR Planets.
As on Earth, Jupiter’s water is moved around by thunderstorms. These are thought to form within the planet’s deep atmosphere, around 50 kilometres below the visible clouds, where the temperature is close to zero degrees Celsius.
When these storms are powerful enough, they carry crystals of water-ice into the upper atmosphere.
In the first paper, researchers from the US and France suggest that when these crystals interact with gaseous ammonia, the ammonia acts as an anti-freeze, changing the ice to a water-ammonia liquid, leading to the creation of the exotic mushballs.
These then fall deeper into the atmosphere until they reach a point where they evaporate. This mechanism drags ammonia and water down to deep levels in the planet’s atmosphere.
Measurements from Juno’s microwave radiometer show that while ammonia is abundant near Jupiter’s equator, it is highly variable and generally depleted elsewhere to very deep pressures. To explain this, the researchers developed an atmospheric mixing model that is described in the second paper.
They show that the presence of thunderstorms and the formation of water-ammonia mushballs dry out the deep atmosphere of its ammonia and account for the variations observed by Juno as a function of latitude.
“A simple model satisfying mass and energy balance accounts for the main features of Juno’s microwave radiometer observations and successfully reproduces the inverse correlation seen between ammonia abundance and the lightning rate as function of latitude,” the researchers write.
“We predict that in regions where ammonia is depleted, water should also be depleted to great depths.”
In the third paper, the authors report observations of Jovian lightning flashes by one of Juno’s cameras. The small flashes appear as bright spots on the cloud tops, with sizes proportional to their depth in the atmosphere.
These flashes come from regions where temperatures are below minus 66 degrees Celsius and where water alone cannot be found in the liquid state – yet the presence of a liquid is thought to be crucial to the lightning-generation process.
Juno’s detection of “shallow lightning” storms at the altitudes where liquid ammonia-water can be created is observational support that the mushball mechanism may indeed be at work in Jupiter’s atmosphere.
In the image above, the central part covers an area of 3200 x 3800 kilometres. White clouds of ammonia can be seen, rotating anticlockwise. Clouds rising as much as 15 kilometres above the others (based on the shadow they cast) are visible in several places, especially in the upper central part of the cyclone.
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