Uranus and Neptune may be hiding slushy hailstones called mushballs deep inside them. These huge clumps of water and ammonia may explain why there is so little ammonia higher up in the ice giants鈥� atmospheres.
Compared with Jupiter and Saturn, the other giant planets in our solar system, Uranus and Neptune seem to contain surprisingly little ammonia 鈥� based on measurements of other compounds, all of the giant planets in our solar system ought to have about the same amount of it. Either Uranus and Neptune formed in some sort of unusual process that included less ammonia in the first place, or something is hiding it.
Recent observations from NASA鈥檚 Juno spacecraft have shown that Jupiter鈥檚 ammonia is buried much deeper inside the planet than expected. at the Observatory of Nice in France calculated that the same may be true on Uranus and Neptune, thanks to mushballs.
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鈥淭hese particles that can reach a kilogram and can zoom through Jupiter鈥檚 clouds are able to transport the ammonia and the water down into the deep reaches of the atmosphere,鈥� said Guillot when he presented the work at a virtual meeting of the Europlanet Science Congress on 16 September. 鈥淭hermodynamics tell us that mushball formation should be more efficient in Uranus and Neptune.鈥�
Both planets are colder than Jupiter, so clumps of liquid ammonia and water are more likely to form an ice shell and become slushy, turning into mushballs. They are then expected to fall like hail deep into the atmosphere, where they are hidden beneath a thick layer of clouds.
Because the mushballs are expected to end up so deep 鈥� hundreds or even thousands of kilometres beneath the clouds 鈥� we cannot detect them from the ground, said Guillot.
鈥淲e really need to go there, not only for the mushballs,鈥� said Guillot. Uranus and Neptune are an especially critical link to help us understand the diversity of atmospheres that can arise on ice giants and giant exoplanets beyond our solar system, he says.
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