Jupiter Weather: Cloudy With a Chance of 'Mushballs'

The weather forecast for Jupiter calls for clouds with a chance of lightning and "mushballs"—a slushy mix of ammonia and water encased in a hard water ice shell that rain down like hail here on Earth.

This is the conclusion of a study by researchers from the University of California, Berkeley who, setting out to show that the "exotic" weather phenomena couldn't exist, ended up proving the exact opposite.

Planetary scientist Chris Moeckel and astronomy professor Imke de Pater said that they initially believed that mushballs would require far too specific atmospheric conditions to form.

"Imke and I both were like, 'There's no way in the world this is true,'" said Moeckel in a statement. "So many things have to come together to explain this. I basically spent three years trying to prove this wrong. And I couldn't prove it wrong."

In fact, the team say, it seems like mushball hailstorms may be a common feature of gaseous planets—including our solar system's Saturn, Uranus and Neptune.

The idea of mushballs was first proposed back in 2020 to explain why—in observations taken by both radio telescopes and NASA's Juno mission—Jupiter's upper atmosphere appeared to be so poorly mixed.

Jupiter has an atmosphere that is radically different to what we are used to here on Earth. While we have air that is mainly nitrogen and oxygen, Jupiter's is primarily composed of hydrogen and helium gas, with trace amounts of molecules like ammonia and water.

Both ammonia gas and water vapour can rise, freeze into droplets and rain down—but at what point do they stop falling on a gas giant?

"On Earth, you have a surface—and rain will eventually hit this surface," said Moeckel.

"The question is: what happens if you take the surface away? How far do the raindrops fall into the planet? This is what we have on the giant planets."

Processes like rain and storms are believed to be the main vertical mixers of planetary atmospheres. For decades, it had been assumed that Jupiter's atmosphere was well-mixed—a belief that then guided inferences about the gas giant's interior makeup.

Instead, however, Jupiter's upper reaches had been creating a misleading impression.

"The turbulent cloud tops would lead you to believe that the atmosphere is well mixed," said Moeckel, comparing the planet to a pot of boiling water. "If you look at the top, you see it boiling and you would assume that the whole pot is boiling.

"But these findings show that even though the top looks like it's boiling, below is a layer that is really steady and sluggish."

What radio observations and Juno revealed about Jupiter is that its atmosphere is depleted in ammonia to depths of around 93 miles. This led to the idea that ammonia could be falling out the upper atmosphere as hail—although such would have to be massive to fall so far.

This led planetary scientist and astronomer Tristan Guillot of the Côte d'Azur Observatory, France, to propose the mushball hypothesis. Under such, strong updrafts during storms in Jupiter's atmosphere would lift tiny particles of ice tens of miles above the clouds, where they mix with ammonia vapour.

The latter works like antifreeze—melting the ice to form a slushy liquid. These mixtures continue to bob up and down in the atmosphere, growing in size until (at about the size of a softball) their mass overcomes the updrafts completely.

At this point, they plummet more than 60 miles deep into the atmosphere—well below the storms that created them—taking both ammonia and water down in the process.

"And so you have, essentially, this weird system that gets triggered far below the cloud deck, goes all the way to the top of the atmosphere and then sinks deep into the planet," said Moeckel.

In this way, the storms that create mushballs help to unmix the atmosphere, explaining how the chemical composition of the cloud tops can differ from deeper in the planet.

As Moeckel and de Pater noted, mushballs do require very specific conditions to form; the storm system causing them need to have updrafts blowing at around 224 miles per hour, the slushy particles must quickly mix with ammonia, and they must grow large enough to survive falling back down through the atmosphere.

In a related study that is undergoing peer review, but published on the arXiv preprint repository, the researchers present the first ever three-dimensional visualization of Jupiter's upper atmosphere—showing that while most of the planet's weather systems are shallow, reaching only depths of around 12 miles—some do emerge from much deeper in the troposphere.

These include hurricane-like vortices, hotspots that couple to ammonia-rich plumes that weave around Jupiter, and the large storms that produce both mushballs and lightning.

As to what finally convinced the team that mushballs do indeed exist—that came in the form of unique signatures in the radio data collected by Juno.

"There was a small spot under the cloud that either looked like cooling—that is, melting ice—or an ammonia enhancement—that is, the release of ammonia," said Moeckel.

"It was the fact that either explanation was only possible with mushballs that eventually convinced me."

He concluded, "This process apparently is true, against my best desire to find a simpler answer."

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References

Moeckel, C., Ge, H., & de Pater, I. (2025). Tempests in the troposphere: Mapping the impact of giant storms on Jupiter's deep atmosphere. Science Advances, 11(13). https://doi.org/10.1126/sciadv.ado9779

Moeckel, C., Pater, I. de, Sault, B., & Butler, B. (2025). The Tropical Atmosphere of Jupiter—Shallow Weather, Deep Plumes, and Vortices (No. arXiv:2504.09943). arXiv. https://doi.org/10.48550/arXiv.2504.09943