Imagine a Slushee™ composed of ammonia and water encased in a tough shell of water ice. Now picture these ice-encrusted slushballs, dubbed “mushballs,” raining down like hailstones during a thunderstorm, illuminated by intense flashes of lightning.
Planetary scientists on the University of California, Berkeley, now say that hailstorms of mushballs accompanied by fierce lightning actually exist on Jupiter. Actually, mushball hailstorms may occur on all gaseous planets within the galaxy, including our solar system’s other giant planets, Saturn, Uranus and Neptune.
The thought of mushballs was initially put forth in 2020 to clarify nonuniformities within the distribution of ammonia gas in Jupiter’s upper atmosphere that were detected each by NASA’s Juno mission and by radio telescopes on Earth.
On the time, UC Berkeley graduate student Chris Moeckel and his adviser, Imke de Pater, professor emerita of astronomy and of earth and planetary science, thought the speculation too elaborate to be real, requiring highly specific atmospheric conditions.
“Imke and I each were like, ‘There is not any way on the planet that is true,'” said Moeckel, who received his UC Berkeley Ph.D. last yr and is now a researcher at UC Berkeley’s Space Sciences Laboratory. “So many things have to return together to really explain this, it seems so exotic. I principally spent three years attempting to prove this fallacious. And I could not prove it fallacious.”
The confirmation, reported March 28 within the journal Science Advances, emerged along with the primary 3D visualization of Jupiter’s upper atmosphere, which Moeckel and de Pater recently created and describe in a paper that’s now undergoing peer review and is posted on the preprint server arXiv.
The 3D picture of Jupiter’s troposphere shows that nearly all of the weather systems on Jupiter are shallow, reaching only 10 to twenty kilometers below the visible cloud deck or “surface” of the planet, which has a radius of 70,000 km. Many of the colourful, swirling patterns within the bands that encircle the planet are shallow.
Some weather, nevertheless, emerges much deeper within the troposphere, redistributing ammonia and water and essentially unmixing what was long considered a uniform atmosphere. The three sorts of weather events responsible are hurricane-like vortices, hotspots coupled to ammonia-rich plumes that wrap across the planet in a wave-like structure, and huge storms that generate mushballs and lightning.
“Each time you have a look at Jupiter, it’s mostly just surface level,” Moeckel said. “It’s shallow, but a number of things — vortices and these big storms — can punch through.”
“Juno really shows that ammonia is depleted in any respect latitudes all the way down to about 150 kilometers, which is admittedly odd,” said de Pater, who discovered 10 years ago that ammonia was depleted all the way down to about 50 km. “That is what Chris is trying to clarify together with his storm systems going much deeper than we expected.”
Inferring planet composition from observations of clouds
Gas giants like Jupiter and Saturn and ice giants like Neptune and Uranus are a significant focus of current space missions and huge telescopes, including the James Webb Space Telescope, partially because they can assist us understand the formation history of our solar system and ground truth observations of distant exoplanets, a lot of that are large and gaseous. Since astronomers can see only the upper atmospheres of faraway exoplanets, knowing easy methods to interpret chemical signatures in these observations can assist scientists infer details of exoplanet interiors, even for Earth-like planets.
“We’re principally showing that the highest of the atmosphere is definitely a reasonably bad representative of what’s contained in the planet,” Moeckel said.
That is because storms like people who create mushballs unmix the atmosphere in order that the chemical composition of the cloud tops doesn’t necessarily reflect the composition deeper within the atmosphere. Jupiter is unlikely to be unique.
“You may just extend that to Uranus, Neptune — actually to exoplanets as well,” de Pater said.
The atmosphere on Jupiter is radically different from that on Earth. It’s primarily made from hydrogen and helium gas with trace amounts of gaseous molecules, like ammonia and water, that are heavier than the majority atmosphere. Earth’s atmosphere is especially nitrogen and oxygen. Jupiter also has storms, just like the Great Red Spot, that last for hundreds of years. And while ammonia gas and water vapor rise, freeze into droplets, like snow, and rain down continually, there is no such thing as a solid surface to hit. At what point do the raindrops stop falling?
“On Earth, you might have a surface, and rain will eventually hit this surface,” Moeckel said. “The query is: What happens should you take the surface away? How far do the raindrops fall into the planet? That is what we’ve got on the large planets.”
That query has piqued the interest of planetary scientists for a long time, because processes like rain and storms are considered the foremost vertical mixers of planetary atmospheres. For a long time, the straightforward assumption of a well-mixed atmosphere guided inferences in regards to the interior makeup of gas giant planets like Jupiter.
Observations by radio telescopes, much of it conducted by de Pater and colleagues, show that this straightforward assumption is fake.
“The turbulent cloud tops would lead you to imagine that the atmosphere is well mixed,” said Moeckel, invoking the analogy of a boiling pot of water. “When you have a look at the highest, you see it boiling, and you’ll assume that the entire pot is boiling. But these findings show that regardless that the highest looks prefer it’s boiling, below is a layer that actually may be very regular and sluggish.”
The microphysics of mushballs
On Jupiter, nearly all of water rain and ammonia snow appears to cycle high up within the cold atmosphere and evaporate because it falls, Moeckel said. Yet, even before Juno’s arrival at Jupiter, de Pater and her colleagues reported an upper atmosphere lacking in ammonia. They were in a position to explain these observations, nevertheless, through dynamic and standard weather modeling, which predicted a rainout of ammonia in thunderstorms all the way down to the water layer, where water vapor condenses right into a liquid.
But radio observations by Juno traced the regions of poor mixing to much greater depths, all the way down to about 150 km, with many areas puzzlingly depleted of ammonia and no known mechanism that would explain the observations. This led to proposals that water and ammonia ice must form hailstones that fall out of the atmosphere and take away the ammonia. But it surely was a mystery how hailstones could form that were heavy enough to fall a whole lot of kilometers into the atmosphere.
To elucidate why ammonia is missing from parts of Jupiter’s atmosphere, planetary scientist Tristan Guillot proposed a theory involving violent storms and slushy hailstones called mushballs. In this concept, strong updrafts during storms can lift tiny ice particles high above the clouds — greater than 60 kilometers up. At those altitudes, the ice mixes with ammonia vapor, which acts like antifreeze and melts the ice right into a slushy liquid. Because the particles proceed to rise and fall, they grow larger — like hailstones on Earth — eventually becoming mushballs the dimensions of softballs.
These mushballs can trap large amounts of water and ammonia with a 3 to 1 ratio. Due to their size and weight, they fall deep into the atmosphere — well below where the storm began — carrying the ammonia with them. This helps explain why ammonia appears to be missing from the upper atmosphere: it’s being dragged down and hidden deep contained in the planet, where it leaves faint signatures to be observed with radio telescopes.
Nevertheless, the method relies on various specific conditions. The storms must have very strong updrafts, around 100 meters per second, and the slushy particles must quickly mix with ammonia and grow large enough to survive the autumn.
“The mushball journey essentially starts about 50 to 60 kilometers below the cloud deck as water droplets. The water droplets get rapidly lofted all of the option to the highest of the cloud deck, where they freeze out after which fall over 100 kilometers into the planet, where they begin to evaporate and deposit material down there,” Moeckel said. “And so you might have, essentially, this weird system that gets triggered far below the cloud deck, goes all of the option to the highest of the atmosphere after which sinks deep into the planet.”
Unique signatures within the Juno radio data for one storm cloud convinced him and his colleagues that that is, indeed, what happens.
“There was a small spot under the cloud that either looked like cooling, that’s, melting ice, or an ammonia enhancement, that’s, melting and release of ammonia,” Moeckel said. “It was the undeniable fact that either explanation was only possible with mushballs that eventually convinced me.”
The radio signature couldn’t have been attributable to water raindrops or ammonia snow, in response to paper co-author Huazhi Ge, an authority in cloud dynamics on giant planets and a postdoctoral fellow on the California Institute of Technology in Pasadena.
“The Science Advances paper shows, observationally, that this process apparently is true, against my best desire to search out an easier answer,” Moeckel said.
Coordinated observations of Jupiter
Scientists around the globe observe Jupiter commonly with ground-based telescopes, timed to coincide with Juno’s closest approach to the planet every six weeks. In February 2017 and April 2019 — the periods covered by the 2 papers — the researchers used data from each the Hubble Space Telescope (HST) and the Very Large Array (VLA) in Latest Mexico to enrich Juno observations in an try and create a 3D picture of the troposphere. The HST, at visible wavelengths, provided measurements of reflected light off the cloud tops, while the VLA, a radio telescope, probed tens of kilometers below the clouds to supply global context. Juno’s Microwave Radiometer explored the deep atmosphere of Jupiter over a limited region of the atmosphere.
“I essentially developed a tomography method that takes the radio observations and turns them right into a three-dimensional rendering of that a part of the atmosphere that’s seen by Juno,” Moeckel said.
The 3D picture of that one swath of Jupiter confirmed that almost all of the weather is going on within the upper 10 kilometers.
“The water condensation layer plays an important role in controlling the dynamics and the weather on Jupiter,” Moeckel said. “Only probably the most powerful storms and waves can break through that layer.
Moeckel noted that his evaluation of Jupiter’s atmosphere was delayed by the dearth of publicly available calibrated data products from the Juno mission. Given the present level of information released, he was forced to independently reconstruct the mission team’s data processing methods — tools, data and discussions that, if shared earlier, could have significantly accelerated independent research and broadened scientific participation. He has since made these resources publicly available to support future research efforts.
The work was funded partially by a Solar System Observations (SSO) award from NASA (80NSSC18K1003).
