Bizarre hailstones composed of icy "mushroom balls" - a slurry mixture of ammonia and water - have been confirmed on Jupiter, with intense lightning lighting up these frozen slush balls as they fall from towering storms. Using the first 3D visualizations of Jupiter's troposphere, researchers found that while most weather systems are surprisingly shallow, deep convective systems can penetrate clouds, separate ammonia and water, and drag them deep into the cloud tops.
This illustration, using data from NASA's Juno mission, depicts high-altitude thunderstorms on Jupiter. The Juno spacecraft's sensitive Stellar Reference Unit camera detected unusual lightning on the far side of Jupiter during a close flyby of the planet. Image credit: NASA/JPL-Caltech/SwRI/MSSS/Gerald Eichstädt
This upends the idea that Jupiter has a well-mixed gas giant, as the mushroom balls act like underground conveyor belts, capturing and transporting chemicals, reshaping our understanding of giant planet atmospheres.
Imagine a giant smoothie composed of ammonia and water, wrapped in a hard shell of ice. Now, imagine these icy slushie balls - known as "mushroom balls" - flying through Jupiter's atmosphere like hailstones, illuminated by intense lightning.
According to planetary scientists at the University of California, Berkeley, this violent weather is not the stuff of science fiction—it really happens on Jupiter. These hailstones accompanied by lightning may also appear on other gas giant planets in the solar system, such as Saturn, Uranus, and Neptune, and may even appear on gas planets throughout the Milky Way.
The "mushroom ball" concept was first proposed in 2020 to explain the puzzling changes in ammonia levels observed in Jupiter's upper atmosphere. The anomalies were detected by NASA's Juno spacecraft and confirmed by ground-based radio telescopes.
At the time, Chris Morkel, a graduate student at the University of California, Berkeley, and his advisor, professor emeritus Imke de Pat, were skeptical. This theory requires very specific and extreme atmospheric conditions to hold.
A cross-section of Jupiter's upper atmosphere (or troposphere) showing the depth of north-south banded storms across Jupiter's equator, or equatorial zone (EZ). Blue and red represent above- and below-normal ammonia abundances, respectively. By tracking ammonia, two new UC Berkeley studies show that most of Jupiter's rapidly changing weather systems are very shallow (left), but two types of storms—rapidly rising ammonia plumes (center) and tornado-like vortices—strike deeper and are responsible for the separation of atmospheric gases. Massive storms can produce mushroom balls that fall deeper than ammonia plumes and vortices. Image credit: Chris Moeckel, University of California, Berkeley
"Imke and I were like, 'This can't be true,'" Morkel said. He received his PhD from UC Berkeley last year and is now a researcher at the UC Berkeley Space Sciences Laboratory. "It takes a lot of factors to really explain this, and it just seems so incredible. I spent basically three years trying to prove it was wrong, and I couldn't prove it was wrong."
The confirmation, published recently in the journal Science Advances, coincides with the first 3D visualization of Jupiter's upper atmosphere, recently created by Moeckel and de Pater and described in a paper that is currently undergoing peer review and posted on the preprint server arXiv.
3D images of Jupiter's troposphere show that most weather systems on Jupiter are shallow, reaching only 10 to 20 kilometers below visible clouds (or the planet's "surface"), which have a radius of 70,000 kilometers. Most of the colorful swirling patterns in the ribbons of clouds surrounding the planet are also shallow.
However, some weather phenomena occur deeper in the troposphere, redistributing ammonia and water and fundamentally disrupting what was long thought to be a homogeneous atmosphere. Three weather events that cause this phenomenon are: hurricane-like vortices, hot spots that combine with ammonia-rich plumes and wrap around the Earth in wave-like structures, and large storms that produce mushroom balls and lightning.
This illustration depicts how violent storms on Jupiter (and possibly other gas giants) create mushroom balls and shallow lightning. Mushroom balls are formed by thunderstorm clouds, which form about 40 miles below cloud tops and drive strong updrafts that carry water ice upward to extremely high altitudes, sometimes even above visible clouds. When they reach an altitude of about 14 miles below the visible clouds, the ammonia acts like antifreeze, melting and combining with the ice to form a layer of viscous ammonia liquid that is covered in water ice - mushroom balls. The mushroom ball keeps rising until it becomes too heavy, then falls back through the atmosphere, growing until it reaches the hydrocondensation layer and evaporates. This ultimately results in the redistribution of ammonia and water from the upper atmosphere (green and blue layers) to deep below the clouds, creating ammonia-depleted regions visible in radio observations. Image source: NASA/JPL-Caltech/SwRI/CNRS
"Every time you look at Jupiter, it's mostly just the surface," Morkel said. "It's shallow, but there are things - eddies and these big storms that can penetrate it."
"The Juno detections showed that ammonia is depleted at all latitudes, down to a depth of about 150 kilometers, which is really strange," said de Pater, who 10 years ago discovered that ammonia is depleted at a depth of about 50 kilometers. "Chris was trying to explain this by saying his storm system was much deeper than we expected."
Gas giants like Jupiter and Saturn, and ice giants like Neptune and Uranus, are the focus of current space missions and large telescopes, including the James Webb Space Telescope, in part because they can help us understand the formation history of the solar system and provide ground-truth observations of distant exoplanets, many of which are massive and gaseous. Because astronomers can only observe the upper atmospheres of distant exoplanets, understanding how to interpret the chemical signatures in these observations can help scientists infer details about the interiors of exoplanets, even Earth-like ones.
"We basically showed that the top of the atmosphere is actually a poor representation of what's going on inside the planet," Morkel said.
That's because storms like those that form mushroom balls separate the atmosphere, so the chemical composition of the cloud tops doesn't necessarily reflect the composition of the deeper layers of the atmosphere. Jupiter is unlikely to be unique.
“This could be extended to Uranus, Neptune — and certainly exoplanets,” DePat said.
Jupiter's atmosphere is completely different from Earth's. It is composed primarily of hydrogen and helium, with trace amounts of gaseous molecules such as ammonia and water, which are heavier than the atmosphere as a whole. Earth's atmosphere is mainly composed of nitrogen and oxygen. Jupiter also has storms like the Great Red Spot that last for centuries. Although ammonia and water vapor will rise, freeze into water droplets like snow, and keep raining, but since there is no solid surface, where will the raindrops fall?
"The Earth has a surface, and rain eventually falls on that surface," Morkel said. "The question is: What happens if you take away this surface? How far will the raindrops fall to Earth? This is what we encounter on giant planets."
This question has piqued the interest of planetary scientists for decades because processes such as rainfall and storms are thought to be major vertical mixers of planetary atmospheres. For decades, people have inferred the interior composition of gas giants like Jupiter based on the simple assumption that the atmosphere is well-mixed.
Observations with radio telescopes, most of them carried out by DePater and colleagues, show that this simple assumption is wrong.
"Turbulent cloud tops lead you to believe that the atmosphere is well mixed," said Morkel, citing the analogy of a pot of boiling water. "If you look at the top of a cloud and you see it boiling, you think the whole pot of water is boiling. But these findings show that even though the top of the cloud looks like it's boiling, underneath it's actually a very stable and slow layer of clouds."
On Jupiter, most of the water rain and ammonia snow appears to circulate high in the cold atmosphere and evaporate as it falls, Morkel said. However, even before Juno arrived at Jupiter, de Pater and her colleagues reported that Jupiter's upper atmosphere lacked ammonia. However, they were able to explain these observations through dynamic and standard weather models, which predicted that ammonia in thunderstorms would fall to the water layer, where the water vapor would condense into liquid.
However, Juno's radio observations tracked poorly mixed regions to much greater depths, up to about 150 kilometers, with many areas puzzlingly lacking in ammonia, and there is currently no known mechanism to explain these observations. This has led to the suggestion that water and ammonia ice must form hail and then fall out of the atmosphere, taking the ammonia with it. But how these hailstones, which are heavy enough to fall hundreds of kilometers into the atmosphere, form remains a mystery.
To explain why parts of Jupiter's atmosphere are lacking ammonia, planetary scientist Tristan Gilot proposed a theory involving violent storms and hailstones known as mushroom balls. According to the theory, strong updrafts during storms can lift tiny ice particles above the clouds - to an altitude of more than 60 kilometers. At these altitudes, ice particles mix with ammonia vapor, which acts like antifreeze, melting the ice into a slurry. As these particles rise and fall, they grow larger and larger—like hailstones on Earth—eventually turning into softball-sized mushroom balls.
These mushroom balls can capture large amounts of water and ammonia in a 3 to 1 ratio. Because of their size and weight, they fall deep into the atmosphere—well below where the storm begins and take the ammonia with them. This helps explain why ammonia seems to be missing from the upper atmosphere: It is dragged down and hidden deep inside the planet, where it leaves a weak signal that can be seen with radio telescopes.
However, this process depends on many specific conditions. The storm needs to have very strong updrafts, on the order of 100 meters per second, and the mud particles must quickly mix with the ammonia and become large enough to survive the fall.
"The mushroom ball's journey essentially begins about 50 to 60 kilometers below the clouds as water droplets. These water droplets quickly rise to the top of the clouds, freeze there, and then fall over a hundred kilometers into the planet's interior, where they begin to evaporate and deposit material," Morkel said. "So, essentially, this is a strange system that is triggered far below the clouds, all the way to the top of the atmosphere, and then sinks deep into the planet."
Unique signatures of a storm cloud in Juno's radio data convinced him and his colleagues that this was indeed what was happening.
"There's a small area underneath the clouds that looks like it's either cooling, which is where the ice is melting, or it's building up ammonia, which is where the ammonia is melting and being released," Morkel said. "Ultimately, I was convinced because both explanations only applied to mushroom balls."
Huazhi Ge, co-author of the paper, an expert in giant planet cloud dynamics and a postdoctoral researcher at the California Institute of Technology in Pasadena, said the radio signal could not have been caused by water raindrops or ammonia snow.
"The Science Advances paper shows from an observational perspective that this process is clearly correct, which goes against my desire to find a simpler answer," Morkel said.
Scientists around the world use ground-based telescopes to observe Jupiter regularly, coinciding with Juno's closest approach to the planet every six weeks. During the time period covered by both papers—February 2017 and April 2019—the researchers used data from the Hubble Space Telescope (HST) and the Very Large Array (VLA) in New Mexico to supplement Juno observations in an attempt to create a three-dimensional image of the troposphere. The Hubble Space Telescope measures light reflected from cloud tops at visible wavelengths, while the radio telescope Very Large Array (VLA) detects tens of kilometers below the clouds, providing global background information. Juno's microwave radiometer probed a limited region of deep atmosphere in Jupiter's atmosphere.
"I developed a tomography method that uses radio observation data and converts it into a three-dimensional rendering of the atmosphere as seen by Juno," Morkel said.
3D images of this region of Jupiter confirm that most weather phenomena occur in regions above 10 kilometers. "The hydrometeor layer plays a crucial role in controlling Jupiter's dynamics and weather, and only the most powerful storms and waves can breach this layer."
Morkel noted that his analysis of Jupiter's atmosphere has been delayed by a lack of publicly available Juno mission calibration data products. Given the level of data currently being released, he was forced to independently rebuild the mission team's approach to data—the tools, data, and discussions. Had they been shared earlier, these tools, data, and discussions could have significantly accelerated independent research and expanded scientific engagement. He has made these resources available to support future research efforts.
Compiled from /ScitechDaily