Photo credit: www.sciencedaily.com
Envision a frozen treat made of ammonia and water encapsulated in a hardened layer of ice. Now, imagine these icy globules, referred to as “mushballs,” cascading from the sky like hail during a tempest, flashing with brilliant lightning.
Research conducted by planetary scientists at the University of California, Berkeley, suggests that such mushball hailstorms, alongside intense lightning, do indeed take place on Jupiter. These meteoric phenomena may also occur on other gas giants throughout the galaxy, including Saturn, Uranus, and Neptune.
The concept of mushballs was initially introduced in 2020 to clarify the uneven distribution of ammonia gas detected in Jupiter’s upper atmosphere, as observed by NASA’s Juno mission and terrestrial radio telescopes.
At the time, UC Berkeley graduate student Chris Moeckel and his advisor, Imke de Pater, an emerita professor of astronomy and earth and planetary science, considered this theory too elaborate to be accurate, as it necessitated very specific atmospheric conditions.
“We both thought there was no way this theory could be valid,” said Moeckel, who completed his Ph.D. at UC Berkeley last year and now works at the university’s Space Sciences Laboratory. “So many variables needed to align for this to hold true, it seemed incredibly exotic. For three years, I tried to disprove it, but I failed.”
On March 28, the existence of these mushball hailstorms was confirmed in a study published in the journal Science Advances. This work presented the first-ever three-dimensional visualization of Jupiter’s upper atmosphere, which Moeckel and de Pater have recently developed, and is currently under peer review while being accessible on the preprint server arXiv.
This 3D model illustrates that most of Jupiter’s weather patterns are relatively shallow, extending only 10 to 20 kilometers below the planet’s visible cloud deck, which has a radius of approximately 70,000 kilometers. The vibrant, swirling patterns observed in its bands are primarily superficial.
However, certain weather phenomena develop much deeper within the troposphere, redistributing ammonia and water, and challenging the long-standing belief in a homogenous atmosphere. The three types of weather systems involved include powerful, hurricane-like vortices; hotspots linked to ammonia-rich plumes that wrap around the planet; and substantial storms producing mushballs and lightning.
“Typically, our view of Jupiter is limited to just the surface level,” Moeckel stated. “It’s mostly shallow, yet a few phenomena—those vortices and significant storms—can penetrate deeper.”
De Pater noted, “Juno has revealed that ammonia diminishes at all latitudes down to about 150 kilometers, which is quite unusual. Ten years prior, I discovered ammonia depleting to around 50 kilometers. Chris is now exploring explanations for this depletion through deeper storm systems.”
Understanding Planetary Composition through Cloud Observations
Current space missions and major telescopes, including the James Webb Space Telescope, primarily target gas giants like Jupiter and Saturn, as well as ice giants like Neptune and Uranus. Analyzing these planets helps astronomers uncover the formation history of our solar system and gain insight into distant exoplanets, many of which are gaseous and amenable to similar studies. Since observations permit a view only of upper atmospheres of such exoplanets, deciphering their chemical signatures becomes crucial for inferring their internal structures, including Earth-like planets.
“Our findings indicate that the atmosphere’s upper layers poorly represent the true composition of the planet’s interior,” Moeckel explained.
Storms like those creating mushballs disrupt the atmosphere, meaning the chemical makeup of the planetary cloud tops may not mirror that found deeper within. Jupiter is likely not an isolated case.
“This principle applies to Uranus and Neptune—and certainly to distant exoplanets,” de Pater remarked.
In contrast to Earth’s atmosphere, which mainly comprises nitrogen and oxygen, Jupiter’s atmosphere consists primarily of hydrogen and helium, with trace amounts of heavier molecules like ammonia and water. Jupiter is also home to long-lived storms, such as the Great Red Spot. As ammonia gas and water vapor rise, they freeze into drop-like structures that repeatedly fall without ever reaching a distinct surface.
“On Earth, rain eventually hits the ground,” Moeckel noted. “But without a solid surface on Jupiter, how far do the raindrops penetrate into the planet? This fundamental question has intrigued planetary scientists for decades since processes like precipitation are believed to be critical for vertically mixing planetary atmospheres. For years, it was simplistically presumed that these atmospheres were well-mixed, guiding assumptions about gas giant interiors.
However, investigations conducted via radio telescopes—largely led by de Pater and colleagues—indicate this assumption is flawed.
“The turbulent appearance of cloud tops might suggest a well-mixed atmosphere,” Moeckel explained. “It’s akin to watching a boiling pot of water; the visible turbulence leads you to presume the entire pot is agitated. Yet, these findings indicate that beneath that chaotic upper layer lies a calmer and more stable environment.”
The Microphysical Properties of Mushballs
On Jupiter, it appears that the majority of water and ammonia precipitation cycles occur high in the cold atmosphere before evaporating as they descend. Prior to Juno’s findings, de Pater and her team had reported a lack of ammonia in the upper atmosphere, which they could explain through conventional weather modeling. This modeling predicted a rainout of ammonia through thunderstorms descending into the water layer, where the vapor condenses into liquid.
However, Juno’s radio observations traced the regions with minimal mixing down to depths of about 150 kilometers, where many areas intriguingly lacked ammonia, with no clear mechanistic explanation. This observation led to suggestions that hailstones formed of water and ammonia ice might fall from the atmosphere, absorbing the ammonia. Yet, it remained unclear how these hailstones could attain sufficient mass to descend hundreds of kilometers into the atmosphere.
Planetary scientist Tristan Guillot proposed a theory that link intense storms with slushy hailstones, referred to as mushballs. According to this theory, strong updrafts during storms could lift tiny ice particles high above the clouds—over 60 kilometers up. Here, the ice interacts with ammonia vapor, which acts similarly to antifreeze, melting into a slushy liquid. As these particles continue their upward and downward journey, they grow larger, ultimately transforming into mushballs the size of softballs.
These mushballs can encase significant quantities of water and ammonia in a 3 to 1 ratio. Due to their size and weight, they delve deep into the atmosphere, dragging ammonia with them, which explains why ammonia is perceived as absent from the upper atmospheres—its signature is hidden deep within the planet, detectable only through radio telescope observations.
This complex process requires meeting specific conditions: storms must possess robust updrafts of around 100 meters per second, and slushy particles must rapidly mix with ammonia and attain sufficient size to survive their descent.
“The journey of these mushballs begins about 50 to 60 kilometers beneath the cloud cover as water droplets,” Moeckel elaborated. “These droplets are rapidly lifted to the cloud’s apex, where they freeze and then plunge deeper into the planet, evaporating and depositing material along the way. This creates an unusual cycling system that starts beneath the cloud deck, resurfaces at the top of the atmosphere, and then descends deep into the planet.”
Distinct signatures found in Juno’s radio data for particular storm clouds led Moeckel and his collaborators to conclude that this process occurs consistently.
“We observed a localized area under the cloud that exhibited either cooling (suggestive of melting ice) or an increase in ammonia (indicative of melting and releasing ammonia),” Moeckel mentioned. “The fact that only mushballs could explain either scenario solidified my conviction.”
The radio signatures could not be attributed to water raindrops or ammonia ice, noted Huazhi Ge, a cloud dynamics expert at the California Institute of Technology and co-author of the study.
“The findings presented in Science Advances illustrate that this hypothesis likely holds true, despite my initial preference for a more straightforward explanation,” Moeckel remarked.
Coordinated Observations of Jupiter
Researchers frequently observe Jupiter using ground-based telescopes that are synchronized with Juno’s closest approaches to the planet every six weeks. Between February 2017 and April 2019—timeframes encompassed by the studies—data from both the Hubble Space Telescope (HST) and the Very Large Array (VLA) in New Mexico were utilized alongside Juno’s data to construct a 3D model of its troposphere. The HST captured measurements of light reflections from the cloud tops, while the VLA, a radio telescope, probed several kilometers beneath the clouds for global context. Juno’s Microwave Radiometer investigated the deeper atmospheric strata over a limited area.
“I essentially devised a tomographic technique that converts radio observations into a three-dimensional representation of the region detectable by Juno,” Moeckel stated.
This three-dimensional view of a segment of Jupiter confirmed that most weather events transpire within the upper 10 kilometers.
“The water condensation layer is vital in driving the dynamics and weather patterns on Jupiter,” Moeckel noted. “Only the most powerful storms and atmospheric waves can breach that layer.”
He also highlighted that his analysis was hampered by the unavailability of publicly calibrated data from the Juno mission. Given the current release status, he needed to independently reconstruct the team’s data processing techniques, emphasizing that such shared resources could have greatly accelerated independent research and expanded scientific engagement. He has since made these tools publicly accessible to aid future exploration.
This research was partially funded by a Solar System Observations (SSO) grant from NASA (80NSSC18K1003).
Source
www.sciencedaily.com