Hail-like “Mushballs” May Fall on All Our Giant Planets

Icy, ammonia-rich “mushballs” could plunge deep beneath the cloud financial institution in Jupiter’s ambiance. Now, a planetary scientist suggests this situation may clarify the surprisingly low ranges of ammonia detected on Uranus and Neptune, too.

uranus and neptune
Uranus (left) and Neptune (proper), each imaged by Voyager 2.
NASA / JPL / Caltech

Tristan Guillot (Côte d’Azur University, France), who shared the outcomes on the digital Europlanet Science Congress, says this will likely additionally assist us perceive the deep hydrogen-rich atmospheres of large exoplanets.

Hailstorms on Jupiter

While astronomers had lengthy noticed an uneven distribution of ammonia on Jupiter, the important thing proof for mushballs’ existence got here from the Juno spacecraft’s dramatic close-ups of violent thunderstorms on Jupiter. Juno revealed faint lightning flashes in areas the place temperatures dip beneath -66ºC. Since lightning flashes require the presence of a liquid, and water is a strong at such low temperatures, the outcome initially baffled astronomers.

But what if the water is blended with anti-freeze? Last year, Guillot and colleagues proposed that ammonia and water mixed might stay slushy sufficient to permit lightning.

Ammonia is “the best antifreeze you can get,” says Guillot. Mixing one half ammonia to 2 components water can preserve the answer liquid right down to -100ºC. Jupiter’s ambiance accommodates sufficient ammonia to type such a mix, producing hail-like mushballs as massive as the largest terrestrial hailstones.

To make a mushball (diagram)
Water ice crystallizes excessive within the ambiance, however it turns into frozen-over slush because it incorporates ammonia on its way down.
NASA / JPLCaltech / SwRI / CNRS

As proven within the above diagram, mushball formation begins when water ice crystallizes excessive within the ambiance and begins drifting downwards. At decrease ranges, ammonia begins mixing with the water ice. Its antifreeze impact melts the water whereas extra ice crystallizes, forming a strong outer shell that thickens because the mushball falls deeper down.

“During strong storms large ammonia-water . . . mushballs can form and fall to the deeper atmosphere,” reaching plenty of a kilogram or extra earlier than they evaporate, says Guillot. The storms thus pull ammonia down from the higher ambiance into the depths, whereas in Jupiter’s calm equatorial area, the place there are fewer storms, ammonia is extra plentiful.

Mushballs and Ice Giants

Recent infrared and radio observations have proven that ammonia is comparatively uncommon on Uranus and Neptune in comparison with different small molecules thought to have been current within the early solar system, when these planets fashioned. At this week’s Europlanet Science Congress, Guillot famous that mushballs might clarify this phenomenon, pulling ammonia deeper than anticipated on the ice giants and making it seem rarer. 

The colder temperatures of Uranus’s and Neptune’s atmospheres favor ammonia’s antifreeze impact. Guillot confirmed that mushballs might exist over a broader vary of situations on the ice giants and on Saturn than they may on Jupiter.

“What may differ between Uranus and Neptune is the frequency and strength of storms,” he says. More plentiful storms would pull extra ammonia deeper. He notes that ammonia abundance on Saturn, as on Jupiter, varies with latitude.

Dave Stevenson (Caltech), who got here up with the time period “mushballs” and labored with Guillot on the Jupiter analysis however not on Uranus and Neptune, is cautious. He says they could not discover any different to mushballs to elucidate the distribution of ammonia on Jupiter. However, with out making an in depth evaluation, he says, “In the case of Uranus and Neptune, the data admit alternatives.”

“We really need to go there,” Guillot says. Only a devoted mission can probe deep into these ice-giant atmospheres. “Neptune and Uranus are a critical link between giant planets, like Jupiter and Saturn, and ice-giant exoplanets that we are discovering in the galaxy.”

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