#astronomy Hail-like “Mushballs” Might Fall on All Our Large Planets – Sky & Telescope

September 24, 2021 - Comment

Icy, ammonia-rich “mushballs” could plunge deep under the cloud financial institution in Jupiter’s ambiance. Now, a planetary scientist suggests this state of affairs may clarify the surprisingly low ranges of ammonia detected on Uranus and Neptune, too. Uranus (left) and Neptune (proper), each imaged by Voyager 2. NASA / JPL / Caltech Tristan Guillot (Côte


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

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

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

Hailstorms on Jupiter

Whereas 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 under -66ºC. Since lightning flashes require the presence of a liquid, and water is a stable at such low temperatures, the end result initially baffled astronomers.

However what if the water is blended with anti-freeze? Final 12 months, Guillot and colleagues proposed that ammonia and water mixed might stay slushy sufficient to permit lightning.

Ammonia is “the most effective antifreeze you may 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 combination, producing hail-like mushballs as huge 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 approach 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 stable outer shell that thickens because the mushball falls deeper down.

“Throughout sturdy storms massive ammonia-water . . . mushballs can type and fall to the deeper ambiance,” reaching lots 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 ample.

Mushballs and Ice Giants

Latest 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 photo voltaic system, when these planets shaped. 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 could differ between Uranus and Neptune is the frequency and power of storms,” he says. Extra ample 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 clarify the distribution of ammonia on Jupiter. Nonetheless, with out making an in depth evaluation, he says, “Within the case of Uranus and Neptune, the information admit alternate options.”

“We actually have to go there,” Guillot says. Solely a devoted mission can probe deep into these ice-giant atmospheres. “Neptune and Uranus are a essential hyperlink between big planets, like Jupiter and Saturn, and ice-giant exoplanets that we’re discovering within the galaxy.”



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