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EI2GYB > ASTRO 27.09.21 08:48l 91 Lines 4644 Bytes #999 (0) @ WW
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Subj: Hail-like "Mushballs" May Fall on All Our Giant Planets
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Hail-like "Mushballs" May Fall on All Our Giant Planets
"Mushballs," first inferred to exist on Jupiter, may fall within the
atmospheres of all the solar system's giant planets.
Icy, ammonia-rich "mushballs" may plunge deep below the cloud bank in Jupiter's
atmosphere. Now, a planetary scientist suggests this scenario might explain the
surprisingly low levels of ammonia detected on Uranus and Neptune, too.
Tristan Guillot (C“te d'Azur University, France), who shared the results at the
virtual Europlanet Science Congress, says this may also help us understand the
deep hydrogen-rich atmospheres of giant exoplanets.
While astronomers had long observed an uneven distribution of ammonia on
Jupiter, the key evidence for mushballs' existence came from the Juno
spacecraft's dramatic close-ups of violent thunderstorms on Jupiter. Juno
revealed faint lightning flashes in regions where temperatures dip below -66§C.
Since lightning flashes require the presence of a liquid, and water is a solid
at such low temperatures, the result initially baffled astronomers.
But what if the water is mixed with anti-freeze? Last year, Guillot and
colleagues proposed that ammonia and water combined could remain slushy enough
to allow lightning.
Ammonia is "the best antifreeze you can get," says Guillot. Mixing one part
ammonia to two parts water can keep the solution liquid down to -100§C.
Jupiter's atmosphere contains enough ammonia to form such a mixture, producing
hail-like mushballs as big as the biggest terrestrial hailstones.
As shown in the above diagram, mushball formation starts when water ice
crystallizes high in the atmosphere and starts drifting downwards. At lower
levels, ammonia begins mixing with the water ice. Its antifreeze effect melts
the water while more ice crystallizes, forming a solid outer shell that
thickens as the mushball falls deeper down.
"During strong storms large ammonia-water . . . mushballs can form and fall to
the deeper atmosphere," reaching masses of a kilogram or more before they
evaporate, says Guillot. The storms thus pull ammonia down from the upper
atmosphere into the depths, while in Jupiter's calm equatorial region, where
there are fewer storms, ammonia is more abundant.
Mushballs and Ice Giants
Recent infrared and radio observations have shown that ammonia is relatively
rare on Uranus and Neptune compared to other small molecules thought to have
been present in the early solar system, when those planets formed. At this
week's Europlanet Science Congress, Guillot noted that mushballs could explain
this phenomenon, pulling ammonia deeper than expected on the ice giants and
making it appear rarer.
The colder temperatures of Uranus's and Neptune's atmospheres favor ammonia's
antifreeze effect. Guillot showed that mushballs could exist over a broader
range of conditions on the ice giants and on Saturn than they could on Jupiter.
"What may differ between Uranus and Neptune is the frequency and strength of
storms," he says. More abundant storms would pull more ammonia deeper. He notes
that ammonia abundance on Saturn, as on Jupiter, varies with latitude.
Dave Stevenson (Caltech), who came up with the term "mushballs" and worked with
Guillot on the Jupiter research but not on Uranus and Neptune, is cautious. He
says they couldn't find any alternative to mushballs to explain the
distribution of ammonia on Jupiter. However, without making a detailed
analysis, he says, "In the case of Uranus and Neptune, the data admit
alternatives."
"We really need to go there," Guillot says. Only a dedicated 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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