Icy, ammonia-rich “snowballs” can plunge deep beneath the cloud bank in Jupiter’s atmosphere. Now, a planetary scientist suggests that this scenario could also explain the surprisingly low levels of ammonia detected on Uranus and Neptune.
Tristan Guillot (Université Côte d’Azur, France), who shared the results at the Europlanet Science virtual congress, says it can also help us understand the deep hydrogen-rich atmospheres of giant exoplanets.
Hailstorms on Jupiter
While astronomers have long observed an uneven distribution of ammonia on Jupiter, the main evidence for the existence of the dumplings came from the dramatic close-ups of the Juno spacecraft on the severe thunderstorms on Jupiter. Juno has revealed weak lightning in areas where temperatures drop below -66 ° C. Since lightning requires 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 antifreeze? Last year, Guillot and his colleagues proposed that ammonia and water combined could remain muddy enough to allow lightning to occur.
Ammonia is “the best antifreeze you can get,” says Guillot. Mixing one part ammonia with two parts water can keep the liquid solution at -100 ° C. Jupiter’s atmosphere contains enough ammonia to form such a mixture, producing hailstones as large as the largest hailstones on Earth.
As shown in the diagram above, dough ball formation begins when water ice crystallizes high in the atmosphere and begins to drift downward. At lower levels, ammonia begins to mix with the water ice. Its anti-freeze effect melts the water as more ice crystallizes, forming a strong outer shell that thickens as the mushball falls deeper.
“During strong storms, large amounts of ammoniacal water… Mushroom balls can form and fall into the deeper atmosphere,” reaching masses of one kilogram or more before evaporating, explains Guillot. Thunderstorms thus carry ammonia from the upper atmosphere to the depths, while in the calm equatorial region of Jupiter, where there are fewer thunderstorms, 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 that would have been present in the early solar system when these planets were formed. At this week’s Europlanet Science congress, Guillot noted that dough balls could explain this phenomenon, pulling ammonia deeper than expected onto ice giants and making it appear rarer.
The colder temperatures of the atmospheres of Uranus and Neptune promote the antifreeze effect of ammonia. Guillot showed that mushroom balls could exist under a wider range of conditions on ice giants and Saturn than on Jupiter.
“What may differ between Uranus and Neptune is the frequency and strength of storms,” he says. More abundant storms would draw more ammonia deeper. He notes that the abundance of ammonia on Saturn, as on Jupiter, varies with latitude.
Dave Stevenson (Caltech), who coined the term “mushballs” and worked with Guillot on research on Jupiter but not Uranus and Neptune, is cautious. He says they haven’t found any alternative to mushroom balls to explain the distribution of ammonia on Jupiter. However, without doing a detailed analysis, he says, “In the case of Uranus and Neptune, the data admits of alternatives.
“You really have to go,” said Guillot. Only a dedicated mission can probe deep into these giant icy atmospheres. “Neptune and Uranus are an essential link between giant planets, like Jupiter and Saturn, and the giant ice exoplanets that we are discovering in the galaxy. “