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Stinky 'mushball' hailstones on Uranus may explain an atmospheric anomaly there

Artist’s impression of a mushball descending through a giant planet’s atmosphere.
Artist’s impression of a mushball descending through a giant planet’s atmosphere. (Image credit: NASA/JPL-Caltech/SwRI/CNRS)

A recent discovery of giant ammonia-rich hailstones, dubbed mushballs, on Jupiter might explain why Uranus and Neptune seem to have no ammonia in their atmospheres. 

Scientists have puzzled for years over the apparent absence of ammonia in the atmospheres of Uranus and Neptune.

Known for its unpleasant smell, ammonia is rather common in the universe. Since the atmospheres of Uranus and Neptune are rich in other chemical compounds known to be present in the primordial cloud from which planets formed, scientists had no good explanation for ammonia's absence in the ice giants' air. 

Related: Could a spaceship fly through a gas giant like Jupiter?

However, a recent discovery of giant ammonia-rich hailstones on Jupiter might shed some light on this mystery. Spoiler alert! The ammonia might not be missing at all; it may just be hidden in deeper layers of the planets' atmospheres, where current scientific instruments cannot reach. 

A possible key to solving this mystery was provided by NASA's Juno mission, which is currently orbiting Jupiter. 

"The Juno spacecraft has shown that in Jupiter, ammonia is present in abundance, but generally much deeper [in the atmosphere] than expected," Tristan Guillot, a researcher at the French National Centre for Scientific Research (CNRS) in Nice, said in a statement.

A study published last year in the journal Nature found that mushballs containing ammonia form high in the atmosphere of Jupiter during thunderstorms thanks to ammonia's ability to melt ice into liquid water even in extremely cold temperatures of around minus 162 degrees Fahrenheit (minus 90 degrees Celsius). 

As these mushballs fall through the atmosphere, they absorb more and more ammonia, eventually accumulating up to 2.2 pounds (1 kilogram) of mass. The ammonia gets transported deep into the atmosphere, where it remains locked below the cloud base. 

"What we have learned at Jupiter can be applied to provide a plausible solution to this mystery at Uranus and Neptune," said Guillot, who presented his theory at Europlanet Science Congress (EPSC) 2021, which was held virtually this year from Sept. 13 to Sept. 24. 

"Thermodynamic chemistry implies that this process is even more efficient in Uranus and Neptune, and the mushball seed region is extended and occurs at greater depths," Guillot added. 

That means that, just like on Jupiter, on Uranus and Neptune ammonia may be simply hidden deep in the atmosphere. Scientists currently measure the atmospheric composition of these distant planets of the solar system by analyzing the infrared and radio signatures of the atmospheres by Earth-based telescopes. 

Both of these planets have so far been visited very briefly by only one spacecraft: NASA's Voyager 2 in the late 1980s. 

The intrigues of the distant planets' atmospheres, Guillot suggests, provide an incentive for a dedicated mission that could enable scientists to fully uncover what's going on.

"To fully understand the processes, we need a dedicated mission to map the deep atmospheric structure and understand mixing in hydrogen atmospheres," said Guillot. "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. We really need to go there!"

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Tereza Pultarova
Tereza is a London-based science and technology journalist, video producer and health blogger. Originally from Prague, the Czech Republic, she spent the first seven years of her career working as a reporter, script-writer and presenter for various TV programmes of the Czech national TV station. She later took a career break to pursue further education and added a Master in Science from the International Space University, France, to her Bachelor's degree in Journalism from Prague's Charles University. She is passionate about nutrition, meditation and psychology, and sustainability.