A recent discovery of sizeable ammonia-rich hail, called mushball, on Jupiter could explain why Uranus and Neptune do not appear to have ammonia in their atmospheres.
Scientists have been perplexed for years as to the apparent absence of ammonia in the atmospheres of Uranus and Neptune.
Known for its annoying smell, ammonia is quite common in the universe. However, 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 reasonable explanation for ammonia's absence in the ice giants' air.
However, a recent discovery of large ammonia-rich hailstones on Jupiter could illuminate this mystery. Spoiler alert! Ammonia may not be missing at all; it may be hidden in deeper layers of the planets' atmosphere, where current science instruments cannot reach.
The NASA Juno mission, which is currently orbiting Jupiter, has solved this mystery.
"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 mushball contained ammonia from high in the atmosphere of Jupiter during thunderstorms thanks to ammonia's ability to melt ice into liquid water even in freezing temperatures of around minus 162 degrees Fahrenheit (minus 90 degrees Celsius).
As these mushrooms drop into the atmosphere, they absorb more and more ammonia and eventually accumulate up to 2.2 pounds (1 kilogram) in mass. Finally, ammonia is carried deep into the atmosphere, where it stays stuck under the base of the clouds.
"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 means that this process is even more efficient at Uranus and Neptune, and the region of mushball seeds is expanded and occurs at greater depths," Guillot added.
This means that, just as on Jupiter, on Uranus, and Neptune, ammonia can be hidden in the depths of the atmosphere. Scientists currently measure the atmospheric composition of these planets far from the solar system by analyzing infra-red and radio signatures of the atmospheres through Earth-based telescopes.
To date, these two planets have only been visited very briefly by a single spaceship: NASA's Voyager 2 in the late 1980s.
The storylines of the atmospheres of distant planets, suggests Guillot, prompt a dedicated mission that could allow scientists to discover what is happening entirely.
"To fully understand the processes, we need a dedicated mission to map the deep air structure and understand the mixture in hydrogen atmospheres," said Mr. Guillot. "Neptune and Uranus constitute a vital link between giant planets, such as Jupiter and Saturn, and the giant ice exoplanets we discover in the galaxy. So we have to get going!"