Mushroom balls hide ammonia missing from Uranus and Neptune

Mushroom balls – giant, muddy hailstones made from a mixture of ammonia and water – could be responsible for an atmospheric anomaly on Neptune and Uranus that has baffled scientists. A study presented by Tristan Guillot at the 2021 Europlanet Science Congress (EPSC) shows that mushballs could be very effective at transporting ammonia deep into the atmosphere of ice giants, hiding the gas of detection under opaque clouds.

Recently, remote observations at infrared and radio wavelengths have shown that Uranus and Neptune lack ammonia in their atmosphere compared to other giant planets in our solar system. This is surprising because they are also very rich in other compounds, such as methane, found in the primordial cloud from which the planets were formed.

Either the planets were formed under special conditions, from materials also poor in ammonia, or an ongoing process must be responsible. Guillot, a researcher at CNRS, Laboratoire Lagrange in Nice, France, turned to a recent discovery in Jupiter for a possible answer to the riddle.

“The Juno spacecraft has shown that in Jupiter ammonia is present in abundance, but usually much deeper than expected, thanks to the formation of mushball balls. I show that what we learned in Jupiter can be applied to bring a plausible solution to this mystery to Uranus and Neptune, ”said Guillot.

Juno’s observations at Jupiter showed that ammonia-water hailstones can form rapidly during storms due to the ability of ammonia to liquefy water ice crystals, even at very low temperatures of around – 90 degrees Celsius. Models indicate that these Jupiter pellets can reach a weight of up to a kilogram or more, slightly higher than the largest hailstones on Earth. By plunging down, they transport ammonia very efficiently to the deep atmosphere, where it ends up trapped under the cloud base.

“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 said. “So ammonia is probably simply hidden in the deep atmospheres of these planets, beyond the reach of current instruments.”

To determine exactly how deep the pellets contain ammonia and water, you may have to wait until an instrumented orbiter can closely probe the atmosphere of the ice giants.

“To fully understand the processes, we need a dedicated mission to map the deep atmospheric structure and understand the mixing in hydrogen atmospheres,” Guillot said. “Neptune and Uranus are a vital link between giant planets, like Jupiter and Saturn, and the giant ice exoplanets that we are discovering in the galaxy. We really have to go.”

Provided by Europlanet