Uranus and Neptune are planets that have intrigued scientists when it comes to ammonia. Is that recent observations, carried out in infrared light and radio waves, have shown that the two planets have smaller amounts of the compound than expected compared to the other gas giants in the Solar System. In a new study, researcher Tristan Guillot has proposed that this mystery may be related to the discovery of “mushballs”, ammonia and water hailstones formed by storms on Jupiter.
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The discrepancy in the amount of ammonia detected surprised scientists, because both planets are rich in other compounds, such as methane, which were part of the cloud of gas and dust that formed the planets. Therefore, the amount of ammonia may indicate that either these planets formed under special conditions, from material that was also poor in ammonia, or there was some other process going on.
So Guillot turned to a recent discovery provided by the scientific team on the Juno spacecraft, which studies Jupiter. “Juno showed that on Jupiter, ammonia is abundant, but it tends to run much deeper than we expected thanks to mushballs,” he explained. So he demonstrated that what was discovered on Jupiter can also be applied to explain what was observed on Uranus and Neptune.
Overall, Juno’s observations have shown that this muddy, ammonia-rich hail (nicknamed “mushballs”) can form quickly during storms due to ammonia’s ability to liquefy water crystals, which can occur even in very low temperatures. Models have shown that mushballs can weigh 1 kg or even more, and they sink and transport the ammonia into the planet’s deep atmosphere, where it is trapped in the underside of Jovian clouds.
According to Guillot, thermodynamic chemistry suggests that this process occurs even more efficiently on Uranus and Neptune, because the region that the mushballs travel through is more extensive and reaches deeper depths. “So ammonia is probably simply hidden in the deep atmosphere of these planets, beyond the reach of the instruments we have today,” he said.
Therefore, the author believes that to determine how far mushballs are carrying ammonia and water, orbiters with appropriate instruments will be needed, so a dedicated mission would be needed to map the atmospheric structure of these planets. “Uranus and Neptune are a unique link between the giant planets, as are Jupiter and Saturn, and the icy giant exoplanets we are discovering; we really need to go there!”, he concluded.
The study was made available on the website of the 2021 edition of the European Planetary Science Congress.