Jupiter’s Clouds Contain Smoggy Ammonium Hydrosulphide, Not Ammonia Ice

Jupiter’s clouds aren’t what we thought they were. Planetary atmosphere experts have studied them for many years, uncovering new and puzzling mysteries. Recently, several researchers banded together to solve a long-standing mystery about those clouds. It turns out they aren’t made of ammonia ice, which is what everyone has thought for years. Instead, they seem to be largely a mix of smog and ammonium hydrosulfide. That compound forms in the atmosphere as hydrogen sulfide gas passes through ammonia.

Most of us are familiar with the Jovian clouds and know that ammonia and water are involved in their formation. There’s precipitation, meaning that ammonia and other substances “rain out.” Then, they evaporate. Most of the clouds we do see are thought to be mainly ammonia ice, contaminated with other materials that lend color to the clouds. Ammonia is an important “tracer” of activity in Jupiter’s atmosphere and scientists have studied its presence for years. Most of those measurements come from spacecraft instruments and large ground-based telescopes outfitted with special filters and spectroscopes. Even those observations, however, are limited when it comes to determining their positions in the atmosphere. Also, temporal coverage is limited.

Getting observation time to track the presence of ammonia, and there are only so many spacecraft to go around. Plus, the methods for analyzing the observations are complex and time-consuming. What if there was a quick and cost-effective way to get continual observations of the Jovian clouds? Could smaller telescopes used by amateur astronomers be effective enough to chart variations in the amounts of ammonia in and above Jupiter’s clouds over time? If so, that would fill in a huge gap in Jupiter atmospheric observations.

Measuring Those Clouds

The saga of the Jovian clouds began when Dr. Steven Hill, a space weather forecasting expert, tried a fresh approach and made backyard observations of the gas giant’s clouds in 2020-2021 and 2022-2023. He was able to compare images that show absorption in the atmosphere due to ammonia and methane gases. He also determined variations in the amount of ammonia in and above the cloud tops.

With time on big observatory scopes at such a premium, Hill used a 0.28-meter Celestron Schmidt-Cassegrain telescope, outfitted with a ZWO ASI120MM CMOS camera. He used a 647-nm ammonia band filter first. Later on he applied a 619-nm methane band filter. The idea was to detect individual ammonia abundance features. “I always like to push my observations to see what physical measurements I can make with modest, commercial equipment,” said Hill. “The hope is that I can find new ways for amateurs to make useful contributions to professional work. But I certainly did not expect an outcome as productive as this project has been!”

Applying Hill’s Approach to Jupiter’s Clouds

It turns out Hill’s technique is easier and less expensive than the more complex observational and statistical methods scientists use to map clouds. It can be used in professional research to zero in on specific regions of the atmosphere. The approach also gives citizen scientists with backyard-type telescopes a way to track ammonia and cloud-top pressure variations across features in Jupiter’s atmosphere. That includes Jupiter’s cloud bands, its fast-moving small storms, and even the larger features such as the Great Red Spot.

Planetary atmosphere expert Professor Patrick Irwin at the University of Oxford in England, who co-wrote a paper with Hill about the observations, emphasized the advantage of doing such observations. “I am astonished that such a simple method is able to probe so deep in the atmosphere and demonstrate so clearly that the main clouds cannot be pure ammonia ice,” he said. “These results show that an innovative amateur using a modern camera and special filters can open a new window on Jupiter’s atmosphere and contribute to understanding the nature of Jupiter’s long-mysterious clouds and how the atmosphere circulates.”

Insights into Jupiter’s Clouds

Hill’s initial results showed that the clouds he studied lay in a region of Jupiter’s warm atmosphere that doesn’t allow ammonia ice to exist. In their follow-up study, Irwin and his colleagues applied Hill’s method to observations using the Multi Unit Spectroscopic Explorer on the Very Large Telescope in Chile. Doing spectroscopy allows scientists to measure the visible light fingerprints of the gases in the Jovian atmosphere and chart the distribution of ammonia and the height of its clouds. They also simulated how light interacts with those gases and clouds using a computer model.

It turns out that the Jovian clouds observed through Hill’s backyard telescope had to be much deeper than previously thought. They lie in an atmospheric region with higher pressures and higher temperatures. That means the region is too warm to allow ammonia to condense. Chemical reactions created by sunlight’s effect on the gases are very active in Jupiter’s atmosphere. In small regions, where convection (heat transport from one region to another) is especially strong, the updrafts may be fast enough to form fresh ammonia ice. Such regions do exist and have been spotted by spacecraft over the years.

Irwin’s team suggests that when moist, ammonia-rich air gets raised upwards, ammonia gets destroyed. It could also be mixed with photochemical products faster than ammonia ice can form. That means the main cloud deck may actually be composed of ammonium hydrosulphide mixed with photochemical, smoggy products. That’s what produces the red and brown colors we see in Jupiter images. And, this method also works for observations of ammonia clouds in Saturn’s atmosphere. Further work should help determine if the same photochemical processes exist there.

For More Information

Citizen Science Reveals Insight into Jupiter
Clouds and Ammonia in the Atmospheres of Jupiter and Saturn Determined From a Band-Depth Analysis of VLT/MUSE Observations
Spatial Variations of Jovian Tropospheric Ammonia via Ground-Based Imaging

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