A collaboration between amateur and professional astronomers has helped solve an old question regarding the composition of Jupiter’s clouds, according to a study published in the Journal of Geophysical Research – Planets. Instead of ammonia ice, it now appears they are likely to be composed of ammonium hydrosulphide mixed with smog.
This work started with amateur astronomer Dr Steven Hill, who showed that the abundance of ammonia and cloud-top pressure in Jupiter’s atmosphere could be analysed using commercially available telescopes and a few specially coloured filters. These initial results showed that the clouds are too deep within Jupiter’s warm atmosphere to be made out of ammonia ice.
In this new study, Professor Patrick Irwin from the University of Oxford’s Department of Physics used Dr Steven Hill’s analytical method with a Multi Unit Spectroscopic Explorer (MUSE) instrument at Chile’s European Southern Observatory’s Very Large Telescope (VLT). MUSE uses spectroscopy to show Jupiter’s gases in visible light at different wavelengths, which then can be used to map the ammonia and cloud heights.
Professor Irwin and his team used a computer model to find that the primary clouds of Jupiter—the ones that can be seen when looking through backyard telescopes—had to be much deeper than previously thought, in a region of higher pressure and higher temperature. This means the conditions are too warm for the condensation of ammonia. Instead, those clouds are made of something different: ammonium hydrosulphide.
Previous analyses of MUSE observations had hinted at these results, but these analyses relied on extremely complex methods that can only be done by a limited number of groups around the world, and it wasn’t easy to corroborate. Now, Irwin’s team found that Dr. Hill’s is a much simpler method that relies on comparing the brightnesses in adjacent, narrow-coloured filters to obtain identical results. As this new method is much faster and very simple, confirming the results is easier. The team concluded that the clouds of Jupiter are at deeper pressures than previously believed, and so cannot be composed of pure ammonia ice.
“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! 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,” said Professor Irwin.
“I always like to push my observations to see what physical measurements I can make with modest, commercial equipment. 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,” added Dr Steven Hill, who has a PhD in Astrophysics from the University of Colorado and works in weather forecasting.
Professor Irwin suggested that in regions where ammonia-rich air is raised upwards, the ammonia is mixed with photochemical products faster than ammonia ice can form. Thus, the leading cloud deck will likely be ammonium hydrosulphide mixed with photochemical, smoggy products, which produce the red and brown colours seen in Jupiter images.
Irwin P et al (2025) Clouds and ammonia in the atmospheres of Jupiter and Saturn determined from a band-depth analysis of VLT/MUSE observations. Journal of Geophysical Research – Planets, https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024JE008622