For decades, scientists believed that Jupiter’s iconic clouds were primarily composed of ammonia ice. This assumption was rooted in early observations of the gas giant’s atmosphere and was supported by theoretical models of cloud formation. However, new collaborative research by both amateur and professional astronomers has led to a groundbreaking discovery: Jupiter’s clouds are unlikely to consist of pure ammonia ice, as previously thought. Instead, they are most likely made up of ammonium hydrosulfide mixed with photochemical products or smog. This revelation, published in the Journal of Geophysical Research: Planets, has rewritten our understanding of the gas giant’s atmosphere.
The initial breakthrough that led to this discovery came from an unlikely source—Dr. Steven Hill, an amateur astronomer based in Colorado. Dr. Hill has long been interested in planetary observation, and using commercially available telescopes combined with specially designed colored filters, he developed an innovative method for mapping ammonia and cloud-top pressures in Jupiter’s atmosphere. His work demonstrated that ammonia could be mapped across Jupiter’s clouds and provided new insights into the deeper layers of the atmosphere.
Dr. Hill’s preliminary findings were startling because they suggested that the ammonia-rich clouds of Jupiter might not be situated at the expected level, where ammonia ice would condense in the atmosphere. In fact, the clouds were found to reside in much deeper, warmer regions than initially hypothesized. This raised significant questions about the conventional view that Jupiter’s clouds were made primarily of ammonia ice.
The potential implications of Dr. Hill’s observations piqued the interest of professional scientists, particularly Professor Patrick Irwin, who is part of the Department of Physics at the University of Oxford. In collaboration with colleagues, Professor Irwin used Dr. Hill’s approach to analyze Jupiter’s atmosphere in greater detail. They applied this method to data collected using the MUSE (Multi Unit Spectroscopic Explorer) instrument, located at the European Southern Observatory’s Very Large Telescope (VLT) in Chile. This instrument allowed the team to conduct highly detailed spectroscopic observations of Jupiter, capturing the subtle differences in light wavelengths reflected by the planet’s atmosphere. These differences act as “fingerprints” that provide insights into the chemical composition and structure of the clouds.
Through sophisticated modeling of these observations, Irwin’s team found that the visible clouds on Jupiter—those most visible through both professional and amateur telescopes—are situated deeper within the planet’s atmosphere than previously imagined. In particular, these clouds reside in areas of higher pressure and temperature, making ammonia ice condensation unlikely. Based on this new data, they concluded that the clouds are not primarily made of ammonia ice as had been the prevailing view for so long. Instead, the clouds are composed of ammonium hydrosulfide, a chemical compound formed by the reaction of ammonia and hydrogen sulfide at the planet’s extreme conditions.
This finding was reinforced by previously collected data, including that from MUSE observations, although previous studies used more complex, highly advanced techniques to arrive at similar conclusions. These earlier analyses were difficult to verify due to their highly technical nature. In contrast, the method proposed by Dr. Hill, which involves simply comparing brightness across adjacent, narrow colored filters, provided the same result with far less complexity. This simplicity makes it easier to replicate the findings and confirms the deeper pressure levels of the clouds, along with their non-ammonia composition. In fact, Irwin and his team were astonished at the clarity and depth of insight gained from such a straightforward approach.
Dr. Steven Hill, a former astrophysics Ph.D. from the University of Colorado who works in space weather forecasting, had set out to push the boundaries of amateur observation and see what physical measurements he could make using inexpensive, commercial equipment. To his delight, his method proved invaluable, offering professional scientists a new way to track atmospheric processes on Jupiter. “I didn’t expect this outcome when I started using commercial equipment, but I’m glad my efforts have contributed meaningfully to understanding one of Jupiter’s greatest mysteries,” said Dr. Hill. The results, published by Irwin’s team, mark a significant step forward in planetary science.
The simplicity and speed of Dr. Hill’s approach have significant implications for the future of astronomical observation. This method can be employed by amateur astronomers and citizen scientists using modest equipment and at a fraction of the computational cost typically involved in more advanced techniques. As a result, the study of Jupiter’s atmospheric phenomena—such as ammonia levels, cloud-top pressures, and weather patterns—can now be conducted more frequently. This increased level of accessibility means that many more people, including non-professional astronomers, could become involved in tracking atmospheric variations on Jupiter.
One of the key insights arising from the study was the role of photochemistry in shaping Jupiter’s clouds. Photochemistry refers to chemical reactions that are initiated by sunlight. These reactions are particularly significant in the upper atmosphere of gas giants like Jupiter, where sunlight causes the breakdown of ammonia and other volatile chemicals. In regions where ammonia-rich air rises, photochemical processes destroy ammonia or convert it into different compounds before it has a chance to condense and form clouds of ammonia ice.
Professor Irwin and his team hypothesize that the main cloud decks of Jupiter are composed of ammonium hydrosulfide, a substance that forms when ammonia reacts with hydrogen sulfide—a common compound in Jupiter’s atmosphere—combined with other photochemically-derived compounds, such as hydrocarbons. These compounds likely contribute to the red and brown colors we observe in Jupiter’s clouds, lending the planet its striking appearance.
Interestingly, in areas of the atmosphere where updrafts and convection are particularly strong, small patches of ammonia ice clouds can form, as they might carry the ammonia high enough to condense before being broken down by the intense sunlight. Such formations have been observed by spacecraft like NASA’s Galileo and Juno, which have recorded occasional small white clouds at high altitudes casting shadows on the main cloud deck below.
Building on their work with Jupiter, Irwin’s team has applied their method to Saturn, another gas giant with a similarly enigmatic atmosphere. Their results have shown comparable results regarding ammonia distribution in Saturn’s clouds. By using the MUSE data, the research team found that the primary reflectivity of Saturn’s clouds occurs at pressure levels deeper than those expected for ammonia condensation, which suggests that photochemical processes on Saturn’s atmosphere are likely similar to those on Jupiter.
Ultimately, this collaborative research by both amateurs and professionals has drastically expanded our understanding of the chemical composition of Jupiter’s clouds. It’s also opened new doors for future contributions by citizen scientists in planetary science. Amateur astronomers and hobbyists, who have traditionally been limited to simpler observations, are now able to make real contributions to answering fundamental questions about the nature of planetary atmospheres.
This breakthrough in our understanding of Jupiter’s clouds invites even deeper questions about the intricate dynamics at play in the atmosphere of gas giants and emphasizes how vital collaboration across scientific communities—from amateurs to professionals—can be in advancing our exploration of the cosmos. By challenging established scientific notions and embracing more accessible, creative methods, the field of planetary science continues to evolve in exciting and unexpected ways.
Reference: Patrick G. J. Irwin et al, 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 (2025). DOI: 10.1029/2024JE008622