The collaborative effort of amateur and professional astronomers has led to a breakthrough in understanding clouds on Jupiter and has overturned a decades-old presumption regarding their nature. Owing to work by the amateur astronomer Dr. Steven Hill, the clouds appear not made of ammonia ice, as everyone had considered. Instead, they are likely composed of ammonium hydrosulphide mixed with smog.
Based in Colorado, Dr. Hill demonstrated that ammonia abundance and cloud-top pressures in Jupiter’s atmosphere could be mapped using commercially available telescopes and special filters. His simple method yielded ammonia maps remarkably similar to those produced by more complex, costly techniques.
These findings suggested that the clouds were located deeper within Jupiter’s atmosphere, where it is too warm for ammonia to condense into ice.
Building on Dr. Hill’s technique, Professor Patrick Irwin of the University of Oxford applied the method to observations made with the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope in Chile.
The team simulated how light interacts with gases and clouds. The results confirmed that the clouds are formed at more profound pressures and are instead likely composed of ammonium hydrosulphide—a compound produced by reactions between ammonia and hydrogen sulfide—and a mix of smoggy photochemical products.
First 3D view of Jupiter’s atmosphere
Based on comparing brightness values from adjacent narrow filters, Dr. Hill’s method produces results consistent with those obtained from far more complex and computationally expensive techniques. Moreover, the simplicity and speed of this approach means that amateur astronomers worldwide can now track ammonia variations and cloud-top pressures across Jupiter’s atmosphere.
Hence, the team concludes that Jupiter’s clouds are at more profound pressures than the expected ammonia clouds at 700 mb and cannot be composed of pure ammonia ice.
“This simple method allows us to probe deeper into Jupiter’s atmosphere than ever before,” said Professor Irwin. “It’s incredible that an amateur astronomer with commercial equipment can contribute to such significant scientific discoveries.”
Dr. Hill added, “I never imagined that my observations would lead to such an impactful discovery, but it shows how amateur astronomers can contribute meaningfully to professional science.”
Citizen scientists could also use the ammonia maps created using this method to track weather patterns on Jupiter, such as the Great Red Spot, and further our understanding of the planet’s dynamic atmosphere.
Why is ammonia not forming the thick clouds that scientists expected?
The answer lies in photochemistry. In Jupiter‘s atmosphere, chemical reactions driven by the Sun’s radiation break down ammonia molecules before they can condense into ice. As moist, ammonia-rich air rises within the planet’s atmosphere, it is quickly destroyed or mixed with photochemical products, such as sulfur compounds, creating a smoggy mixture that forms the planet’s characteristic cloud colors.
The updrafts may be fast enough to form fresh ammonia ice in small regions with extreme convection. Spacecrafts like NASA’s Galileo and NASA’s Juno have occasionally seen such regions, where a few small high white clouds have been seen, casting their shadows down on the main cloud deck below.
Professor Irwin and his team also applied the method to VLT/MUSE observations of Saturn and have found similar agreement in the derived ammonia maps with other studies. Similarly, they have found the main level of reflection to be well below the expected ammonia condensation level, suggesting that similar photochemical processes are occurring in Saturn’s atmosphere.
Journal 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. DOI: 10.1029/2024JE008622
