A team of scientists and amateur astronomers has challenged a long-held belief regarding Jupiter's clouds, traditionally thought to be primarily composed of frozen ammonia. This breakthrough alters the understanding of the gas giant's atmospheric composition.
Utilizing commercial telescopes and spectral filters, amateur astronomer Steve Hill gathered data to map ammonia abundance in Jupiter's atmosphere. However, Hill's findings contradicted previous models of the planet's atmospheric makeup.
Patrick Irwin from Oxford University expressed initial skepticism about Hill's old technique but soon recognized its significance. The analysis revealed that Jupiter's atmosphere comprises mostly hydrogen and helium, alongside trace amounts of ammonia, methane, water vapor, and other gases, which form clouds that reflect sunlight, giving the planet its striking appearance.
Traditionally, scientists assumed that the upper clouds were mainly composed of ammonia ice due to its presence in the atmosphere and its tendency to condense at relatively low pressures. Irwin explained that astronomers often adopt a simplified model unless compelling evidence suggests otherwise.
In 2023, Irwin connected with Hill through a mutual acquaintance after Hill presented his intriguing observations. Hill employed a technique from the 1970s and 1980s, known as absorption band depth analysis, to estimate gas concentrations based on light absorption at specific wavelengths, successfully calculating ammonia distribution above Jupiter's clouds.
Further analysis indicated that the reflected light originated from cloud layers where atmospheric pressure was too high and temperatures too elevated for ammonia to condense. Irwin noted that the primary reflective layer is significantly deeper than where ammonia should condense, situated at 2-3 bars.
Consequently, it was concluded that Jupiter's clouds are likely composed of ammonium hydrosulfide and possibly particles produced by photochemical reactions, although the exact composition remains uncertain. Another hypothesis suggests that the clouds could be an exotic mix of water and ammonia.
These findings point to complex photochemical processes occurring in Jupiter's atmosphere. Irwin remarked that in most regions, ammonia is destroyed faster than it can be transported upward, making pure ammonia ice clouds relatively rare and confined to areas with intense convection.
To validate Hill's observations, the team compared the data with advanced analyses conducted using the MUSE instrument on the Very Large Telescope (VLT) of ESO, alongside VLA and NASA's Juno mission. This comparison corroborates the findings and opens new avenues for more accessible observations of Jupiter and similar planets like Saturn.
Hill emphasized that understanding where ammonia exists provides vital clues about meteorological processes on Jupiter, which is crucial for comprehending this planet and others like it. Despite the significant progress, limitations remain, including the assumed 'vertical' profile of ammonia, which may vary with altitude.
Irwin added that closer comparisons of VLT/MUSE, Juno, and VLA results are necessary to understand ammonia distribution at different altitudes. The contributions of both amateur and professional astronomers highlight the value of even seemingly 'simple' observations in expanding knowledge about the cosmos.