Jupiter's Clouds Not What They Seem, Study Finds Deeper Ammonia Presence

In a surprising turn of events that could change how we understand one of our solar system’s most iconic planets, scientists have uncovered new insights into the atmospheric composition of Jupiter. Using data from the European Southern Observatory’s Very Large Telescope (VLT) and its instrument MUSE, researchers have discovered that Jupiter’s clouds are not what they seem, harboring ammonia at much deeper levels than previously thought.

The study, led by Patrick G. J. Irwin from the University of Oxford, along with colleagues Steven M. Hill, Leigh N. Fletcher, Charlotte Alexander, and John H. Rogers, focused on analyzing the visible spectrum of Jupiter. They used the absorption bands of methane at 619 nm and ammonia at 647 nm to probe the planet’s cloud layers. Traditionally, it was believed that ammonia would condense into clouds at around 0.7 bar pressure, creating a visible layer. However, this new research points to a different story.

The team applied a method known as the band-depth approximation, which had previously been shown to work for amateur observations with modest telescopes. This technique, when applied to the high-resolution spectra from MUSE, confirmed that the main level of reflection in Jupiter’s atmosphere occurs much deeper, between 2 and 3 bar. This is significantly below the expected ammonia condensation level, suggesting that the clouds we see are not primarily composed of pure ammonia ice.

One of the most interesting findings was the spatial variation of ammonia. The study mapped how ammonia abundance at these deeper levels correlates with thermal-infrared observations from spacecraft like Juno and microwave observations from the Very Large Array (VLA). “We found that the ammonia distribution at 2-3 bar is strongly correlated with what we’ve seen in other spectral regions,” says Irwin. The data showed a significant enhancement of ammonia around 2-4 degrees north latitude, with a notable depletion in the North Equatorial Belt, ranging from 7 to 20 degrees north. This pattern matches previous observations, reinforcing the reliability of the method used.

But why does this matter? The distribution of ammonia can tell scientists about the dynamics of Jupiter’s atmosphere. Areas with high ammonia might indicate upwelling motion where deep gases are brought to higher altitudes, while low ammonia regions could suggest downwelling, where gases are pushed deeper into the atmosphere.

The research also sheds light on one of Jupiter’s most famous features, the Great Red Spot (GRS). The study found a slight depletion of ammonia within this massive storm, suggesting that the air might be trapped within the vortex, allowing more time for photochemical processes to alter the ammonia into other compounds or what are known as “chromophores,” which give the GRS its distinctive color.

Moreover, the study tackled the issue of “North Equatorial Dark Features” (NEDFs), also known as 5-micron hotspots. These areas, which appeared dark in visible light but bright in thermal infrared due to less cloud cover, were found to have complex ammonia dynamics. The researchers noted higher ammonia to the south and east of these features, adding another layer to understanding Jupiter’s atmospheric circulation.

Reference

J. Irwin, P. G., Hill, S. M., Fletcher, L. N., Alexander, C., & Rogers, J. H. (2024). 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, 130(1), e2024JE008622. https://doi.org/10.1029/2024JE008622

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