Clouds and Ammonia in the Atmospheres of Jupiter and Saturn Determined From a Band-Depth Analysis of VLT/MUSE Observations

Abstract

The visible spectrum of Jupiter contains absorption bands of methane (619 nm) and ammonia (647 nm) that can be used to probe the cloud-top pressures and ammonia abundance in Jupiter's atmosphere. Recently, it has been shown that filter-averaged observations of Jupiter made with telescopes and filters accessible to backyard astronomers can be reduced to yield ammonia maps that bear a remarkable similarity with distributions derived using more complex radiative transfer methods. Here, we determine the reliability of this method by applying it to observations made with the MUSE instrument at ESO's Very Large Telescope, and find excellent correspondence with the retrieved products from multiple-scattering retrieval model analyses. We find that the main level of reflection in Jupiter's atmosphere is at 2–3 bar, which is far beneath the anticipated ammonia ice condensation level at $\sim$ 0.7 bar, and conclude that pure ammonia ice cannot be the main cloud constituent. We show that the spatial variations of ammonia determined at 2–3 bar are strongly correlated with those determined from thermal-infrared observations, and microwave observations by the Very Large Array and the Juno spacecraft. Finally, we show that the same technique can be applied to observations of Saturn, again yielding maps of ammonia abundance at 2–3 bar that are well-correlated with thermal-IR observations made near 5 $\mu$m by Cassini/VIMS and JWST/MIRI. Similarly, the main level of reflectivity is found to be lie far beneath the expected condensation level of ammonia in Saturn's atmosphere at $\sim$ 1.8 bar.