The Cloud Structure and Ammonia Abundance in Jupiter’s Equatorial Zone Derived from 5-µm Spectra

Abstract:

We have acquired ground-based spectra with the Keck 2 and IRTF telescopes of Jupiter's equatorial regions at 5 µm each year between 2017 and 2020. In 2018, a quasi-periodic disturbance in Jupiter’s Equatorial Zone (EZ) developed, resulting in a narrow region in latitude and longitude of enhanced 5-μm radiance. We have modeled spectra near the equator to characterize the deep cloud structure and ammonia abundance. Regions that are very bright at 5 µm lack water clouds, whereas dark regions have opaque clouds at the 4- to 5-bar level. In 2017 the brightest region at the equator was at the tip of a visibly dark, chevron-shaped feature that extended into the North Equatorial Belt. However, by 2019 there were several discrete bright spots centered on the normally uniformly cold equator that clearly lack water clouds, while in 2020 they had extended in longitude and faded in brightness. The EZ disturbance may be associated with a slowly descending wave of dry air that reduces the opacity of one or more cloud layers. Spectra at 5 µm reveal the extent to which the EZ disturbance affected Jupiter's water clouds and the time scale over which this occurred. The EZ is the region on Jupiter where the deep atmosphere is best constrained by the microwave radiometer investigation on the Juno spacecraft, and our interest in this phenomenon is motivated by large-scale changes in past episodes (Antuñano et al. 2018). We will compare ammonia abundances retrieved from ground-based spectra with the Juno NH₃ abundance of 350 ppm at 3°N reported by Li et al. (2020).

References

Antuñano, A. et al. Geophys. Res. Lett. 45, 10987-10995. (2018).

Li, C. et al. Nature Astronomy, doi:10.1038/s41550-020-1009-3. (2020).