The north-south variation of water clouds, NH₃, and H₂O on Jupiter

We obtained spectra of Jupiter at 5 microns at a resolving power of 20,000 using the NIRSPEC spectrometer on the Keck telescope in Hawaii. The data were acquired near the time of the 4^th perijove of the Juno spacecraft in February 2017. With the slit aligned north-south we obtained spectra near the ground track of the Microwave Radiometer on Juno. Method: Cloud top pressures between 3 and 7 bars were derived using the strength of CH₃D absorption at 4.66 microns. The fraction of reflected sunlight was measured by comparing the strength of a Fraunhofer line on Jupiter at 4.67 microns with its counterpart on the Sun, as measured by the ATMOS investigation (Farmer 1987). Column abundances of NH₃ and H₂O were measured using absorption features near 4.96 microns. These were converted to mole fractions using the cloud structure derived from CH₃D and the Fraunhofer line.

Results: 1) Water clouds are present in both belts and zones; they are the source of most of the variation in continuum brightness in images at 5 microns. Thick water clouds are present at 30^oS, the Equator, 23^oN, and 35^oN in zones, but they are also present at certain longitudes within belts, especially in the NEB at 8^oN. 2) Our technique allows us to determine, in certain locations, whether 5-micron continuum absorption is due to upper clouds, such as NH₃ and NH₄SH, or due to water clouds. This has not been possible using 5-micron imaging alone. 3) The fraction of reflected sunlight in the darkest regions at 5 microns can exceed 20%. In the Equatorial Zone this value can reach 60%. The emission from brighter regions, such as belts and Hot Spots, consists entirely of thermal radiation. 4) We obtained good agreement with previous retrievals of NH₃ derived from both the JIRAM instrument on Juno (Grassi et al. 2020) and from ground-based observations. We obtained good agreement with the spatial variation of H₂O reported by Grassi et al. and by Giles et al (2015) using Cassini/VIMS data for the 4-bar level in Jupiter’s atmosphere.

References: Farmer, C. B. 1987. High resolution infrared spectroscopy of the Sun and the Earth’s atmosphere from space. Mikrochim. Acta III, 189-214.

Giles, R.S.; Fletcher, L.N.; Irwin, P.G.J. 2015. Cloud structure and composition of Jupiter’s troposphere from 5-μm Cassini VIMS spectroscopy. Icarus 257, 457–470, doi:10.1016/j.icarus.2015.05.030.

Grassi, D. and the JIRAM team 2020. On the Spatial Distribution of Minor Species in Jupiter’s Troposphere as Inferred from Juno JIRAM data. J. Geophys. Res. doi.org/10.1029/2019JE006206