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

Presentation #302.01 in the session A Wet and Cloudy Jupiter.

Published onOct 20, 2022
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 4th 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 30oS, the Equator, 23oN, and 35oN in zones, but they are also present at certain longitudes within belts, especially in the NEB at 8oN. 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

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