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 along the ground track of the Microwave Radiometer on Juno. When the slit was oriented east-west, we investigated spatial variability in the North and South Equatorial Belts (NEB, SEB). Method: Cloud top pressures between 4 and 8 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 the NEB at 8oN. We found only a few regions which are dark at 5 microns and lack water clouds. Here we attribute the continuum absorption to upper clouds, such as NH3 and NH4SH. 2) 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. 3) We obtained good agreement with the retrieved abundances of NH3 derived from the JIRAM instrument on Juno (Grassi et al. 2020). 4) We obtained qualitative agreement with the spatial variation of H2O reported by Grassi et al., but there are differences in retrieved vertical profiles. Keck data require a more rapid falloff in H2O for pressures less than 5 bars in order to fit the wings of strong water lines.
References: Farmer, C. B. (1987). “High resolution infrared spectroscopy of the Sun and the Earth’s atmosphere from space.” Mikrochim. Acta III, 189-214.
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