The Spatial Variation of Water Clouds, NH3, and H2O on Jupiter Using Keck Data at 5 Microns

Abstract

We obtained high-resolution spectra of Jupiter between 4.6 and 5.4 µm using NIRSPEC on the Keck 2 telescope in February 2017. We measured the spatial variation of NH3, H2O, and the pressure level of deep (p > 3 bar) clouds using two geometries. We aligned the slit north–south on Jupiter’s Central Meridian to measure the spatial variation of the gas composition and cloud structure between 66°N and 70°S. With the slit aligned east–west, we also examined the longitudinal variation at two regions of the North Equatorial Belt (NEB) at 18°N and at 8°N near the latitude of the Galileo Probe entry site. We used the integrated line absorption, also known as the equivalent width, of deuterated methane (CH3D) at 4.66 µm to derive the pressure level of deep clouds between 3 and 7 bar. From thermochemical models, these are most likely water clouds. At the location of a deep cloud revealed by HST methane-band imaging, we found spectroscopic evidence for an opaque cloud at the 5 bar level. We also identified regions on Jupiter that lacked deep clouds but exhibited evidence for upper clouds and enhanced NH3. We estimated column-averaged mole fractions of H2O and NH3 above the opaque lower boundary of the deep cloud. The meridional scan exhibited significant belt-zone structure with retrieved NH3 abundances in the 200–400 ppm range above the opaque lower cloud, except for a depletion (down to 90 ppm) in the NEB. Water in Jupiter’s belts varies from a maximum of 7 ppm at 8°S to a minimum of 1.5 ppm at 23°S. We found evidence for water clouds and enhanced NH3 and H2O in the South Equatorial Belt Outbreak region at 13°S. The NEB is a heterogeneous region with significant variation in all of these quantities. The NH3 abundance at 18°N and 8°N varies with the longitude with mole fractions between 120 and 300 ppm. The H2O abundance at these same latitudes varies with the longitude with mole fractions between 3 and 10 ppm. Our volatile mole fractions apply to the 5 to 8 bar pressure range (or to the level of an opaque cloud top where found at shallower pressure); therefore, they imply a deeper gradient continuing to increase toward higher concentrations detected by the Galileo Probe Mass Spectrometer at 11 and 20 bar. Hot Spots in the NEB exhibit minimal cloud opacity; however, they lack prominent anomalies in the concentrations of NH3 or H2O.