Cloud structure and composition of Jupiter’s troposphere from 5-μm Cassini VIMS spectroscopy

Abstract

Jupiter's tropospheric composition and cloud structure are studied using Cassini VIMS 4.5-5.1 µm thermal emission spectra from the 2000-2001 flyby. We make use of both nadir and limb darkening observations on the planet's nightside, and compare these with dayside observations. Although there is significant spatial variability in the 5-μm brightness temperatures, the shape of the spectra remain very similar across the planet, suggesting the presence of a spectrally-flat, spatially inhomogeneous cloud deck. We find that a simple cloud model consisting of a single, compact cloud is able to reproduce both nightside and dayside spectra, subject to the following constraints: (i) the cloud base is located at pressures of 1.2 bar or lower; (ii) the cloud particles are highly scattering; (iii) the cloud is sufficiently spectrally flat. Using this cloud model, we search for global variability in the cloud opacity and the phosphine deep volume mixing ratio. We find that the vast majority of the 5-μm inhomogeneity can be accounted for by variations in the thickness of the cloud decks, with huge differences between the cloudy zones and the relatively cloud-free belts. The relatively low spectral resolution of VIMS limits reliable retrievals of gaseous species, but some evidence is found for an enhancement in the abundance of phosphine at high latitudes.

Preliminary results on the composition of Jupiter's troposphere in hot spot regions from the JIRAM/Juno instrument

“…Its abundance increases from north moving toward the equator, but with a local minimum (9°N, 120°E) just north of the brightest region of the hot spot. Overall, the phosphine values are consistent with the global latitudinal trends inferred from VIMS-Cassini data [Giles et al, 2015] and ground-based observations [Giles et al, 2016]. The phosphine maps presented by Fletcher et al [2009] also suggest an increase of the mixing ratio from latitude 10°N toward the equator.…”

“…Albeit the high-spectral resolution measurements discussed by Giles et al [2016] suggest that a water deep cloud can be present at most latitudes on Jupiter, it would in any case tend to vanish at the typical hot spot latitudes. The large-scale latitudinal trends for water vapor presented by Giles et al [2015] in their Figure 13 is also only marginally consistent (1.2 ± 0.5%) with our retrievals in the regions surrounding the brightest part of the hot spot.…”

“…Jupiter's emission at 5 μm has been measured in the past at low latitude and midlatitude from space [Carlson et al, 1993;Roos-Serote et al, 1999;Giles et al, 2015] and ground-based observations [e.g., Fletcher et al, 2016;Giles et al, 2016]. At these wavelengths, the Jupiter disk presents strong signal contrasts, exceeding a factor 100 in radiance.…”

Detection of H3+auroral emission in Jupiter’s 5-micron window

“…We find that the 5-micron brightness temperatures vary between 200 K in the coolest parts of the planet and 260 K in the brightest regions. This is roughly consistent with the Cassini VIMS 5-µm observations, which showed a 190−240 K range (Giles et al 2015). Any differences are likely to be due to how centrally the standard star was located within the narrow slit.…”

Characterization of the white ovals on Jupiter's southern hemisphere using the first data by the Juno/JIRAM instrument

“…Two tropospheric clouds and a stratospheric haze describe the atmospheric particulate component. The properties of the deepest NH 4 SH cloud we considered are the ones suggested by Giles et al [2015]. They are assumed to have single scattering albedo ω = 0.9 and asymmetry factor g = 0.7 for the Henyey-Greenstein phase function in the entire spectral range of our interest.…”