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Register a new userColour and Tropospheric Cloud Structure of Jupiter from MUSE/VLT: Retrieving a Universal Chromophore
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Recent work by Sromovsky et al. (2017, Icarus 291, 232-244) suggested that all red colour in Jupiters atmosphere could be explained by a single colour-carrying compound, a so-called universal chromophore. We tested this hypothesis on ground-based spectroscopic observations in the visible and near-infrared (480-930 nm) from the VLT/MUSE instrument between 2014 and 2018, retrieving a chromophore absorption spectrum directly from the North Equatorial Belt, and applying it to model spatial variations in colour, tropospheric cloud and haze structure on Jupiter. We found that we could model both the belts and the Great Red Spot of Jupiter using the same chromophore compound, but that this chromophore must exhibit a steeper blue-absorption gradient than the proposed chromophore of Carlson et al. (2016, Icarus 274, 106-115). We retrieved this chromophore to be located no deeper than 0.2+/-0.1 bars in the Great Red Spot and 0.7+/-0.1 bars elsewhere on Jupiter. However, we also identified some spectral variability between 510 nm and 540 nm that could not be accounted for by a universal chromophore. In addition, we retrieved a thick, global cloud layer at 1.4+/-0.3 bars that was relatively spatially invariant in altitude across Jupiter. We found that this cloud layer was best characterised by a real refractive index close to that of ammonia ice in the belts and the Great Red Spot, and poorly characterised by a real refractive index of 1.6 or greater. This may be the result of ammonia cloud at higher altitude obscuring a deeper cloud layer of unknown composition.
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A new laboratory-generated chemical compound made from photodissociated ammonia (NH3) molecules reacting with acetylene (C2H2) was suggested as a possible coloring agent for Jupiters Great Red Spot (GRS) by Carlson et al. (2016, Icarus 274, 106-115). Baines et al. (2016, Icarus, submitted) showed that the GRS spectrum measured by the visual channels of the Cassini VIMS instrument in 2000 could be accurately fit by a cloud model in which the chromophore appeared as a physically thin layer of small particles immediately above the main cloud layer of the GRS. Here we show that the same chromophore and same layer location can also provide close matches to the short wave spectra of many other cloud features on Jupiter, suggesting this material may be a nearly universal chromophore that could explain the various degrees of red coloration on Jupiter. This is a robust conclusion, even for 12% changes in VIMS calibration and large uncertainties in the refractive index of the main cloud layer due to uncertain fractions of NH4SH and NH3 in its cloud particles. The chromophore layer can account for color variations among north and south equatorial belts, equatorial zone, and the Great Red Spot, by varying particle size from 0.12 microns to 0.29 microns and 1-micron optical depth from 0.06 to 0.76. The total mass of the chromophore layer is much less variable, ranging from 18 to 30 micrograms/cm^2, except in the equatorial zone, where it is only 10-13 micrograms/cm^2. We also found a depression of the ammonia volume mixing ratio in the two belt regions, which averaged 0.4-0.5 X 10^{-4} immediately below the ammonia condensation level, while the other regions averaged twice that value.
Radar data collected at an experimental facility arranged on purpose suggest that the footprint of atmospheric turbulence might be encoded in the radar signal statistics. Radar data probability distributions are calculated and nicely fitted by a one parameter family of generalized Gumbel (GG) distributions, G(a). A relation between the wind strength and the measured shape parameter a is obtained. Strong wind fluctuations return pronounced asymmetric leptokurtic profiles, while Gaussian profiles are eventually recovered as the wind fluctuations decrease. Besides stressing the crucial impact of air turbulence for radar applications, we also confirm the adequacy of G(a) statistics for highly correlated complex systems.
We revisit the issue of sensitivity to initial flow and intrinsic variability in hot-Jupiter atmospheric flow simulations, originally investigated by Cho et al. (2008) and Thrastarson & Cho (2010). The flow in the lower region (~1 to 20 MPa) `dragged to immobility and uniform temperature on a very short timescale, as in Liu & Showman (2013), leads to effectively a complete cessation of variability as well as sensitivity in three-dimensional (3D) simulations with traditional primitive equations. Such momentum (Rayleigh) and thermal (Newtonian) drags are, however, ad hoc for 3D giant planet simulations. For 3D hot-Jupiter simulations, which typically already employ strong Newtonian drag in the upper region, sensitivity is not quenched if only the Newtonian drag is applied in the lower region, without the strong Rayleigh drag: in general, both sensitivity and variability persist if the two drags are not applied concurrently in the lower region. However, even when the drags are applied concurrently, vertically-propagating planetary waves give rise to significant variability in the ~0.05 to 0.5 MPa region, if the vertical resolution of the lower region is increased (e.g. here with 1000 layers for the entire domain). New observations on the effects of the physical setup and model convergence in `deep atmosphere simulations are also presented.
We extracted a Martian water-ice cloud climatology from OMEGA data covering 7 Martian years (MY 26-32). We derived two products, the Reversed Ice Cloud Index (ICIR) and the Percentage of Cloudy Pixels (PCP), indicating the mean cloud thickness and nebulosity over a regular grid (1{deg} longitude x 1{deg} latitude x 1{deg} Ls x 1 h Local Time). The ICIR has been shown to be a proxy of the water-ice column derived from the Mars Climate Database. The PCP confirms the location of the main cloud structures mapped with the ICIR, and gives a more accurate image of the cloud cover. We observed a denser cloud coverage over Hellas Planitia, Lunae Planum and over large volcanoes in the aphelion belt. For the first time, thanks to the fact that Mars Express is not in Sun-synchronous orbit, we can explore the cloud diurnal cycle at a given season by combining 7 years of observations. However, because of the eccentric orbit, the temporal coverage remains limited. Other limitations of the dataset are its small size, the difficult distinction between ice clouds and frosts, and the impact of surface albedo on data uncertainty. We could nevertheless study the diurnal cloud life cycle by averaging the data over larger regions: from specific topographic features (covering a few degrees in longitude and latitude) up to large climatic bands (all longitudes). We found that in the tropics around northern summer solstice, the diurnal thermal tide modulates the abundance of clouds, which is reduced around noon. At northern midlatitudes, clouds corresponding to the edge of the north polar hood are observed mainly in the morning and around noon during northern winter (Ls=260-30{deg}). Over Chryse Planitia, low lying morning fogs dissipate earlier and earlier in the afternoon during northern winter. Over Argyre, clouds are present over all daytime during two periods, around Ls = 30 and 160{deg}.
Context. The tropospheric wind pattern in Jupiter consists of alternating prograde and retrograde zonal jets with typical velocities of up to 100 m/s around the equator. At much higher altitudes, in the ionosphere, strong auroral jets have been discovered with velocities of 1-2 km/s. There is no such direct measurement in the stratosphere of the planet. Aims. In this paper, we bridge the altitude gap between these measurements by directly measuring the wind speeds in Jupiters stratosphere. Methods. We use the Atacama Large Millimeter/submillimeter Arrays very high spectral and angular resolution imaging of the stratosphere of Jupiter to retrieve the wind speeds as a function of latitude by fitting the Doppler shifts induced by the winds on the spectral lines. Results. We detect for the first time equatorial zonal jets that reside at 1 mbar, i.e. above the altitudes where Jupiters Quasi-Quadrennial Oscillation occurs. Most noticeably, we find 300-400 m/s non-zonal winds at 0.1 mbar over the polar regions underneath the main auroral ovals. They are in counter-rotation and lie several hundreds of kilometers below the ionospheric auroral winds. We suspect them to be the lower tail of the ionospheric auroral winds. Conclusions. We detect directly and for the first time strong winds in Jupiters stratosphere. They are zonal at low-to-mid latitudes and non-zonal at polar latitudes. The wind system found at polar latitudes may help increase the effciency of chemical complexification by confining the photochemical products in a region of large energetic electron precipitation.
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