Gas Giants

The gas giants (Jupiter and Saturn) and icy giants (Uranus and Neptune) are fluid planets with atmospheres primarily made of hydrogen and helium. The part of their atmospheres accessible to remote sensing occupies only a small fraction of their radii (0.05%). Clouds and hazes form around the 1 bar altitude pressure level and extend vertically, according to the thermochemical models, in a layer with a thickness of 200_500 km where temperature increases with depth (usually known as the "weather layer"). Clouds made of NH3, NH4SH, H2O (in Jupiter and Saturn), with the addition of CH4 (in Uranus and Neptune), cover the planet in stratified layers that are mixed with unknown hromophore agents. Dynamical phenomena in the weather layer shape different cloud patterns that define the visible appearance of these planets. In the thermal part of the spectrum clouds act as opacity sources providing brightness contrasts. The ensemble of cloud morphologies in terms of shapes, sizes and albedos allows their use as tracers of the atmospheric motions in the weather layer (Fig. 4.1). This is the main tool employed so far to study the winds on these fourplanets

An enduring rapidly moving storm as a guide to Saturn’s Equatorial jet’s complex structure

Saturn has an intense and broad eastward equatorial jet with a complex three-dimensional structure mixed with time variability. The equatorial region experiences strong seasonal insolation variations enhanced by ring shadowing, and three of the six known giant planetary-scale storms have developed in it. These factors make Saturn's equator a natural laboratory to test models of jets in giant planets. Here we report on a bright equatorial atmospheric feature imaged in 2015 that moved steadily at a high speed of 450 ms(-1) not measured since 1980-1981 with other equatorial clouds moving within an ample range of velocities. Radiative transfer models show that these motions occur at three altitude levels within the upper haze and clouds. We find that the peak of the jet ( latitudes 10 degrees N to 10 degrees S) suffers intense vertical shears reaching + 2.5 ms(-1) km(1), two orders of magnitude higher than meridional shears, and temporal variability above 1 bar altitude level. Palabras claveThis work is based on observations and analysis from Hubble Space Telescope (GO/DD program 14064), Cassini ISS images (NASA pds), and Calar Alto Observatory (CAHA-MPIA). A.S.-L. and UPV/EHU team are supported by the Spanish projects AYA2012-36666 and AYA2015-65041-P with FEDER support, Grupos Gobierno Vasco IT-765-13, Universidad del Pais Vasco UPV/EHU program UFI11/55, and Diputacion Foral Bizkaia (BFA). We acknowledge the contribution of Saturn images by T. Olivetti, M. Kardasis, A. Germano, A. Wesley, P. Miles, M. Delcroix, C. Go, T. Horiuchi and P. Maxon. We also acknowledge the wind model data provided by J. Friedson

Cassini observations reveal a regime of zonostrophic macroturbulence on Jupiter

In December 2000, the Cassini fly-by near Jupiter delivered high-resolution images of Jupiter’s clouds over the entire planet in a band between 50°N and 50°S. Three daily-averaged two-dimensional velocity snapshots extracted from these images are used to perform spectral analysis of jovian atmospheric macroturbulence. A similar analysis is also performed on alternative data documented by Choi and Showman (Choi, D., Showman, A. [2011]. Icarus 216, 597–609), based on a different method of image processing. The inter-comparison of the products of both analyses ensures a better constraint of the spectral estimates. Both analyses reveal strong anisotropy of the kinetic energy spectrum. The zonal spectrum is very steep and most of the kinetic energy resides in slowly evolving, alternating zonal (west–east) jets, while the non-zonal, or residual spectrum obeys the Kolmogorov–Kraichnan law specific to two-dimensional turbulence in the range of the inverse energy cascade. The spectral data is used to estimate the inverse cascade rate ∊ and the zonostrophy index Rβ for the first time. Although both datasets yield somewhat different values of ∊, it is estimated to be in the range 0.5–1.0 × 10−5 m2 s−3. The ensuing values of Rβ ≳ 5 belong well in the range of zonostrophic turbulence whose threshold corresponds to Rβ ≃ 2.5. We infer that the large-scale circulation is maintained by an anisotropic inverse energy cascade. The removal of the Great Red Spot from both datasets has no significant effect upon either the spectra or the inverse cascade rate. The spectral data are used to compute the rate of the energy exchange, W, between the non-zonal structures and the large-scale zonal flow. It is found that instantaneous values of W may exceed ∊ by an order of magnitude. Previous numerical simulations with a barotropic model suggest that W and ∊ attain comparable values only after averaging of W over a sufficiently long time. Near-instantaneous values of W that have been routinely used to infer the rate of the kinetic energy supply to Jupiter’s zonal flow may therefore significantly overestimate ∊. This disparity between W and ∊ may resolve the long-standing conundrum of an unrealistically high rate of energy transfer to the zonal flow. The meridional diffusivity Kϕ in the regime of zonostrophic turbulence is given by an expression that depends on ∊. The value of Kϕ estimated from the spectra is compared against data from the dispersion of stratospheric gases and debris resulting from the Shoemaker-Levy 9 comet and Wesley asteroid impacts in 1994 and 2009 respectively. Not only is Kϕ found to be consistent with estimates for both impacts, but the eddy diffusivity found from observations appears to be scale-independent. This behaviour could be a consequence of the interaction between anisotropic turbulence and Rossby waves specific to the regime of zonostrophic macroturbulence

Long-Term Evolution of the Aerosol Debris Cloud Produced by the 2009 Impact on Jupiter

We present a study of the long-term evolution of the cloud of aerosols produced in the atmosphere of Jupiter by the impact of an object on 19 July 2009. The work is based on images obtained during 5 months from the impact to 31 December 2009 taken in visible continuum wavelengths and from 20 July 2009 to 28 May 2010 taken in near-infrared deep hydrogen-methane absorption bands at 2.1-2.3 micron. The impact cloud expanded zonally from approximately 5000 km (July 19) to 225,000 km (29 October, about 180 deg in longitude), remaining meridionally localized within a latitude band from 53.5 deg S to 61.5 deg S planetographic latitude. During the first two months after its formation the site showed heterogeneous structure with 500-1000 km sized embedded spots. Later the reflectivity of the debris field became more homogeneous due to clump mergers. The cloud was mainly dispersed in longitude by the dominant zonal winds and their meridional shear, during the initial stages, localized motions may have been induced by thermal perturbation caused by the impact's energy deposition. The tracking of individual spots within the impact cloud shows that the westward jet at 56.5 deg S latitude increases its eastward velocity with altitude above the tropopause by 5- 10 m/s. The corresponding vertical wind shear is low, about 1 m/s per scale height in agreement with previous thermal wind estimations. We found evidence for discrete localized meridional motions with speeds of 1-2 m/s. Two numerical models are used to simulate the observed cloud dispersion. One is a pure advection of the aerosols by the winds and their shears. The other uses the EPIC code, a nonlinear calculation of the evolution of the potential vorticity field generated by a heat pulse that simulates the impact. Both models reproduce the observed global structure of the cloud and the dominant zonal dispersion of the aerosols, but not the details of the cloud morphology. The reflectivity of the impact cloud decreased exponentially with a characteristic timescale of 15 days; we can explain this behavior with a radiative transfer model of the cloud optical depth coupled to an advection model of the cloud dispersion by the wind shears. The expected sedimentation time in the stratosphere (altitude levels 5-100 mbar) for the small aerosol particles forming the cloud is 45-200 days, thus aerosols were removed vertically over the long term following their zonal dispersion. No evidence of the cloud was detected 10 months after the impact

Evolution of the cloud field and wind structure of Jupiter's highest speed jet during a huge disturbance

Aims. Despite the banded visual aspect of cloud patterns in Jupiter, high resolution images indicate that these regions are markedly turbulent. One region of particular interest is the north temperate belt (NTB) at 21° N planetocentric latitude, where the most intense Jovian jet resides with eastward peak speeds of 160-180 m s-1. Almost every 15 years, the NTB is known to experience an eruption or disturbance that dramatically changes its appearance, a phenomenon known as NTB disturbance (NTBD). In this work, we characterize the morphology of the disturbed cloud field in the wake of the plumes that caused the perturbation, and check for changes in the velocity or shape of the jet. Methods. The 2007 disturbance was witnessed with unprecedented resolution by the Hubble Space Telescope and by a long-term survey based on the “International Outer Planet Watch” (IOPW) network. Our analysis is based on the brightness spectral distribution to characterize both the typical spatial frequency of the perturbation and its turbulent and wavy nature. We also compare our characterization with non-linear dynamical simulations of the disturbance using the EPIC dynamical model. Finally, we obtain a renewed wind profile for the region of interest by cloud tracking. Results. We detect a change in the power spectral slope of the cloud brightness following the disturbance that is related to a change in the typical size of the observed structures. We model the initial disturbance as a Rossby wave. A comparison of the jet profile in the NTB just after the disturbance ended (June 2007) with one observed a year later (July 2008), illustrates a net change occurred in the westward jet at 16° N with a speed change of 25 m s-1. As implied by the power spectra analysis, the disturbance and its related Rossby wave dissipate. We propose that this dissipation produced a momentum transfer to the anticyclonic side of the NTB jet increasing the speed of the westward jet at 16° N as also supported by numerical simulations

A large active wave trapped in Jupiter’s equator

Context. A peculiar atmospheric feature was observed in the equatorial zone (EZ) of Jupiter between September and December 2012 in ground-based and Hubble Space Telescope (HST) images. This feature consisted of two low albedo Y-shaped cloud structures (Y1 and Y2) oriented along the equator and centred on it (latitude 0.5°−1°N). Aims. We wanted to characterize these features, and also tried to find out their properties and understand their nature. Methods. We tracked these features to obtain their velocity and analyse their cloud morphology and the interaction with their surroundings. We present numerical simulations of the phenomenon based on one- and two-layer shallow water models under a Gaussian pulse excitation. Results. Each Y feature had a characteristic zonal length of ~15° (18 000 km) and a meridional width (distance between the north-south extremes of the Y) of 5° (6000 km), and moved eastward with a speed of around 20−40 m s-1 relative to Jupiter’s mean flow. Their lifetime was 90 and 60 days for Y1 and Y2, respectively. In November, both Y1 and Y2 exhibited outbursts of rapidly evolving bright spots emerging from the Y vertex. The Y features were not visible at wavelengths of 255 or 890 nm, which suggests that they were vertically shallow and placed in altitude between the upper equatorial hazes and the main cloud deck. Numerical simulations of the dynamics of the Jovian equatorial region generate Kelvin and Rossby waves, which are similar to those in the Matsuno-Gill model for Earth’s equatorial dynamics, and reproduce the observed cloud morphology and the main properties the main properties of the Y features

Jupiter’s zonal winds and their variability studied with small-size telescopes

International audienc

Color and aerosol changes in Jupiter after a North Temperate Belt disturbance

The banded appearance of Jupiter’s atmosphere shows significant changes over time, sometimes even transforming the reflectivity of a whole latitudinal band in a few weeks, and staying for years with an aspect different from the usual one. The origin of some of these disturbances may be associated with the creation and destruction of the chromophore species that provides Jovian clouds their reddish coloration. In this work, we have focused on the North Temperate Belt (NTB) disturbance detected during the second flyby of Juno mission (NASA) on October 2016, as a series of convective storms interacted with the fastest zonal jet on Jupiter at 24N over months and left a quiet belt characterized by an intense red coloration Sánchez-Lavega et al. (2017). In order to determine the corresponding changes in the upper clouds and hazes we have used images taken in 2016 and 2017 with the Hubble Space Telescope Wide Field Camera 3. Such images were acquired before and after the outbreak, showing an intense color change in a narrow latitude band. The images cover the wavelength range from 250 nm up to the methane absorption band at 890 nm, thus sensitive to a number of atmospheric levels from the lower stratosphere to the upper troposphere where the ammonia condensation cloud is expected to be located. Here we use the radiative transfer suite NEMESIS Irwin et al. (2008) to determine the vertical distribution and properties of the upper hazes that best match the observed dependence of reflectivity with wavelength and geometry. We use two models for the Jovian chromophore: (A) an extended layer whose imaginary refractive index is left as a free parameter; and (B) a concentrated chromophore as in Sromovsky et al. (2017) using the optical properties by Carlson et al. (2016). Both scenarios show an increase in the number of particles responsible for the blue absorption approximately by a factor of 2, and require only small changes in the rest of the atmospheric parameters. We find that, even though results provided by scenario B are also compatible with observations, the limb-darkening is better described by scenario A, where there is also an increase of the particle absorption at the shortest wavelengths. In this work, we also provide an extension of the expected imaginary refractive indices to wavelengths beyond those covered in previous laboratory works, which will be useful for future studies