Deep jets on gas-giant planets

Three-dimensional numerical simulations of the atmospheric flow on giant planets using the primitive equations show that shallow thermal forcing confined to pressures near the cloud tops can produce deep zonal winds from the tropopause all the way down to the bottom of the atmosphere. These deep winds can attain speeds comparable to the zonal jet speeds within the shallow, forced layer; they are pumped by Coriolis acceleration acting on a deep meridional circulation driven by the shallow-layer eddies. In the forced layer, the flow reaches an approximate steady state where east-west eddy accelerations balance Coriolis accelerations acting on the meridional flow. Under Jupiter-like conditions, our simulations produce 25 to 30 zonal jets, similar to the number of jets observed on Jupiter and Saturn. The simulated jet widths correspond to the Rhines scale; this suggests that, despite the three-dimensional nature of the dynamics, the baroclinic eddies energize a quasi-two-dimensional inverse cascade modified by the β effect (where β is the gradient of the Coriolis parameter). In agreement with Jupiter, the jets can violate the barotropic and Charney-Stern stability criteria, achieving curvatures ^∂2u/∂y² of the zonal wind u with northward distance y up to 2β. The simulations exhibit a tendency toward neutral stability with respect to Arnol'd's second stability theorem in the upper troposphere, as has been suggested for Jupiter, although deviations from neutrality exist. When the temperature varies strongly with latitude near the equator, our simulations can also reproduce the stable equatorial superrotation with wind speeds greater than . Diagnostics show that barotropic eddies at low latitudes drive the equatorial superrotation. The simulations also broadly explain the distribution of jet-pumping eddies observed on Jupiter and Saturn. While idealized, these simulations therefore capture many aspects of the cloud-level flows on Jupiter and Saturn.     

Changes in Jupiter’s zonal velocity between 1979 and 2008

We show that the peak velocity of Jupiter’s visible-cloud-level zonal winds near 24°N (planetographic) increased from 2000 to 2008. This increase was the only change in the zonal velocity from 2000 to 2008 for latitudes between ±70° that was statistically significant and not obviously associated with visible weather. We present the first automated retrieval of fast (∼130 m s⁻¹) zonal velocities at 8°N planetographic latitude, and show that some previous retrievals incorrectly found slower zonal winds because the eastward drift of the dark projections (associated with 5-μm hot spots) “fooled” the retrieval algorithms.We determined the zonal velocity in 2000 from Cassini images from NASA’s Planetary Data System using a global method similar to previous longitude-shifting correlation methods used by others, and a new local method based on the longitudinal average of the two-dimensional velocity field. We obtained global velocities from images acquired in May 2008 with the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). Longer-term variability of the zonal winds is based on comparisons with published velocities based on 1979 Voyager 2 and 1995-1998 HST images. Fluctuations in the zonal wind speeds on the order of 10 m s⁻¹ on timescales ranging from weeks to months were found in the 1979 Voyager 2 and the 1995-1998 HST velocities. In data separated by 10 h, we find that the east-west velocity uncertainty due to longitudinal fluctuations are nearly 10 m s⁻¹, so velocity fluctuations of 10 m s⁻¹ may occur on timescales that are even smaller than 10 h. Fluctuations across such a wide range of timescales limit the accuracy of zonal wind measurements. The concept of an average zonal velocity may be ill-posed, and defining a “temporal mean” zonal velocity as the average of several zonal velocity fields spanning months or years may not be physically meaningful.At 8°N, we use our global method to find peak zonal velocities of ∼110 m s⁻¹ in 2000 and ∼130 m s⁻¹ in 2008. Zonal velocities from 2000 Cassini data produced by our local and global methods agree everywhere, except in the vicinity of 8°N. There, the local algorithm shows that the east-west velocity has large variations in longitude; vast regions exceed ∼140 m s⁻¹. Our global algorithm, and all of the velocity-extraction algorithms used in previously-published studies, found the east-west drift velocities of the visible dark projections, rather than the true zonal velocity at the visible-cloud level. Therefore, the apparent increase in zonal winds between 2000 and 2008 at 8°N is not a true change in zonal velocity.At 7.3°N, the Galileo probe found zonal velocities of 170 m s⁻¹ at the 3-bar level. If the true zonal velocity at the visible-cloud level at this latitude is ∼140 m s⁻¹ rather than ∼105 m s⁻¹, then the vertical zonal wind shear is much less than the currently accepted value.     

Constraints on deep-seated zonal winds inside Jupiter and Saturn

The atmospheres of Jupiter and Saturn exhibit strong and stable zonal winds. How deep the winds penetrate unabated into each planet is unknown. Our investigation favors shallow winds. It consists of two parts. The first part makes use of an Ohmic constraint; Ohmic dissipation associated with the planet's magnetic field cannot exceed the planet's net luminosity. Application to Jupiter (J) and Saturn (S) shows that the observed zonal winds cannot penetrate below a depth at which the electrical conductivity is about six orders of magnitude smaller than its value at the molecular-metallic transition. Measured values of the electrical conductivity of molecular hydrogen yield radii of maximum penetration of 0.96RJ and 0.86RS, with uncertainties of a few percent of R. At these radii, the magnetic Reynolds number based on the zonal wind velocity and the scale height of the magnetic diffusivity is of order unity. These limits are insensitive to difficulties in modeling turbulent convection. They permit complete penetration along cylinders of the equatorial jets observed in the atmospheres of Jupiter and Saturn. The second part investigates how deep the observed zonal winds actually do penetrate. As it applies heuristic models of turbulent convection, its conclusions must be regarded as tentative. Truncation of the winds in the planet's convective envelope would involve breaking the Taylor-Proudman constraint on cylindrical flow. This would require a suitable nonpotential acceleration which none of the obvious candidates appears able to provide. Accelerations arising from entropy gradients, magnetic stresses, and Reynolds stresses appear to be much too weak. These considerations suggest that strong zonal winds are confined to shallow, stably stratified layers, with equatorial jets being the possible exception.     

Galileo in-situ dust measurements in Jupiter’s gossamer rings

Galileo was the first artificial satellite to orbit Jupiter. During its late orbital mission the spacecraft made two passages through the giant planet’s gossamer ring system. The impact-ionization dust detector on board successfully recorded dust impacts during both ring passages and provided the first in-situ measurements from a dusty planetary ring. During the first passage—on 5 November 2002 while Galileo was approaching Jupiter—dust measurements were collected until a spacecraft anomaly at 2.33R_J (Jupiter radii) just 16 min after a close flyby of Amalthea put the spacecraft into a safing mode. The second ring passage on 21 September 2003 provided ring dust measurements down to about 2.5R_J and the Galileo spacecraft was destroyed shortly thereafter in a planned impact with Jupiter. In all, a few thousand dust impacts were counted with the instrument accumulators during both ring passages, but only a total of 110 complete data sets of dust impacts were transmitted to Earth. Detected particle sizes range from about 0.2 to 5 μm, extending the known size distribution by an order of magnitude towards smaller particles than previously derived from optical imaging [Showalter, M.R., de Pater, I., Verbanac, G., Hamilton, D.P., Burns, J.A., 2008. Icarus 195, 361-377; de Pater, I., Showalter, M.R., Macintosh, B., 2008. Icarus 195, 348-360]. The grain size distribution increases towards smaller particles and shows an excess of these tiny motes in the Amalthea gossamer ring compared to the Thebe ring. The size distribution for the Amalthea ring derived from our in-situ measurements for the small grains agrees very well with the one obtained from images for large grains. Our analysis shows that particles contributing most to the optical cross-section are about 5 μm in radius, in agreement with imaging results. The measurements indicate a large drop in particle flux immediately interior to Thebe’s orbit and some detected particles seem to be on highly-tilted orbits with inclinations up to 20°. Finally, the faint Thebe ring extension was detected out to at least 5R_J, indicating that grains attain higher eccentricities than previously thought. The drop interior to Thebe, the excess of submicron grains at Amalthea, and the faint ring extension indicate that grain dynamics is strongly influenced by electromagnetic forces. These findings can all be explained by a shadow resonance as detailed by Hamilton and Krüger [Hamilton, D.P., Krüger, H., 2008. Nature 453, 72-75].     

Jupiter’s moment of inertia: A possible determination by Juno

The moment of inertia of a giant planet reveals important information about the planet’s internal density structure and this information is not identical to that contained in the gravitational moments. The forthcoming Juno mission to Jupiter might determine Jupiter’s normalized moment of inertia NMoI = C/MR² by measuring Jupiter’s pole precession and the Lense–Thirring acceleration of the spacecraft (C is the axial moment of inertia, and M and R are Jupiter’s mass and mean radius, respectively). We investigate the possible range of NMoI values for Jupiter based on its measured gravitational field using a simple core/envelope model of the planet assuming that J₂ and J₄ are perfectly known and are equal to their measured values. The model suggests that for fixed values of J₂ and J₄ a range of NMoI values between 0.2629 and 0.2645 can be found. The Radau–Darwin relation gives a NMoI value that is larger than the model values by less than 1%. A low NMoI of ∼0.236, inferred from a dynamical model (Ward, W.R., Canup, R.M. [2006]. Astrophys. J. 640, L91–L94) is inconsistent with this range, but the range is model dependent. Although we conclude that the NMoI is tightly constrained by the gravity coefficients, a measurement of Jupiter’s NMoI to a few tenths of percent by Juno could provide an important constraint on Jupiter’s internal structure. We carry out a simplified assessment of the error involved in Juno’s possible determination of Jupiter’s NMoI.     

The source of widespread 3-μm absorption in Jupiter’s clouds: Constraints from 2000 Cassini VIMS observations

The Cassini flyby of Jupiter in 2000 provided spatially resolved spectra of Jupiter’s atmosphere using the Visual and Infrared Mapping Spectrometer (VIMS). A prominent characteristic of these spectra is the presence of a strong absorption at wavelengths from about 2.9 μm to 3.1 μm, previously noticed in a 3-μm spectrum obtained by the Infrared Space Observatory (ISO) in 1996. While Brooke et al. (Brooke, T.Y., Knacke, R.F., Encrenaz, T., Drossart, P., Crisp, D., Feuchtgruber, H. [1998]. Icarus 136, 1-13) were able to fit the ISO spectrum very well using ammonia ice as the sole source of particulate absorption, Sromovsky and Fry (Sromovsky, L.A., Fry, P.M. [2010]. Icarus 210, 211-229), using significantly revised NH₃ gas absorption models, showed that ammonium hydrosulfide (NH₄SH) provided a better fit to the ISO spectrum than NH₃, but that the best fit was obtained when both NH₃ and NH₄SH were present in the clouds. Although the large FOV of the ISO instrument precluded identification of the spatial distribution of these two components, the VIMS spectra at low and intermediate phase angles show that 3-μm absorption is present in zones and belts, in every region investigated, and both low- and high-opacity samples are best fit with a combination of NH₄SH and NH₃ particles at all locations. The best fits are obtained with a layer of small ammonia-coated particles (r ∼ 0.3 μm) overlying but often close to an optically thicker but still modest layer of much larger NH₄SH particles (r ∼ 10 μm), with a deeper optically thicker layer, which might also be composed of NH₄SH. Although these fits put NH₃ ice at pressures less than 500 mb, this is not inconsistent with the lack of prominent NH₃ features in Jupiter’s longwave spectrum because the reflectivity of the core particles strongly suppresses the NH₃ absorption features, at both near-IR and thermal wavelengths. Unlike Jupiter, Saturn lacks the broad 3-μm absorption feature, but does exhibit a small absorption near 2.965 μm, which resembles a similar jovian feature and suggests that both planets contain upper tropospheric clouds of sub-micron particles containing ammonia as a minor fraction.     

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 (Sánchez-Lavega, A. et al. [2010]. Astrophys. J. 715, L155-L159). 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 μm. The impact cloud expanded zonally from ∼5000 km (July 19) to 225,000 km (29 October, about 180° in longitude), remaining meridionally localized within a latitude band from 53.5°S to 61.5°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°S latitude increases its eastward velocity with altitude above the tropopause by 5-10 m s⁻¹. The corresponding vertical wind shear is low, about 1 m s⁻¹ 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⁻¹. 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.     

Compressible convection in the deep atmospheres of giant planets

Fast rotating giant planets such as Jupiter and Saturn possess alternate prograde and retrograde zonal winds which are stable over long periods of time. We consider a compressible model of convection in a spherical shell with rapid rotation, using the anelastic approximation, to explore the parameter range for which such zonal flows can be produced.We consider models with a large variation in density across the layer. Our models are based only on the molecular H/He region above the metallic hydrogen transition at about 2 Mbar, and we do not include the hydromagnetic effects which may be important if the electrical conductivity is significant. We find that the convective velocities are significantly higher in the low density regions of the shell, but the zonal flow is almost independent of the z-coordinate parallel to the rotation axis. We analyse how this behaviour is consistent with the Proudman-Taylor theorem.We find that deep prograde zonal flow near the equator is a very robust feature of our models. Prograde and retrograde jets alternating in latitude can occur inside the tangent cylinder in compressible as well as Boussinesq models, particularly at lower Prandtl numbers. However, the zonal jets inside the tangent cylinder are suppressed if a no-slip condition is imposed at the inner boundary. This suggests that deep high latitude jets may be suppressed if there is significant magnetic dissipation.Our compressible calculations include the viscous dissipation in the entropy equation, and we find this is comparable to, and in some cases exceeds, the total heat flux emerging from the surface. For numerical reasons, these simulations cannot reach the extremely low Ekman number found in giant planets, and they necessarily also have a much larger heat flux than planets. We therefore discuss how our results might scale down to give solutions with lower dissipation and lower heat flux.     

Jupiter and Saturn rotation periods

Anderson and Schubert [2007. Saturn's Gravitational field, internal rotation, and interior structure. Science 317, 1384-1387 (paper I)] proposed that Saturn's rotation period can be ascertained by minimizing the dynamic heights of the 100 mbar isosurface with respect to the geoid; they derived a rotation period of 10 h 32 m 35 s. We investigate the same approach for Jupiter to see if the Jovian rotation period is predicted by minimizing the dynamical heights of its isobaric (1 bar pressure level) surface using zonal wind data. A rotation period of 9 h 54 m 29.7 s is found. Further, we investigate the minimization method by fitting Pioneer and Voyager occultation radii for both Jupiter and Saturn. Rotation periods of 9 h 55 m 30 s and 10 h 32 m 35 s are found to minimize the dynamical heights for Jupiter and Saturn, respectively. Though there is no dynamical principle requiring the minimization of the dynamical heights of an isobaric surface, the successful application of the method to Jupiter lends support to its relevance for Saturn.We derive Jupiter and Saturn rotation periods using equilibrium theory to explain the difference between equatorial and polar radii. Rotation periods of 9 h 55 m 20 s and 10 h 31 m 49 s are found for Jupiter and Saturn, respectively. We show that both Jupiter's and Saturn's shapes can be derived using solid-body rotation, suggesting that zonal winds have a minor effect on the planetary shape for both planets.The agreement in the values of Saturn's rotation period predicted by the different approaches supports the conclusion that the planet's period of rotation is about 10 h 32 m.     

Formation of Jupiter using opacities based on detailed grain physics

Numerical simulations, based on the core-nucleated accretion model, are presented for the formation of Jupiter at 5.2 AU in three primordial disks with three different assumed values of the surface density of solid particles. The grain opacities in the envelope of the protoplanet are computed using a detailed model that includes settling and coagulation of grains and that incorporates a recalculation of the grain size distribution at each point in time and space. We generally find lower opacities than the 2% of interstellar values used in previous calculations (Hubickyj, O., Bodenheimer, P., Lissauer, J.J. [2005]. Icarus 179, 415-431; Lissauer, J.J., Hubickyj, O., D’Angelo, G., Bodenheimer, P. [2009]. Icarus 199, 338-350). These lower opacities result in more rapid heat loss from and more rapid contraction of the protoplanetary envelope. For a given surface density of solids, the new calculations result in a substantial speedup in formation time as compared with those previous calculations. Formation times are calculated to be 1.0, 1.9, and 4.0 Myr, and solid core masses are found to be 16.8, 8.9, and 4.7 M_⊕, for solid surface densities, σ, of 10, 6, and 4 g cm⁻², respectively. For σ = 10 and σ = 6 g cm⁻², respectively, these formation times are reduced by more than 50% and more than 80% compared with those in a previously published calculation with the old approximation to the opacity.     

Hydrogen Eos at Megabar Pressures and the Search for Jupiter’s Core

The interior structure of Jupiter serves as a benchmark for an entire astrophysical class of liquid–metallic hydrogen-rich objects with masses ranging from ~0.1M _J to ~80M _J (1M _J = Jupiter mass = 1.9e30 g), comprising hydrogen-rich giant planets (mass < 13M _J) and brown dwarfs (mass > 13M _J but ~ < 80M _J), the so-called substellar objects (SSOs). Formation of giant planets may involve nucleated collapse of nebular gas onto a solid, dense core of mass ~0.04M _J rather than a stellar-like gravitational instability. Thus, detection of a primordial core in Jupiter is a prime objective for understanding the mode of origin of extrasolar giant planets and other SSOs. A basic method for core detection makes use of direct modeling of Jupiter’s external gravitational potential terms in response to rotational and tidal perturbations, and is highly sensitive to the thermodynamics of hydrogen at multi-megabar pressures. The present-day core masses of Jupiter and Saturn may be larger than their primordial core masses due to sedimentation of elements heavier than hydrogen. We show that there is a significant contribution of such sedimented mass to Saturn’s core mass. The sedimentation contribution to Jupiter’s core mass will be smaller and could be zero.     

Dynamics of Jupiter’s equatorial region at cloud top level from Cassini and HST images

We present a study of the equatorial region of Jupiter, between latitudes ∼15°S and ∼15°N, based on Cassini ISS images obtained during the Jupiter flyby at the end of 2000, and HST images acquired in May and July 2008. We examine the structure of the zonal wind profile and report the detection of significant longitudinal variations in the intensity of the 6°N eastward jet, up to 60 m s⁻¹ in Cassini and HST observations. These longitudinal variations are, in the HST case, associated with different cloud morphology. Photometric and radiative transfer analysis of the cloud features used as tracers in HST images show that at most there is only a small height difference, no larger than ∼0.5-1 scale heights, between the slow (∼100 m s⁻¹) and fast (∼150 m s⁻¹) moving features. This suggests that speed variability at 6°N is not dominated by vertical wind shears but instead we propose that Rossby wave activity is the responsible for the zonal variability. Removing this variability, we find that Jupiter’s equatorial jet is actually symmetric relative to equator with two peaks of ∼140-150 m s⁻¹ located at latitudes 6°N and 6°S and at a similar pressure level. We also study the local dynamics of particular equatorial features such as several dark projections associated with 5 μm hot spots and a large, long-lived feature called the White Spot (WS) located at 6°S. Convergent flow at the dark projections appears to be a characteristic which depends on the particular morphology and has only been detected in some cases. The internal flow field in the White Spot indicates that it is a weakly rotating quasi-equatorial anticyclone relative to the ambient meridionally sheared flow.     

Long slit spectropolarimetry of Jupiter and Saturn

We present ground-based limb polarization measurements of Jupiter and Saturn consisting of full disk imaging polarimetry for the wavelength 7300 Å and spatially resolved (long-slit) spectropolarimetry covering the wavelength range 5200-9350 Å.For the polar region of Jupiter we find for λ = 6000 Å a very strong radial (perpendicular to the limb) fractional polarization with a seeing corrected maximum of about +11.5% in the South and +10.0% in the North. This indicates that the polarizing haze layer is thicker at the South pole. The polar haze layers extend down to 58° in latitude. The derived polarization values are much higher than reported in previous studies because of the better spatial resolution of our data and an appropriate consideration of the atmospheric seeing. Model calculations demonstrate that the high limb polarization can be explained by strongly polarizing (p ≈ 1.0), high albedo (ω ≈ 0.98) haze particles with a scattering asymmetry parameter of g ≈ 0.6 as expected for aggregate particles of the type described by West and Smith (West, R.A., Smith, P.H. [1991]. Icarus 90, 330-333). The deduced particle parameters are distinctively different when compared to lower latitude regions.The spectropolarimetry of Jupiter shows a decrease in the polar limb polarization towards longer wavelengths and a significantly enhanced polarization in strong methane bands when compared to the adjacent continuum. This is a natural outcome for a highly polarizing haze layer above an atmosphere where multiple scatterings are suppressed in absorption bands. For lower latitudes the fractional polarization is small, negative, and it depends only little on wavelength except for the strong CH₄-band at 8870 Å.The South pole of Saturn shows a lower polarization (p ≈ 1.0-1.5%) than the poles of Jupiter. The spectropolarimetric signal for Saturn decrease rapidly with wavelength and shows no significant enhancements in the fractional polarization in the absorption bands. These properties can be explained by a vertically extended stratospheric haze region composed of small particles <100 nm as suggested previously by Karkoschka and Tomasko (Karkoschka, E., Tomasko, M. [2005]. Icarus 179, 195-221).In addition we find in the V- and R-band a previously not observed strong polarization feature (p = 1.5-2.0%) near the equator of Saturn. The origin of this polarization signal is unclear but it could be related to a seasonal effect.Finally we discuss the potential of ground-based limb polarization measurements for the investigation of the scattering particles in the atmospheres of Jupiter and Saturn.     

The deep water abundance on Jupiter: New constraints from thermochemical kinetics and diffusion modeling

We have developed a one-dimensional thermochemical kinetics and diffusion model for Jupiter’s atmosphere that accurately describes the transition from the thermochemical regime in the deep troposphere (where chemical equilibrium is established) to the quenched regime in the upper troposphere (where chemical equilibrium is disrupted). The model is used to calculate chemical abundances of tropospheric constituents and to identify important chemical pathways for CO-CH₄ interconversion in hydrogen-dominated atmospheres. In particular, the observed mole fraction and chemical behavior of CO is used to indirectly constrain the jovian water inventory. Our model can reproduce the observed tropospheric CO abundance provided that the water mole fraction lies in the range (0.25-6.0) × 10⁻³ in Jupiter’s deep troposphere, corresponding to an enrichment of 0.3-7.3 times the protosolar abundance (assumed to be H₂O/H₂ = 9.61 × 10⁻⁴). Our results suggest that Jupiter’s oxygen enrichment is roughly similar to that for carbon, nitrogen, and other heavy elements, and we conclude that formation scenarios that require very large (>8× solar) enrichments in water can be ruled out. We also evaluate and refine the simple time-constant arguments currently used to predict the quenched CO abundance on Jupiter, other giant planets, and brown dwarfs.     

The evolving flow of Jupiter’s White Ovals and adjacent cyclones

We present results regarding the dynamical meteorology of Jupiter’s White Ovals at different points in their evolution. Starting from the era with three White Ovals FA, BC, and DE (Galileo), continuing to the post-merger epoch with only one Oval BA (Cassini), and finally to Oval BA’s current reddened state (New Horizons), we demonstrate that the dynamics of their flow have similarly evolved along with their appearance. In the Galileo epoch, Oval DE had an elliptical shape with peak zonal wind speeds of ∼90 m s⁻¹ in both its northern and southern peripheries. During the post-merger epoch, Oval BA’s shape was more triangular and less elliptical than Oval DE; in addition to widening in the north-south direction, its northern periphery was 20 m s⁻¹ slower, and its southern periphery was 20 m s⁻¹ faster than Oval DE’s flow during the Galileo era. Finally, in the New Horizons era, the reddened Oval BA had evolved back to a classical elliptical form. The northern periphery of Oval BA increased in speed by 20 m s⁻¹ from Cassini to New Horizons, ending up at a speed nearly identical to that of the northern periphery of Oval DE during Galileo. However, the peak speeds along the southern rim of the newly formed Oval BA were consistently faster than the corresponding speeds in Oval DE, and they increased still further between Cassini and New Horizons, ending up at ∼140-150 m s⁻¹. Relative vorticity maps of Oval BA reveal a cyclonic ring surrounding its outer periphery, similar to the ring present around the Great Red Spot. The cyclonic ring around Oval BA in 2007 appears to be moderately stronger than observed in 1997 and 2001, suggesting that this may be associated with the coloration of the vortex. The modest strengthening of the winds in Oval BA, the appearance of red aerosols, and the appearance of a turbulent, cyclonic feature to Oval BA’s northwest create a strong resemblance with the Great Red Spot from both a dynamical and morphological perspective.In addition to the White Ovals, we also measure the winds within two compact cyclonic regions, one in the Galileo data set and one in the Cassini data set. In the images, these cyclonic features appear turbulent and filamentary, but our wind field reveals that the flow manifests as a coherent high-speed collar surrounding relatively quiescent interiors. Our relative vorticity maps show that the vorticity likewise concentrates in a collar near the outermost periphery, unlike the White Ovals which have peak relative vorticity magnitudes near the center of the vortex. The cyclones contain several localized bright regions consistent with the characteristics of thunderstorms identified in other studies. Although less studied than their anticyclonic cousins, these cyclones may offer crucial insights into the planet’s cloud-level energetics and dynamical meteorology.     

Instability of Zonal Flows in Rotating Spherical Shells: An Application to Jupiter

Measurements from the Galileo probe suggest that the zonal winds are deep rooted. Jupiter's high rotation rate makes it likely that the whole outer molecular H/He layer is involved in these long-lived jet flows. Assuming that the primary flows are geostrophic, and that the banded surface structure stretches right through the molecular H/He layer, we examine the conditions for such flows to be stable. As a first step, the linear stability of some prescribed banded zonal flows in a rotating spherical shell is explored. Incompressibility is assumed for simplicity, and the boundary condition is stress-free. We compare solutions for two aspect ratios, appropriate for the molecular H/He layers of Jupiter and Saturn, and two Ekman numbers (E=10⁻² and E=10⁻⁴). Convective and shear flow instabilities compete in our system. The convective instabilities are of the well-known columnar structure. Shear flow instabilities for the larger Ekman number are similar to the Taylor-Couette instability in rotating annuli. At the lower Ekman number, shear flow instabilities adopt a geostrophic character, assuming the form of rotating columns, similar to the convective instabilities. While the convective instability always sets in outside the tangent cylinder, shear instability can become unstable inside the tangent cylinder. If even a weak zonal flow is present inside the tangent cylinder, the flow is unstable to shear instability. This offers an explanation why the jovian zonal jet structure is much weaker at the higher latitudes that correspond to locations inside the tangent cylinder.     

Jovian temperature and cloud variability during the 2009-2010 fade of the South Equatorial Belt

Mid-infrared 7-20 μm imaging of Jupiter from ESO’s Very Large Telescope (VLT/VISIR) demonstrate that the increased albedo of Jupiter’s South Equatorial Belt (SEB) during the ‘fade’ (whitening) event of 2009-2010 was correlated with changes to atmospheric temperature and aerosol opacity. The opacity of the tropospheric condensation cloud deck at pressures less than 800 mbar increased by 80% between May 2008 and July 2010, making the SEB (7-17°S) as opaque in the thermal infrared as the adjacent equatorial zone. After the cessation of discrete convective activity within the SEB in May 2009, a cool band of high aerosol opacity (the SEB zone at 11-15°S) was observed separating the cloud-free northern and southern SEB components. The cooling of the SEBZ (with peak-to-peak contrasts of 1.0 ± 0.5 K), as well as the increased aerosol opacity at 4.8 and 8.6 μm, preceded the visible whitening of the belt by several months. A chain of five warm, cloud-free ‘brown barges’ (subsiding airmasses) were observed regularly in the SEB between June 2009 and June 2010, by which time they too had been obscured by the enhanced aerosol opacity of the SEB, although the underlying warm circulation was still present in July 2010. Upper tropospheric temperatures (150-300 mbar) remained largely unchanged during the fade, but the cool SEBZ formation was detected at deeper levels (p > 300 mbar) within the convectively-unstable region of the troposphere. The SEBZ formation caused the meridional temperature gradient of the SEB to decrease between 2008 and 2010, reducing the vertical thermal windshear on the zonal jets bounding the SEB. The southern SEB had fully faded by July 2010 and was characterised by short-wave undulations at 19-20°S. The northern SEB persisted as a narrow grey lane of cloud-free conditions throughout the fade process.The cool temperatures and enhanced aerosol opacity of the SEBZ after July 2009 are consistent with an upward flux of volatiles (e.g., ammonia-laden air) and enhanced condensation, obscuring the blue-absorbing chromophore and whitening the SEB by April 2010. These changes occurred within cloud decks in the convective troposphere, and not in the radiatively-controlled upper troposphere. NH₃ ice coatings on aerosols at p < 800 mbar are plausible sources of the suppressed 4.8 and 8.6-μm emission, although differences in the spatial distribution of opacity at these two wavelengths suggest that enhanced attenuation by a deeper cloud (p > 800 mbar) also occurred during the fade. Revival of the dark SEB coloration in the coming months will ultimately require sublimation of these ices by subsidence and warming of volatile-depleted air.     

High-resolution 3-μm spectra of Jupiter: Latitudinal spectral variations influenced by molecules, clouds, and haze

We present latitudinally-resolved high-resolution (R = 37,000) pole-to-pole spectra of Jupiter in various narrow longitudinal ranges, in spectral intervals covering roughly half of the spectral range 2.86-3.53 μm. We have analyzed the data with the aid of synthetic spectra generated from a model jovian atmosphere that included lines of CH₄, CH₃D, NH₃, C₂H₂, C₂H₆, PH₃, and HCN, as well as clouds and haze. Numerous spectral features of many of these molecular species are present and are individually identified for the first time, as are many lines of and a few unidentified spectral features. In both polar regions the 2.86-3.10-μm continuum is more than 10 times weaker than in spectra at lower latitudes, implying that in this wavelength range the single-scattering albedos of polar haze particles are very low. In contrast, the 3.24-3.53 μm the weak polar and equatorial continua are of comparable intensity. We derive vertical distributions of NH₃, C₂H₂ and C₂H₆, and find that the mixing ratios of NH₃ and C₂H₆ show little variation between equatorial and polar regions. However, the mixing ratios of C₂H₂ in the northern and southern polar regions are ∼6 and ∼3 times, respectively, less than those in the equatorial regions. The derived mixing ratio curves of C₂H₂ and C₂H₆ extend up to the 10⁻⁶ bar level, a significantly higher altitude than most previous results in the literature. Further ground-based observations covering other longitudes are needed to test if these mixing ratios are representative values for the equatorial and polar regions.     

H3 as a trap for noble gases: 1—The case of Argon

The possibility of H₃⁺ playing a role as a sink for noble gases has been investigated in the case of Argon. Elaborate quantum methods (ab initio Coupled Cluster and density functional BH&HLYP levels of theory) have been shown to reproduce the rotational constants within 0.3% together with the only known IR frequency on the test case of Ar…D₃⁺. Dissociation energies of (Ar)_n…H₃⁺ as a function of cluster size, i.e. 7.2 (n=1), 3.7 (n=2), 3.6 (n=3), 1.6 (n=4), 1.7 (n=5) kcal/mol, follow the pattern established experimentally for (Ar)_n…H₃⁺ and (H₂)_n…H₃⁺ series. Rotational constants and harmonic frequencies of (Ar)_n…H₃⁺ (n=1-3) are presented.     

Jovian chromophore characteristics from multispectral HST images

The chromophores responsible for coloring the jovian atmosphere are embedded within Jupiter’s vertical aerosol structure. Sunlight propagates through this vertical distribution of aerosol particles, whose colors are defined by ?₀(λ), and we remotely observe the culmination of the radiative transfer as I/F(λ). In this study, we employed a radiative transfer code to retrieve ?₀(λ) for particles in Jupiter’s tropospheric haze at seven wavelengths in the near-UV and visible regimes. The data consisted of images of the 2008 passage of Oval BA to the south of the Great Red Spot obtained by the Wide Field Planetary Camera 2 on-board the Hubble Space Telescope. We present derived particle colors for locations that were selected from 14 weather regions, which spanned a large range of observed colors. All ?₀(λ) curves were absorbing in the blue, and ?₀(λ) increased monotonically to approximately unity as wavelength increased. We found accurate fits to all ?₀(λ) curves using an empirically derived functional form: ?₀(λ) = 1 − A exp(−Bλ). The best-fit parameters for the mean ?₀(λ) curve were A = 25.4 and B = 0.0149 for λ in units of nm. We performed a principal component analysis (PCA) on our ?₀(λ) results and found that one or two independent chromophores were sufficient to produce the variations in ?₀(λ). A PCA of I/F(λ) for the same jovian locations resulted in principal components (PCs) with roughly the same variances as the ?₀(λ) PCA, but they did not result in a one-to-one mapping of PC amplitudes between the ?₀(λ) PCA and I/F(λ) PCA. We suggest that statistical analyses performed on I/F(λ) image cubes have limited applicability to the characterization of chromophores in the jovian atmosphere due to the sensitivity of I/F(λ) to horizontal variations in the vertical aerosol distribution.