The 10 October 1999 HIP 9369 occultation by the northern polar region of Jupiter: ingress and egress lightcurves analysis

The occultation of bright star HIP9369 by the northern polar region of Jupiter was observed from four locations in North and South America, providing four data sets for ingress and egress. The inversion of the eight occultation lightcurves provides temperature profiles at different latitudes ranging from 55°N to 73.2°N. We estimate the errors on the profiles due to the uncertainties of the inversion method and compare the value of the temperature at the deepest level probed (∼ 50 μbar) with previous observations. The shape of the temperature gradient profile is found similar to previous investigations of planetary atmospheres with propagating and breaking gravity waves. We analyze the small scale structures in both lightcurves and temperature profiles using the continuous wavelet transform. The calculated power spectra of localized fluctuations in the temperature profiles show slopes close to −3 for all eight profiles. We also isolate and reconstruct the three-dimensional geometry of a single wave mode with vertical and horizontal wavelengths of respectively 3 and 70 km. The identified wave is consistent with the gravity wave regime, with a horizontal phase speed nearly parallel to the planetary meridian. Nevertheless, the dissipation of the corresponding wave in Jupiter’s stratosphere should preclude its detection at the observed levels and an acoustic wave cannot be ruled out.     

New Horizons Alice ultraviolet observations of a stellar occultation by Jupiter’s atmosphere

The Alice ultraviolet spectrograph onboard the New Horizons spacecraft observed two occultations of the bright star χ Ophiucus by Jupiter’s atmosphere on February 22 and 23, 2007 during the approach phase of the Jupiter flyby. The ingress occultation probed the atmosphere at 32°N latitude near the dawn terminator, while egress probed 18°N latitude near the dusk terminator. A detailed analysis of both the ingress and egress occultations, including the effects of molecular hydrogen, methane, acetylene, ethylene, and ethane absorptions in the far ultraviolet (FUV), constrains the eddy diffusion coefficient at the homopause level to be  cm² s⁻¹, consistent with Voyager measurements and other analyses (Festou, M.C., Atreya, S.K., Donahue, T.M., Sandel, B.R., Shemansky, D.E., Broadfoot, A.L. [1981]. J. Geophys. Res. 86, 5717-5725; Vervack Jr., R.J., Sandel, B.R., Gladstone, G.R., McConnell, J.C., Parkinson, C.D. [1995]. Icarus 114, 163-173; Yelle, R.V., Young, L.A., Vervack Jr., R.J., Young, R., Pfister, L., Sandel, B.R. [1996]. J. Geophys. Res. 101 (E1), 2149-2162). However, the actual derived pressure level of the methane homopause for both occultations differs from that derived by [Festou et al., 1981] and [Yelle et al., 1996] from the Voyager ultraviolet occultations, suggesting possible changes in the strength of atmospheric mixing with time. We find that at 32°N latitude, the methane concentration is  cm⁻³ at 70,397 km, the methane concentration is  cm⁻³ at 70,383 km, the acetylene concentration is  cm⁻³ at 70,364 km, and the ethane concentration is  cm⁻³ at 70,360 km. At 18°N latitude, the methane concentration is  cm⁻³ at 71,345 km, the methane concentration is  cm⁻³ at 71,332 km, the acetylene concentration is cm⁻³ at 71,318 km, and the ethane concentration is  cm⁻³ at 71,315 km. We also find that the H₂ occultation light curve is best reproduced if the atmosphere remains cold in the microbar region such that the base of the thermosphere is located at a lower pressure level than that determined by in situ instruments aboard the Galileo probe (Seiff, A., Kirk, D.B., Knight, T.C.D., Young, R.E., Mihalov, J.D., Young, L.A., Milos, F.S., Schubert, G., Blanchard, R.C., Atkinson, D. [1998]. J. Geophys. Res. 103 (E10), 22857-22889) - the Sieff et al. temperature profile leads to too much absorption from H₂ at high altitudes. However, this result is highly model dependent and non-unique. The observations and analysis help constrain photochemical models of Jupiter’s atmosphere.     

Simulations of high-latitude spots on Jupiter: Constraints on vortex strength and the deep wind

We combine high-resolution observations of the dynamical behavior of small vortices (diameters ?5000 km) located at latitude 60°N on Jupiter with forward modeling, using the EPIC atmospheric model, to address two open questions: the dependence of the zonal winds with depth, and the strength of vortices that are too small to apply cloud tracking to their internal structure. The observed drift rates of the vortices can only be reproduced in the model when the zonal winds increase slightly with depth below the cloud tops, with a vertical shear that is less than was measured at 7°N at the southern rim of a 5-μm hotspot by the Galileo Probe Doppler Wind Experiment (DWE). This supports the idea that Jupiter's vertical shear may vary significantly with latitude. Our simulations suggest that the morphology of the mergers between vortices mainly depends on their maximum tangential velocities, the best results occurring when the tangential velocity is close to the velocity difference of the alternating jets constraining the zone in which the vortices are embedded. We use this correlation, together with the high-resolution data available for the White Ovals, to derive an empirical relationship between the maximum tangential velocity of a jovian vortex and its size, normalized by the strength and size of the encompassing shear zone. The Great Red Spot stands out as a significant anomaly to this relationship, but interestingly it is becoming less so with time.     

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.     

The 2003 November 14 occultation by Titan of TYC 1343-1865-1: II. Analysis of light curves

We observed a stellar occultation by Titan on 2003 November 14 from La Palma Observatory using ULTRACAM with three Sloan filters: u^′, g^′, and i^′ (358, 487, and 758 nm, respectively). The occultation probed latitudes 2° S and 1° N during immersion and emersion, respectively. A prominent central flash was present in only the i^′ filter, indicating wavelength-dependent atmospheric extinction. We inverted the light curves to obtain six lower-limit temperature profiles between 335 and 485 km (0.04 and 0.003 mb) altitude. The i^′ profiles agreed with the temperature measured by the Huygens Atmospheric Structure Instrument [Fulchignoni, M., and 43 colleagues, 2005. Nature 438, 785–791] above 415 km (0.01 mb). The profiles obtained from different wavelength filters systematically diverge as altitude decreases, which implies significant extinction in the light curves. Applying an extinction model [Elliot, J.L., Young, L.A., 1992. Astron. J. 103, 991–1015] gave the altitudes of line of sight optical depth equal to unity: 396±7 and 401±20 km (u^′ immersion and emersion); 354±7 and 387±7 km (g^′ immersion and emersion); and 336±5 and 318±4 km (i^′ immersion and emersion). Further analysis showed that the optical depth follows a power law in wavelength with index 1.3±0.2. We present a new method for determining temperature from scintillation spikes in the occulting body's atmosphere. Temperatures derived with this method are equal to or warmer than those measured by the Huygens Atmospheric Structure Instrument. Using the highly structured, three-peaked central flash, we confirmed the shape of Titan's middle atmosphere using a model originally derived for a previous Titan occultation [Hubbard, W.B., and 45 colleagues, 1993. Astron. Astrophys. 269, 541–563].     

Phosphine on Jupiter and Saturn from Cassini/CIRS

The global distribution of phosphine (PH₃) on Jupiter and Saturn is derived using 2.5 cm⁻¹ spectral resolution Cassini/CIRS observations. We extend the preliminary PH₃ analyses on the gas giants [Irwin, P.G.J., and 6 colleagues, 2004. Icarus 172, 37-49; Fletcher, L.N., and 9 colleagues, 2007a. Icarus 188, 72-88] by (a) incorporating a wider range of Cassini/CIRS datasets and by considering a broader spectral range; (b) direct incorporation of thermal infrared opacities due to tropospheric aerosols and (c) using a common retrieval algorithm and spectroscopic line database to allow direct comparison between these two gas giants.The results suggest striking similarities between the tropospheric dynamics in the 100-1000 mbar regions of the giant planets: both demonstrate enhanced PH₃ at the equator, depletion over neighbouring equatorial belts and mid-latitude belt/zone structures. Saturn's polar PH₃ shows depletion within the hot cyclonic polar vortices. Jovian aerosol distributions are consistent with previous independent studies, and on Saturn we demonstrate that CIRS spectra are most consistent with a haze in the 100-400 mbar range with a mean optical depth of 0.1 at 10 μm. Unlike Jupiter, Saturn's tropospheric haze shows a hemispherical asymmetry, being more opaque in the southern summer hemisphere than in the north. Thermal-IR haze opacity is not enhanced at Saturn's equator as it is on Jupiter.Small-scale perturbations to the mean PH₃ abundance are discussed both in terms of a model of meridional overturning and parameterisation as eddy mixing. The large-scale structure of the PH₃ distributions is likely to be related to changes in the photochemical lifetimes and the shielding due to aerosol opacities. On Saturn, the enhanced summer opacity results in shielding and extended photochemical lifetimes for PH₃, permitting elevated PH₃ levels over Saturn's summer hemisphere.     

Titan solar occultation observed by Cassini/VIMS: Gas absorption and constraints on aerosol composition

A solar occultation by Titan's atmosphere has been observed through the solar port of the Cassini/VIMS instrument on January 15th, 2006. Transmission spectra acquired during solar egress probe the atmosphere in the altitude range 70 to 900 km at the latitude of 71° S. Several molecular absorption bands of CH₄ and CO are visible in these data. A line-by-line radiative transfer calculation in spherical geometry is used to model three methane bands (1.7, 2.3, 3.3 μm) and the CO 4.7 μm band. Above 200 km, the methane 2.3 μm band is well fit with constant mixing ratio between 1.4 and 1.7%, in agreement with in situ and other Cassini measurements. Under 200 km, there are discrepancies between models and observations that are yet fully understood. Under 480 km, the 3.3 μm CH₄ band is mixed with a large and deep additional absorption. It corresponds to the C-H stretching mode of aliphatic hydrocarbon chains attached to large organic molecules. The CO 4.7 μm band is observed in the lower stratosphere (altitudes below 150 km) and is well fit with a model with constant mixing ratio of 33±10 ppm. The continuum level of the observed transmission spectra provides new constraints on the aerosol content of the atmosphere. A model using fractal aggregates and optical properties of tholins produced by Khare et al. [Khare, B.N., Sagan, C., Arakawa, E.T., Suits, F., Callcott, T.A., Williams, M.W., 1984. Icarus 60, 127-137] is developed. Fractal aggregates with more than 1000 spheres of radius 0.05 μm are needed to fit the data. Clear differences in the chemical composition are revealed between tholins and actual haze particles. Extinction and density profiles are also retrieved using an inversion of the continuum values. An exponential increase of the haze number density is observed under 420 km with a typical scale height of 60 km.     

Merging circulations on Jupiter: Observed differences between cyclonic and anticyclonic mergers

The dynamics of mergers of large circulations in Jupiter's atmosphere may permit different models of the atmosphere to be tested. We report well-resolved observations of such events at visible wavelengths: three anticyclonic and three cyclonic events. A merger of anticyclonic white ovals in the South South Temperate domain (2002 March) is compared with the previously reported merger of ovals BE and FA in the South Temperate domain (2000 March). In each case, the two similar-sized ovals converged rapidly once they were separated by less than the sum of their diameters; they orbited around each other anticyclonically during the merger; the merged oval initially had the same rapid drift as the western parent; and, in an unexpected similarity, a cyclonic oval emerged westward from the point of merger. Evidence suggests that a merger of smaller ovals in the North North Temperate domain (2002 February) had similar dynamics. In contrast, mergers of cyclonic ovals in the North Equatorial Belt (‘barges’: 2001 November, 2005 May) proceeded in a different manner. The two parent barges showed no consistent acceleration towards each other as they converged; on contact there was no obvious sign of mutual circulation, and the low-albedo regions had almost passed each other before they finally merged; and the resulting barge had a drift rate intermediate between the two parents, and a length that was greater than either parent. Again, a third such event involving a smaller barge (2002 December) showed many of the same characteristics. These observations define different dynamical behaviour during anticyclonic and cyclonic mergers.     

Potential for stratospheric Doppler windspeed measurements of Jupiter by sub-millimetre spectroscopy

The sub-millimetre/microwave range of the spectrum has been exploited in the field of Earth observation by many instruments over the years and has provided a plethora of information on atmospheric chemistry and dynamics—however, this spectral range has not been fully explored in planetary science, having been exclusively employed to carry out ground-based measurements. To this end, a sub-millimetre instrument, the Orbiter Terahertz Infrared Spectrometer (ORTIS), is studied by the University of Oxford and the Rutherford Appleton Laboratory, to meet the requirements of the European Space Agency's Cosmic Visions 2015-2025 programme—in particular, the Europa Jupiter System Mission (EJSM), which has the European Space Agency and the National Aeronautics and Space Administration as partners. ORTIS is designed to measure atmospheric temperature, the abundance of stratospheric water vapour and other jovian gases, and is intended to be capable of retrieving vertical profiles of horizontal windspeed in the stratosphere for the first time, from Doppler-shifted emission lines measured at high spectral resolution. In this work, a preliminary study and implementation of the estimation of windspeed profiles on simulated spectra representative of Jupiter is presented, detailing the development of the retrieval algorithm, showing that a sub-millimetre instrument such as ORTIS should be able to retrieve windspeed profiles to an accuracy of about 15 m/s between 70 and 200 km/0.1-10 mb using a single near-limb measurement, for expected noise amplitudes.     

Jupiter’s South South Temperate Zone vortices: Observations and simulations

The region in Jupiter’s atmosphere with the highest density of anticyclonic spot-like vortices is the region known as the South South Temperate Zone (SSTZ), which is located between the eastward jet at ≈−42.9° latitude and the westward jet at ≈−39.2° latitude. We present a characterization of the spots found in this region based on ground-based and Hubble Space Telescope observations from the years 1993 to 2007. Mergers have been reported between spots in this region, similar to those observed for the White Ovals in the latitudinal domain immediately equatorward (northward). We use a multilayer model to perform numerical simulations that capture the details of a well-observed merger event involving multiple interacting vortices. We find that the vertical stratification has an important effect in the outcome of the interaction between spots. In particular it can play a determining role on whether or not a cyclone embedded between two approaching anticyclones can inhibit their merging. From our simulations we conclude that the background static stability of the atmosphere in the SSTZ is better characterized by an average value of .     

An investigation of Pluto’s troposphere using stellar occultation light curves and an atmospheric radiative-conductive-convective model

We use a radiative-conductive-convective model to assess the height of Pluto’s troposphere, as well as surface pressure and surface radius, from stellar occultation data from the years 1988, 2002, and 2006. The height of the troposphere, if it exists, is less than 1 km for all years analyzed. Pluto has at most a planetary boundary layer and not a troposphere. As in previous analyses of Pluto occultation light curves, we find that the surface pressure is increasing with time, assuming that latitude and longitude variations in Pluto’s atmosphere are negligible. The surface pressure is found to be slightly higher ( μbar in 1988,  μbar in 2002, and 18.5 ± 4.7 μbar in 2006) than in our previous analyses with the troposphere excluded. The surface radius is determined to be . Comparison of the minimum reduced chi-squared values between the best-fit radiative-conductive-convective (i.e., troposphere-included) model and best-fit radiative-conductive (i.e., troposphere-excluded) shows that the troposphere-included model is only a slightly better fit to the data for all 3 years. Uncertainties in the small-scale physical processes of Pluto’s lower atmosphere and consequently the functional form of the model troposphere lend more confidence to the troposphere-excluded results.     

On the long-term variability of Jupiter’s winds and brightness as observed from Hubble

Hubble Space Telescope Wide Field Planetary Camera 2 imaging data of Jupiter were combined with wind profiles from Voyager and Cassini data to study long-term variability in Jupiter’s winds and cloud brightness. Searches for evidence of wind velocity periodicity yielded a few latitudes with potential variability; the most significant periods were found nearly symmetrically about the equator at 0°, 10-12°N, and 14-18°S planetographic latitude. The low to mid-latitude signals have components consistent with the measured stratospheric temperature Quasi-Quadrennial Oscillation (QQO) period of 4-5 years, while the equatorial signal is approximately seasonal and could be tied to mesoscale wave formation. Robustness tests indicate that a constant or continuously varying periodic signal near 4.5 years would appear with high significance in the data periodograms as long as uncertainties or noise in the data are not of greater magnitude. However, the lack of a consistent signal over many latitudes makes it difficult to interpret as a QQO-related change. In addition, further analyses of calibrated 410-nm and 953-nm brightness scans found few corresponding changes in troposphere haze and cloud structure on QQO timescales. However, stratospheric haze reflectance at 255-nm did appear to vary on seasonal timescales, though the data do not have enough temporal coverage or photometric accuracy to be conclusive. Sufficient temporal coverage and spacing, as well as data quality, are critical to this type of search.     

The aftermath of the July 2009 impact on Jupiter: Ammonia, temperatures and particulates from Gemini thermal infrared spectroscopy

We obtained longitudinally resolved thermal infrared spectra (8-13 μm and 17-25 μm) of Jupiter’s impact debris at the Gemini South Telescope on July 24, 2009; five days after the July 19th collision. These were used to study the mechanisms responsible for the redistribution of thermal energy and material (ammonia and stratospheric particulates) following the impact. Upwelling of (8.5 ± 4.1) × 10¹⁴ g of tropospheric air was sufficient to deposit (6.7 ± 4.1) × 10¹² g of NH₃ over a 6° longitude range above the impact core. The NH₃ was distributed over the 20-80 mbar region with a peak abundance of 1.0 ± 0.6 ppm at 45 mbar. Only a 10th of this abundance was observed over the western ejecta, and it is unlikely that these observations were sensitive to NH₃ entrained in the ballistic plume itself. The pattern of excess thermal energy was markedly different from that of Shoemaker-Levy 9 (SL9), with a localized tropospheric perturbation of 2.0 ± 1.0 K at 200-300 mbar and a broader stratospheric warming of up to 3.5 ± 2.0 K at 10-30 mbar. We find no evidence of residual warmth at p < 1 mbar five days after the impact. The excess thermal energy places lower limits on the total energy of the impact (1.8-15.7 × 10²⁶ ergs), which limits the impactor diameter to 70-510 m (depending on the bulk density chosen for the material).The models of the Gemini spectra required three distinct aerosol features, indicative of the mineralogy of the dark particulate debris, centred at 9.1, 10.0 and 18.5 μm. The retrieved opacities for each of these features were distributed over a larger area (9-10° longitude) and at higher altitudes (above the 10-mbar level) than the stratospheric NH₃, and they are more spatially inhomogeneous. This implies the particulates were either entrained with the rising hot plume or created upon plume re-entry and are subsequently redistributed by stratospheric winds. The three particulate features were consistent with a mixture of amorphous iron and magnesium-rich silicates and silicas in the debris field. A broad 10-μm signature was coincident with peaks expected from material rich in amorphous olivines (but poor in pyroxenes), and similar to silicate features observed during SL9. A narrow 9.1-μm signature was interpreted as a combination of amorphous and crystalline silica. Finally, a broad 18.5-μm emitter was not adequately reproduced by a mixture of simple olivines and pyroxenes and remains to be identified.     

The density and temperature structure near the exobase of Saturn from Cassini UVIS solar occultations

We analyzed 15 solar occultations observed by the Cassini UVIS instrument to constrain the density and temperature structure near the exobase of Saturn. We retrieved the density of H₂ and thus the temperature at altitudes higher than 1900 km above the 1 bar level by analyzing the ionization continuum of H₂ at wavelengths shorter than 804 Å. We find that the exospheric temperature ranges from 370 K to 540 K, with a typical uncertainty of less than 20 K. According to our data the temperature increases with latitude from the equator to the poles by 100–150 K. At similar latitudes, the temperature varies by 20–50 K at different times with no evidence for any systematic diurnal trend so far. Based on our data, the exobase of Saturn is 2700–3000 km above the 1 bar level and the thermal escape parameter near the exobase ranges from 260 to 340, implying that thermal escape from Saturn is firmly in the Jeans regime. The mixing ratio of H₂ is close to unity at all altitudes below the exobase. We find that the pressure levels in the thermosphere deviate significantly from a simple spheroid predicted by potential theory. This is consistent with significant meridional temperature variations in the lower thermosphere. A global analysis of the temperature structure at different depths in the atmosphere is required to constrain both the shape and the deposition and redistribution of energy in the upper atmosphere further.     

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 cloud structure of the jovian atmosphere as seen by the Cassini/CIRS experiment

We analyze the thermal infrared spectra of Jupiter obtained by the Cassini-CIRS instrument during the 2000 flyby to infer temperature and cloud density in the jovian stratosphere and upper troposphere. We use an inversion technique to derive zonal mean vertical profiles of cloud absorption coefficient and optical thickness from a narrow spectral window centered at 1392 cm⁻¹ (7.18 μm). At this wavenumber atmospheric absorption due to ammonia gas is very weak and uncertainties in the ammonia abundance do not impact the cloud retrieval results. For cloud-free conditions the atmospheric transmission is limited by the absorption of molecular hydrogen and methane. The gaseous optical depth of the atmosphere is of order unity at about 1200 mbar. This allows us to probe the structure of the atmosphere through a layer where ammonia cloud formation is expected. The results are presented as height vs latitude cross-sections of the zonal mean cloud optical depth and cloud absorption coefficient. The cloud optical depth and the cloud base pressure exhibit a significant variability with latitude. In regions with thin cloud cover (cloud optical depth less than 2), the cloud absorption coefficient peaks at 1.1±0.05 bar, whereas in regions with thick clouds the peak cloud absorption coefficient occurs in the vicinity of 900±50 mbar. If the cloud optical depth is too large the location of the cloud peak cannot be identified. Based on theoretical expectations for the ammonia condensation pressure we conclude that the detected clouds are probably a system of two different cloud layers: a top ammonia ice layer at about 900 mbar covering only limited latitudes and a second, deeper layer at 1100 mbar, possibly made of ammonium hydrosulfide.     

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.     

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.     

Power spectral analysis of Jupiter’s clouds and kinetic energy from Cassini

We present suggestive evidence for an inverse energy cascade within Jupiter’s atmosphere through a calculation of the power spectrum of its kinetic energy and its cloud patterns. Using Cassini observations, we composed full-longitudinal mosaics of Jupiter’s atmosphere at several wavelengths. We also utilized image pairs derived from these observations to generate full-longitudinal maps of wind vectors and atmospheric kinetic energy within Jupiter’s troposphere. We computed power spectra of the image mosaics and kinetic energy maps using spherical harmonic analysis. Power spectra of Jupiter’s cloud patterns imaged at certain wavelengths resemble theoretical spectra of two-dimensional turbulence, with power-law slopes near −5/3 and −3 at low and high wavenumbers, respectively. The slopes of the kinetic energy power spectrum are also near −5/3 at low wavenumbers. At high wavenumbers, however, the spectral slopes are relatively flatter than the theoretical prediction of −3. In addition, the image mosaic and kinetic energy power spectra differ with respect to the location of the transition in slopes. The transition in slope is near planetary wavenumber 70 for the kinetic energy spectra, but is typically above 200 for the image mosaic spectra. Our results also show the importance of calculating spectral slopes from full 2D velocity maps rather than 1D zonal mean velocity profiles, since at large wavenumbers the spectra differ significantly, though at low wavenumbers, the 1D zonal and full 2D kinetic energy spectra are practically indistinguishable. Furthermore, the difference between the image and kinetic energy spectra suggests some caution in the interpretation of power spectrum results solely from image mosaics and its significance for the underlying dynamics. Finally, we also report prominent variations in kinetic energy within the equatorial jet stream that appear to be associated with the 5 μm hotspots. Other eddies are present within the flow collar of the Great Red Spot, suggesting caution when interpreting snapshots of the flow inside these features as representative of a time-averaged state.     

Turbulent convection in rapidly rotating spherical shells: A model for equatorial and high latitude jets on Jupiter and Saturn

The origin of zonal jets on the jovian planets has long been a topic of scientific debate. In this paper we show that deep convection in a spherical shell can generate zonal flow comparable to that observed on Jupiter and Saturn, including a broad prograde equatorial jet and multiple alternating jets at higher latitudes. We present fully turbulent, 3D spherical numerical simulations of rapidly rotating convection with different spherical shell geometries. The resulting global flow fields tend to be segregated into three regions (north, equatorial, and south), bounded by the tangent cylinder that circumscribes the inner boundary equator. In all of our simulations a strong prograde equatorial jet forms outside the tangent cylinder, whereas multiple jets form in the northern and southern hemispheres, inside the tangent cylinder. The jet scaling of our numerical models and of Jupiter and Saturn is consistent with the theory of geostrophic turbulence, which we extend to include the effect of spherical shell geometry. Zonal flow in a spherical shell is distinguished from that in a full sphere or a shallow layer by the effect of the tangent cylinder, which marks a reversal in the sign of the planetary β-parameter and a jump in the Rhines length. This jump is manifest in the numerical simulations as a sharp equatorward increase in jet widths—a transition that is also observed on Jupiter and Saturn. The location of this transition gives an estimate of the depth of zonal flow, which seems to be consistent with current models of the jovian and saturnian interiors.