Michael E. D. Allison

Titan's Rotation Reveals an Internal Ocean and Changing Zonal Winds

Cassini radar observations of Saturns moon Titan over several years show that its rotational period is changing and is different from its orbital period. The present-day rotation period difference from synchronous spin leads to a shift of ∼0.36° per year in apparent longitude and is consistent with seasonal exchange of angular momentum between the surface and Titans dense superrotating atmosphere, but only if Titans crust is decoupled from the core by an internal water ocean like that on Europa.

Planetary waves in Jupiter's equatorial atmosphere

Abstract Voyager infrared, radio occultation, and imaging measurements provide a variety of observational evidence and constraints for planetary-scale waves latitudinally trapped within Jupiters equatorial jet. The visually prominent plumes, encircling the planet at 8 deg north latitude, show a longitudinal organization of wavenumber n ≈11–13. An apparently solitary plume also appears at about 8 deg south along the with other smaller periodic cloud features. Deviations in cloud-tracked driff speed (including differences in mean values to the north and south of the equator) in excess of 25 m sec−1 may be crudely indicative of the magnitude of the associated wave phase speeds. Radio occultation retrievals of the temperature profiles at two equatorial longitudes show harmonic variation consistent with the stratospheric propagation of two components with vertical wavelengths of approximately 3 and 0.5 scale heights. These observations may be consistently interpreted in terms of specific classes of equatorially trapped wave modes. The gravest equatorial Rossby mode with an “equivalent depth” of h ≈ 2–4 km (with a stratospheric wavelength of 3–4 scale heights) represents the most consistent interpretation of the plumes. The inferred vetical eigenmode could be ducted by a deep, statically stable layer, possibly imposed by a roughly solar abundant water cloud, depending upon the actual details of the moist convection. An estimate of the relative growth rates for the near-neutral forcing of Rossby modes of the inferred equivalent depth implies a maximum for zonal wavenumber 11. The solitary plume feature to the south of the equator may be interpreted as a wavenumber 1 Rossby mode with the same vertical scale, possibly forced by the interaction between the equatorial jet and the Great Red Spot. Smaller visual features may correspond to inertia-gravity modes with vertical wavelengths comparable to the small harmonic of the occultation profiles. Galileo orbiter and probe observations will provide an essential test of the wave analysis and substantially enhance its diagnostic utility.

Jupiter’s interior and deep atmosphere: The initial pole-to-pole passes with the Juno spacecraft

Juno swoops around giant Jupiter Jupiter is the largest and most massive planet in our solar system. NASAs Juno spacecraft arrived at Jupiter on 4 July 2016 and made its first close pass on 27 August 2016. Bolton et al. present results from Junos flight just above the cloud tops, including images of weather in the polar regions and measurements of the magnetic and gravitational fields. Juno also used microwaves to peer below the visible surface, spotting gas welling up from the deep interior. Connerney et al. measured Jupiters aurorae and plasma environment, both as Juno approached the planet and during its first close orbit. Science, this issue p. 821, p. 826 Juno’s first close pass over Jupiter provides answers and fresh questions about the giant planet. On 27 August 2016, the Juno spacecraft acquired science observations of Jupiter, passing less than 5000 kilometers above the equatorial cloud tops. Images of Jupiter’s poles show a chaotic scene, unlike Saturn’s poles. Microwave sounding reveals weather features at pressures deeper than 100 bars, dominated by an ammonia-rich, narrow low-latitude plume resembling a deeper, wider version of Earth’s Hadley cell. Near-infrared mapping reveals the relative humidity within prominent downwelling regions. Juno’s measured gravity field differs substantially from the last available estimate and is one order of magnitude more precise. This has implications for the distribution of heavy elements in the interior, including the existence and mass of Jupiter’s core. The observed magnetic field exhibits smaller spatial variations than expected, indicative of a rich harmonic content.

A Wave Dynamical Interpretation of Saturn's Polar Hexagon

The hexagonal, pole-centered cloud feature in Saturns northern atmosphere, as revealed in Voyager close-encounter imaging mosaics, may be interpreted as a stationary Rossby wave. The wave is embedded within a sharply peaked eastward jet (of 100 meters per second) and appears to be perturbed by at least one anticyclonic oval vortex immediately to the south. The effectively exact observational determination of the horizontal wave number and phase speed, applied to a simple model dispersion relation, suggests that the wave is vertically trapped and provides a diagnostic template for further modeling of the deep atmospheric stratification.

THE HUYGENS DOPPLER WIND EXPERIMENT Titan Winds Derived from Probe Radio Frequency Measurements

A Doppler Wind Experiment (DWE) will be performed during the Titan atmospheric descent of the ESA Huygens Probe. The direction and strength of Titans zonal winds will be determined with an accuracy better than 1 m s−1 from the start of mission at an altitude of ∼160 km down to the surface. The Probes wind-induced horizontal motion will be derived from the residual Doppler shift of its S-band radio link to the Cassini Orbiter, corrected for all known orbit and propagation effects. It is also planned to record the frequency of the Probe signal using large ground-based antennas, thereby providing an additional component of the horizontal drift. In addition to the winds, DWE will obtain valuable information on the rotation, parachute swing and atmospheric buffeting of the Huygens Probe, as well as its position and attitude after Titan touchdown. The DWE measurement strategy relies on experimenter-supplied Ultra-Stable Oscillators to generate the transmitted signal from the Probe and to extract the frequency of the received signal on the Orbiter. Results of the first in-flight checkout, as well as the DWE Doppler calibrations conducted with simulated Huygens signals uplinked from ground (Probe Relay Tests), are described. Ongoing efforts to measure and model Titans winds using various Earth-based techniques are briefly reviewed.

A similarity model for the windy jovian thermocline

Abstract A new conceptual model for the observed banded wind structure of the giant outer planets is proposed, assuming a smoothly distributed potential vorticity (PV) atop a layer of deeply seated, latitudinally variable stratification. With a vanishing horizontal entropy contrast presumably enforced at the bottom of the flow layer by the underlying convection, the thermal-wind balance of observed cloud-top motions implies a mapping of constant potential temperature surfaces, nearly vertical at upper tropospheric levels, down to a deeper, flatter, but variably sloped “thermocline”. For a generally colder-poleward distribution of isentropes consistent with the strong equatorial jets of both Jupiter and Saturn, this kind of mapping would also imply a poleward decreasing static stability. The proposed temperature-stability distribution is just the reverse of the Earth’s troposphere, where the strongest latitudinal potential temperature gradients are at the bottom instead of the top, with static stability generally increasing toward the pole. The warmer–stabler correlation for Jupiter would be consistent, however, with a nearly monotonic distribution of PV from low to high latitudes, over planetary scales for which the planetary vorticity dominates the relative vorticity of the jets. In this way efficient PV mixing for the isentropically bounded thermocline can account for the dynamical maintenance of the cyclonic flanks of the equatorial jet. Over latitudinal intervals comparable to the internal deformation scale, however, the gradient of the absolute vorticity is dominated by the flow curvature, correlated for a well-mixed PV distribution with the local latitudinal gradient of the static stability, itself proportional for the assumed thermocline structure to the local potential temperature gradient and therefore the local departures in geostrophic velocity. The resulting correlation of velocities and vorticity gradients would imply an alternation of the flow over the internal deformation radius set by the vertical entropy contrast. The requisite size of the deep stability is itself roughly consistent with the diagnostic interpretation of the apparent dispersive character and vertical wavelength of observed temperature oscillations in Jupiter’s upper troposphere and stratosphere, as well as a plausible extrapolation of the deep sub-adiabatic lapse rate detected by the Galileo probe.

Zero potential vorticity envelopes for the zonal-mean velocity of the Venus/Titan atmospheres

Abstract The diagnostic analysis of numerical simulations of the Venus/Titan wind regime reveals an overlooked constraint upon the latitudinal structure of their zonal-mean angular momentum. The numerical experiments, as well as the limited planetary observations, are approximately consistent with the hypothesis that within the latitudes bounded by the wind maxima the total Ertel potential vorticity associated with the zonal-mean motion is approximately well mixed with respect to the neutral equatorial value for a stable circulation. The implied latitudinal profile of angular momentum is of the form M ≤ Me(cosλ)2/Ri, where λ is the latitude and Ri the local Richardson number, generally intermediate between the two extremes of uniform angular momentum (Ri → ∞) and uniform angular velocity (Ri = 1). The full range of angular momentum profile variation appears to be realized within the observed meridional–vertical structure of the Venus atmosphere, at least crudely approaching the implied relationship betwee...

Richardson number constraints for the Jupiter and outer planet wind regime

Present theories of the dynamics of the outer planet atmospheres are limited by the absence of direct observations of the wind and temperature layering below their visible cloud decks. We show that if the potential vorticity is assumed to be small at low latitudes, then cloud-tracked wind measurements of the equatorial flow on Jupiter, Saturn, Uranus, and Neptune can be used to constrain their vertical stratification at deeper levels. A Richardson number between 1.2 and 4 is indicated for all the giant planets, which at their rapid rotation implies a much more vertical isentropic structure at these levels than is realized in the terrestrial atmospheres. These conditions signify the likely presence of a unique but as yet unstudied regime for the jovian meteorology, illustrated by a new “Richardson-Rossby” diagram for comparative planetary circulations. The Jupiter case will be directly tested by the Galileo atmospheric entry probe.

The Huygens Doppler Wind Experiment

A Doppler Wind Experiment (DWE) will be performed during the Titan atmospheric descent of the ESA Huygens Probe. The direction and strength of Titan’s zonal winds will be determined with an accuracy better than 1 m s−1 from the start of mission at an altitude of ~ 160 km down to the surface. The Probe’s wind-induced horizontal motion will be derived from the residual Doppler shift of its S-band radio link to the Cassini Orbiter, corrected for all known orbit and propagation effects. It is also planned to record the frequency of the Probe signal using large ground-based antennas, thereby providing an additional component of the horizontal drift. In addition to the winds, DWE will obtain valuable information on the rotation, parachute swing and atmospheric buffeting of the Huygens Probe, as well as its position and attitude after Titan touchdown. The DWE measurement strategy relies on experimenter-supplied Ultra-Stable Oscillators to generate the transmitted signal from the Probe and to extract the frequency of the received signal on the Orbiter. Results of the first in-flight checkout, as well as the DWE Doppler calibrations conducted with simulated Huygens signals uplinked from ground (Probe Relay Tests), are described. Ongoing efforts to measure and model Titan’s winds using various Earth-based techniques are briefly reviewed.

The distribution of ammonia on Jupiter from a preliminary inversion of Juno Microwave Radiometer data

The Juno microwave radiometer measured the thermal emission from Jupiters atmosphere from the cloud tops at about 1 bar to as deep as a hundred bars of pressure during its first flyby over Jupiter (PJ1). The nadir brightness temperatures show that the Equatorial Zone is likely to be an ideal adiabat, which allows a determination of the deep ammonia abundance in the range 362^(+33)_(-33) ppm. The combination of Markov chain Monte Carlo method and Tikhonov regularization is studied to invert Jupiters global ammonia distribution assuming a prescribed temperature profile. The result shows (1) that ammonia is depleted globally down to 50–60 bars except within a few degrees of the equator, (2) the North Equatorial Belt is more depleted in ammonia than elsewhere, and (3) the ammonia concentration shows a slight inversion starting from about 7 bars to 2 bars. These results are robust regardless of the choice of water abundance.