F. Michael Flasar

Thermal structure and dynamics of Saturn's northern springtime disturbance

Satellite and ground-based observations characterize a massive storm on Saturn and its effects on the atmosphere. Saturn’s slow seasonal evolution was disrupted in 2010–2011 by the eruption of a bright storm in its northern spring hemisphere. Thermal infrared spectroscopy showed that within a month, the resulting planetary-scale disturbance had generated intense perturbations of atmospheric temperatures, winds, and composition between 20° and 50°N over an entire hemisphere (140,000 kilometers). The tropospheric storm cell produced effects that penetrated hundreds of kilometers into Saturn’s stratosphere (to the 1-millibar region). Stratospheric subsidence at the edges of the disturbance produced “beacons” of infrared emission and longitudinal temperature contrasts of 16 kelvin. The disturbance substantially altered atmospheric circulation, transporting material vertically over great distances, modifying stratospheric zonal jets, exciting wave activity and turbulence, and generating a new cold anticyclonic oval in the center of the disturbance at 41°N.

Active upper-atmosphere chemistry and dynamics from polar circulation reversal on Titan

Saturn’s moon Titan has a nitrogen atmosphere comparable to Earth’s, with a surface pressure of 1.4 bar. Numerical models reproduce the tropospheric conditions very well but have trouble explaining the observed middle-atmosphere temperatures, composition and winds. The top of the middle-atmosphere circulation has been thought to lie at an altitude of 450 to 500 kilometres, where there is a layer of haze that appears to be separated from the main haze deck. This ‘detached’ haze was previously explained as being due to the co-location of peak haze production and the limit of dynamical transport by the circulation’s upper branch. Here we report a build-up of trace gases over the south pole approximately two years after observing the 2009 post-equinox circulation reversal, from which we conclude that middle-atmosphere circulation must extend to an altitude of at least 600 kilometres. The primary drivers of this circulation are summer-hemisphere heating of haze by absorption of solar radiation and winter-hemisphere cooling due to infrared emission by haze and trace gases; our results therefore imply that these effects are important well into the thermosphere (altitudes higher than 500 kilometres). This requires both active upper-atmosphere chemistry, consistent with the detection of high-complexity molecules and ions at altitudes greater than 950 kilometres, and an alternative explanation for the detached haze, such as a transition in haze particle growth from monomers to fractal structures.

Saturn Atmospheric Structure and Dynamics

Saturn inhabits a dynamical regime of rapidly rotating, internally heated atmospheres similar to Jupiter. Zonal winds have remained fairly steady since the time of Voyager except in the equatorial zone and slightly stronger winds occur at deeper levels. Eddies supply energy to the jets at a rate somewhat less than on Jupiter and mix potential vorticity near westward jets. Convective clouds exist preferentially in cyclonic shear regions as on Jupiter but also near jets, including major outbreaks near 35°S associated with Saturn electrostatic discharges, and in sporadic giant equatorial storms perhaps generated from frequent events at depth. The implied meridional circulation at and below the visible cloud tops consists of upwelling (downwelling) at cyclonic (anti-cyclonic) shear latitudes. Thermal winds decay upward above the clouds, implying a reversal of the circulation there. Warm-core vortices with associated cyclonic circulations exist at both poles, including surrounding thick high clouds at the south pole. Disequilibrium gas concentrations in the tropical upper troposphere imply rising motion there. The radiative-convective boundary and tropopause occur at higher pressure in the southern (summer) hemisphere due to greater penetration of solar heating there. A temperature “knee” of warm air below the tropopause, perhaps due to haze heating, is stronger in the summer hemisphere as well. Saturns south polar stratosphere is warmer than predicted by radiative models and enhanced in ethane, suggesting subsidence-driven adiabatic warming there. Recent modeling advances suggest that shallow weather layer theories of jet pumping may be viable if water condensation is the source of energy input driving the flow, and that deep convective cylinder models with a sufficiently large tangent cylinder radius can reproduce observed flow features as well.

Dynamics of Saturn's South Polar Vortex

The camera onboard the Cassini spacecraft has allowed us to observe many of Saturns cloud features. We present observations of Saturns south polar vortex (SPV) showing that it shares some properties with terrestrial hurricanes: cyclonic circulation, warm central region (the eye) surrounded by a ring of high clouds (the eye wall), and convective clouds outside the eye. The polar location and the absence of an ocean are major differences. It also shares properties with the polar vortices on Venus, such as polar location, cyclonic circulation, warm center, and long lifetime, but the Venus vortices have cold collars and are not associated with convective clouds. The SPVs combination of properties is unique among vortices in the solar system

Saturn's emitted power

Long-term (2004–2009) on-orbit observations by Cassini Composite Infrared Spectrometer are analyzed to precisely measure Saturns emitted power and its meridional distribution. Our evaluations suggest that the average global emitted power is 4.952 ± 0.035 W m^(−2) during the period of 2004–2009. The corresponding effective temperature is 96.67 ± 0.17 K. The emitted power is 16.6% higher in the Southern Hemisphere than in the Northern Hemisphere. From 2005 to 2009, the global mean emitted power and effective temperature decreased by ~2% and ~0.5%, respectively. Our study further reveals the interannual variability of emitted power and effective temperature between the epoch of Voyager (~1 Saturn year ago) and the current epoch of Cassini, suggesting changes in the cloud opacity from year to year on Saturn. The seasonal and interannual variability of emitted power implies that the energy balance and internal heat are also varying.

Atmospheric Structure and Composition

Titans atmosphere is predominantly N2 with CH4 the next most abundant molecule. It has a mole fraction of 0.05 just above the surface decreasing to 0.014 in the stratosphere. Above the homopause (~800–850 km), it increases to 0.12 at the exobase. The third abundant molecule is H2 with a tropospheric mole fraction of 0.001 increasing to 0.004 at ~1000 km and ~0.02 at the exobase (~1500– 1600 km). This chapter reviews the various measurements acquired by the Voyager flybys, Huygens Probe, orbiting Cassini spacecraft, ground-based and orbiting telescopes of the large suite of hydrocarbons, nitriles, other nitrogen and also oxygen bearing compounds. Titan possesses a mostly stable troposphere with a well defined tropopause (T ~ 70 K at ~44 km) and a lower stratosphere with a high static stability, which is extremely cold over the winter polar region (currently northern hemisphere) and warm over the summer pole. Remarkably in the middle stratosphere, the warmest temperatures occur at the equator and the largest meridional temperature gradients are found in the winter hemisphere. The stratopause from the summer pole to about 45° N remains at a relatively constant pressure of 0.1 mbar/300 km and then it rises rapidly upward to ~0.01 mbar/400 km at the winter north pole, where it is the warmest region in the entire atmosphere. One possible interpretation of the Huygens Atmospheric Structure Instrument (HASI) temperature profile is that Titans atmosphere is essentially isothermal ~170 K from 500–1100 km, with large amplitude thermal waves (10 K) superimposed. The existence and location of a well defined mesopause is an open question.

Infrared limb sounding of Titan with the Cassini Composite InfraRed Spectrometer: effects of the mid-IR detector spatial responses

The composite infrared spectrometer (CIRS) instrument on board the Cassini Saturn orbiter employs two 1×10 HgCdTe detector arrays for mid-infrared remote sensing of Titans and Saturns atmospheres. In this paper we show that the real detector spatial response functions, as measured in ground testing before launch, differ significantly from idealized “boxcar” responses. We further show that neglecting this true spatial response function when modeling CIRS spectra can have a significant effect on interpretation of the data, especially in limb-sounding mode, which is frequently used for Titan science. This result has implications not just for CIRS data analysis but for other similar instrumental applications.

Strong jet and a new thermal wave in Saturn's equatorial stratosphere

The strong jet, with a speed between 500 and 600 m/s, is inferred in the equatorial region of Saturn by combining the nadir and limb observations of Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft. A similar jet was discovered on Jupiter (F. M. Flasar et al., 2004a). These discoveries raise the possibility that intense jets are common in the equatorial stratospheres of giant planets. An equatorial wave with wavenumber ~9 is revealed in the stratosphere of Saturn by the CIRS high spatial-resolution thermal maps. Our discussion based on the phase velocity suggests that the equatorial wave is probably a Rossby-gravity wave. The discovery of an equatorial wave in the stratosphere suggests that Saturns equatorial oscillations (T. Fouchet et al., 2008; G. S. Orton et al., 2008) may be driven by vertically propagating waves, the same mechanism that drives the quasi-biennial oscillation (QBO) on Earth.

Upper limits for undetected trace species in the stratosphere of Titan

In this paper we describe the first quantitative search for several molecules in Titans stratosphere in Cassini CIRS infrared spectra. These are: ammonia (NH3), methanol (CH3OH), formaldehyde (H2CO), and acetonitrile (CH3CN), all of which are predicted by photochemical models but only the last of which has been observed, and not in the infrared. We find non-detections in all cases, but derive upper limits on the abundances from low-noise observations at 25 degrees S and 75 degrees N. Comparing these constraints to model predictions, we conclude that CIRS is highly unlikely to see NH3 or CH3OH emissions. However, CH3CN and H2CO are closer to CIRS detectability, and we suggest ways in which the sensitivity threshold may be lowered towards this goal.

Strong Temporal Variation Over One Saturnian Year: From Voyager to Cassini

Here we report the combined spacecraft observations of Saturn acquired over one Saturnian year (~29.5 Earth years), from the Voyager encounters (1980–81) to the new Cassini reconnaissance (2009–10). The combined observations reveal a strong temporal increase of tropic temperature (~10 Kelvins) around the tropopause of Saturn (i.e., 50 mbar), which is stronger than the seasonal variability (~a few Kelvins). We also provide the first estimate of the zonal winds at 750 mbar, which is close to the zonal winds at 2000 mbar. The quasi-consistency of zonal winds between these two levels provides observational support to a numerical suggestion inferring that the zonal winds at pressures greater than 500 mbar do not vary significantly with depth. Furthermore, the temporal variation of zonal winds decreases its magnitude with depth, implying that the relatively deep zonal winds are stable with time.