In December 2010, a major storm erupted in Saturn’s northern hemisphere near 37° planetographic latitude. This rather surprising event, occurring at an unexpected latitude and time, is the sixth “Great White Spot” (GWS) storm observed over the last century and a half. Such GWS events are extraordinary, planetary-scale atmospheric phenomena that dramatically change the typically bland appearance of the planet. Occurring while the Cassini mission was on orbit at Saturn, the Great Storm of 2010–2011 was well suited for intense scrutiny by the suite of sophisticated instruments onboard the Cassini spacecraft as well by modern instrumentation on ground-based telescopes and onboard the Hubble Space Telescope. This GWS erupted on 5 December close to the peak of a westward jet and generated a major dynamical disturbance that affected the whole latitude band from 25° to 48°N. At the upper cloud level, following the rapid growth of the bright outbreak spot, a blunt aerodynamic-shaped head formed due to interaction of the spot with the westward zonal jet, with the winds reaching velocities of 160 m s−1 along the periphery of the arc. Eastward of the head, the disturbance progressed in the following months forming a turbulent wake or tail with growing vortices, one of them a major enduring anticyclone (called AV) with a size of ~11,000 km. Lightning events were prominent and detected as outbursts and flashes at the head and along the disturbance at both optical and radio wavelengths. The activity of the head ceased after about seven months when AV reached it, leaving the cloud structure and ambient winds perturbed. The tops of the optically dense clouds of the head reached the 300-mbar altitude level (~50 km below tropopause), where a mixture of ices was detected, including (1) a component of water ice lofted over 200 km altitude from its 10-bar condensation level, (2) ammonia ice as the predominant component and (3) a component that might be ammonium hydrogen sulfide ice. The energetics of the frequency and power of lightning, as well as the estimated power generated by the latent heat released in the water-based convection to create the observed dynamical three-dimensional flows, both indicate that the power released for much of the 7-month lifetime of the storm (~1017 Watts) was a significant fraction of Saturn’s total radiated power (~2.2 1017 W). A post-storm depletion of ammonia vapour was also measured in the upper troposphere. The effects of the storm propagated into the stratosphere, forming two warm air masses at the ~0.5- to 5-mbar pressure level altitude that later merged into a so-called “beacon” because of its 80 K temperature excess relative to its surroundings. Related to the stratospheric disturbance, hydrocarbon composition excesses were found, in particular for ethylene (C2H4), in the high stratosphere at the ~0.1- to 0.5-mbar altitude level. Numerical models of the storm dynamics explain the major observed features that essentially result from two processes: (1) a huge and sustained, moist, convective storm at the water clouds (altitude level 10–12 bar, or ~250–275 km below the tropopause) and (2) the interaction of the updraft columns with the ambient winds that generates the turbulent wake consisting of vortices and waves. Model simulations of the GWS require a low vertical shear of the zonal winds and low static stability across the weather layer where the disturbance develops. Its upward propagation into the stratosphere involves Rossby waves and their breaking and energy deposition to form the beacon and induce chemical changes.
The decades-long interval between storms is probably related to the insolation cycle and the long radiative time constant of Saturn’s atmosphere, and several theories for temporarily storing energy have been proposed.
This chapter reviews the state of our knowledge about Saturn’s polar atmosphere that has been revealed through Earth- and space-based observation as well as theoretical and numerical modeling. In particular, the Cassini mission to Saturn, which has been in orbit around the ringed planet since 2004, has revolutionized our understanding of the planet. The current review updates a previous review by Del Genio et al. (2009), written after Cassini’s primary mission phase that ended in 2008, by focusing on the north polar region of Saturn and comparing it to the southern high latitudes. Two prominent features in the northern high latitudes are the northern hexagon and the north polar vortex; we extensively review observational and theoretical investigations to date of both features. We also review the seasonal evolution of the polar regions using the observational data accumulated during the Cassini mission since 2004 (shortly after the northern winter solstice in 2002), through the equinox in 2009, and approaching the next solstice in 2017. We conclude the current review by listing unanswered questions and describing the observations of the polar regions planned for the Grand Finale phase of the Cassini mission between 2016 and 2017.
The longevity of Cassini’s exploration of Saturn’s atmosphere (a third of a Saturnian year) means that we have been able to track the seasonal evolution of atmospheric temperatures, chemistry and cloud opacity over almost every season, from solstice to solstice and from perihelion to aphelion. Cassini has built upon the decades-long ground-based record to observe seasonal shifts in atmospheric temperature, finding a thermal response that lags behind the seasonal insolation with a lag time that increases with depth into the atmosphere, in agreement with radiative climate models. Seasonal hemispheric contrasts are perturbed at smaller scales by atmospheric circulation, such as belt/zone dynamics, the equatorial oscillations and the polar vortices. Temperature asymmetries are largest in the middle stratosphere and become insignificant near the radiative-convective boundary. Cassini has also measured southern-summertime asymmetries in atmospheric composition, including ammonia (the key species forming the topmost clouds), phosphine and para-hydrogen (both disequilibrium species) in the upper troposphere; and hydrocarbons deriving from the UV photolysis of methane in the stratosphere (principally ethane and acetylene). These chemical asymmetries are now altering in subtle ways due to (i) the changing chemical efficiencies with temperature and insolation and (ii) vertical motions associated with large-scale overturning in response to the seasonal temperature contrasts. Similarly, hemispheric contrasts in tropospheric aerosol opacity and coloration that were identified during the earliest phases of Cassini’s exploration have now reversed, suggesting an intricate link between the clouds and the temperatures. Finally, comparisons of observations between Voyager and Cassini (both observing in early northern spring, one Saturn year apart) show tantalizing suggestions of non-seasonal variability. Disentangling the competing effects of radiative balance, chemistry and dynamics in shaping the seasonal evolution of Saturn’s temperatures, clouds and composition remains the key challenge for the next generation of observations and numerical simulations.
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