The Great Saturn Storm of 2010–2011

Abstract

In December 2010, a major storm erupted in Saturn's northern hemisphere near 37 o 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 December 5 th 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 shape head formed due to interaction of the spot with the westward zonal jet, with the winds reaching velocities of 160 ms -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 3-dimensional flows, both indicate that the power released for much of the 7-month lifetime of the storm (~ 10 17 W) was a significant fraction of Saturn's total radiated power (~ 2.2 10 17 W). A post storm depletion of ammonia vapor was also measured in the upper troposphere. The effects of the storm propagated into the stratosphere forming two warm airmasses at the  0.5-5 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 (C 2 H 4 ), in the high stratosphere at the  0.1-0.5 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 a...

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