Saturn's Shifting Seasons from a Decade of Cassini Observations

Last year the Cassini spacecraft celebrated the end of a decade of exploration of the gas giant Saturn, yet another milestone in a mission that is finally set to end in a fiery plunge into Saturn’s atmosphere in September 2017.  When that time comes, Cassini will have been orbiting the ringed world for half of a Saturnian year, from perihelion to aphelion and from northern winter solstice to northern summer solstice.  Saturn’s 27-degree axial tilt subjects the atmosphere to seasonal shifts in sunlight over its 30-year orbit of the Sun.  The atmosphere responds to this changing solar energy deposition, generating hemispheric asymmetries in temperature, composition and cloud coverage that alter over time.  Large-scale circulation patterns move energy from one hemisphere to the other, and the associated meteorology and chemistry lead to changes in the soup of species that make up Saturn’s atmospheric composition. 

The unprecedented duration of Cassini’s reconnaissance of Saturn is allowing planetary scientists to study the implications of the shifting seasons for the first time.  Dr. Leigh Fletcher, Senior Research Fellow in RSPP and co-investigator on Cassini, has recently published a new study of Saturn’s shifting temperatures and composition from Cassini in the journal Icarus (Fletcher et al. 2016, Icarus 264, p137-159, http://arxiv.org/abs/1509.02281), using a thermal-infrared instrument called CIRS (the Composite Infrared Spectrometer).  By considering many thousands of spectra taken since 2004, Fletcher and colleagues were able to reconstruct Saturn’s changing climate in three dimensions – latitude, altitude and time – constructing the first ever movies of Saturn’s changing environmental conditions. 

The movies revealed the slow warming of Saturn’s northern hemisphere as it emerged from the darkness of polar winter into spring sunlight.  Air began sinking over Saturn’s northern polar region, carrying chemicals along with it so that these species (notably spin isomers of hydrogen and hydrocarbons produced by methane photochemistry) became enriched in the northern hemisphere as spring progressed.  Likewise, as Saturn’s autumnal hemisphere descended into winter darkness, the movies revealed the cooling and disappearance of a large warm seasonal vortex over Saturn’s south pole.   The results also showed that Saturn doesn’t respond instantaneously to the sunlight changes – the atmosphere has such a large thermal inertia that the coldest northern conditions are actually found in springtime rather than at winter solstice, lagging almost a season behind the solar insolation.

Zonal mean cross-sections of atmospheric temperatures before, during, and after the equinox of 2009, showing seasonal cooling in the autumnal southern hemisphere and warming in the springtime northern hemisphere.  These large-scale asymmetries are superimposed onto the belt/zone structure of cool zones and warmer belts that typify the atmospheres of Jupiter and Saturn.

Fletcher and colleagues used the movies to investigate the dynamical, meteorological and chemical consequences of the shifting seasons, including the ability of Saturn’s atmosphere to host planetary wave activity.  These waves, potentially launched into the stratosphere by powerful weather activity at deeper levels, can only propagate under certain atmospheric conditions.  One striking example of that was the intense springtime storm, which launched waves high into the overlying stratosphere in 2010-11.   These storms appear to happen once per year during spring or summer, but how these enormous storms might be linked to the seasonal cycle remains a topic of ongoing investigation.  Finally, the shifting temperature was shown to alter the atmospheric chemistry and the production of hazes.  These hazes are responsible for Saturn’s familiar ochre appearance, and were notably absent from Saturn’s blue northern hemisphere early in Cassini’s mission.  As the temperatures warmed, aerosol particles grew in size and Saturn’s ‘blues’ dissipated, providing a direct connection between Saturn’s shifting temperatures and the colours we can see in the clouds. 

These results form a part of a new book on Cassini’s discoveries on Saturn that will soon be available from Cambridge University Press.  Fletcher’s chapter on Saturn’s seasons is available as a preprint (http://arxiv.org/abs/1510.05690).