Seasons on Saturn
Saturn during southern summer just before Cassini arrived in May 2004, showing the familiar yellow-ochre appearance of its cloud tops, and a faintly banded structure less prominent than that of Jupiter. The rings cast long shadows on the northern winter hemisphere, where a hint of blue colours can be observed. Saturn is 95 times the mass of Earth, 9 times the diameter, only 12.5% of the density and receives around 1% of the solar illumination compared to the Earth. (Credit: NASA/JPL/Space Science Institute) http://photojournal.jpl.nasa.gov/catalog/PIA06077
A better view of the northern blue hues from November 2004, showing tiny Mimas against the ring shadows. Saturn's atmosphere responds to the different levels of sunlight, with aerosols growing larger and more opaque in the spring and summer, but vanishing over the winter, explaining this asymmetry between the hemispheres. Where there are fewer scattering hazes in the north, light has to travel through longer paths of atmospheric methane before it reflects from the cloud tops. As methane absorbs red light very strongly, the remaining light is mostly blue, just like on Uranus and Neptune. (Credit: NASA/JPL/Space Science Institute) http://photojournal.jpl.nasa.gov/catalog/PIA06142
Saturn orbits the Sun once every 30 Earth years, so the seasons are around 7.5 years long. Saturn's obliquity of 26 degrees is slightly larger than that of the Earth (23 degrees). As northern winter (2002) marched on to northern spring (2009), the north pole emerged from the shroud of winter darkness, and aerosols grew to give Saturn its typical yellow-ochre appearance, as in this image captured at the equinox. Here, the Sun equally illuminates the northern spring and the southern autumn hemispheres, and the rings would have vanished to a thin line as viewed from Earth. (Credit: NASA/JPL/Space Science Institute) http://photojournal.jpl.nasa.gov/catalog/PIA11667
At the equinox, the shadow of the rings drops to a tiny line at Saturn's equator. This shift in illumination from south to north seems to have coincided with a number of changes in Saturn's weather, generating more convective, turbulent activity in the north where the most dramatic changes have taken place. Note Rhea on the far right of this image. (Credit: NASA/JPL/Space Science Institute) http://photojournal.jpl.nasa.gov/catalog/PIA12513 Emily Lakdawalla has a great blog post explaining some of these changes in more detail.
One method Cassini uses to diagnose these seasonal changes are images taken across lots of different wavelengths, from the ultraviolet to the far-infrared. This image from Cassini's Visual and Infrared Mapping Spectrometer brings together a blue 2.3 µm image (water ice in the rings is very reflective, atmospheric methane very absorbing), a green 3.0 µm image (water ice rings absorbing, but lots of reflection from the sunlit portion of Saturn) and a red 5.1 µm image (showing thermal emission from the planet itself). Note that you can see the thermal emission from the non-illuminated side of Saturn, and all the fine cloud structures are seen in silhouette against the deep internal red glow. (Credit: NASA/JPL/University of Arizona) http://photojournal.jpl.nasa.gov/catalog/PIA09212
Saturn's Seasonal Stormshttp://photojournal.jpl.nasa.gov/catalog/PIA08411
my blog post here. (Credit: University of Oxford/L.N. Fletcher/ESO) http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=50997 and http://photojournal.jpl.nasa.gov/catalog/PIA16190 for the movie.
Saturn's Polar Atmospherehttp://photojournal.jpl.nasa.gov/catalog/PIA11215
This more oblique view, also from 2008, shows the shadows cast by these concentric eyewalls around Saturn's south polar vortex. These images of the southern pole were obtained while it was still in sunlight, before it disappeared into darkness in August 2009, not to be seen again in reflected sunlight for the remainder of the Cassini mission. With sunlight now returning to the northern hemisphere, Cassini has begun to capture images of the northern pole. (Credit: NASA/JPL/Space Science Institute) http://photojournal.jpl.nasa.gov/catalog/PIA11103
Universe Today), and the result is stunning - swirling clouds in the heart of Saturn's north polar vortex. Even more amazing is the 7-frame animation compiled by Bjorn Jonsson and seen here. (Credit: NASA/JPL/Space Science Institute)
Without a doubt Cassini's most stunning image of Saturn, obtained back in September 2006 as Cassini moved into Saturn's shadow. The sun can be seen refracted through Saturn's upper atmosphere, and the pale dot of Earth can be seen just interior to the G ring, from a robotic vantage point over a billion kilometres from home. The diffuse E ring, being actively vented from icy Enceladus, encircles the planet; the narrowly-confined G ring is easily seen just beyond the main rings; and these images even allowed astronomers to discover two faint new rings around the planet associated with satellites Janus, Epimetheus, and Pallene. (Credit: NASA/JPL/Space Science Institute) http://photojournal.jpl.nasa.gov/catalog/PIA08329
http://photojournal.jpl.nasa.gov/catalog/PIA14934). No Earth this time (although you can just make out Tethys and Enceladus on the left of the planet), but another image in November 2012 was able to spy bright Venus between the planet and the innermost rings (http://photojournal.jpl.nasa.gov/catalog/PIA14935) (Credit: NASA/JPL/Space Science Institute).