Seasons in the Saturn System

Cassini gazing down into Saturn’s spectacular north polar vortex, revealed in reflected sunlight as the north pole enters spring time conditions after years of winter darkness, November 2012. Credit:  NASA/JPL

The EPSC in Madrid in September 2012 featured sessions on giant planets, satellites and seasonal processes in our solar system, all of which provided insights into some of the latest discoveries by Cassini in the Saturn system.  Cassini has been orbiting Saturn since mid winter in the northern hemisphere (when Saturn's northern clouds had a distinctly blue, Neptune like appearance).  The spring equinox was in 2009, when the sun passed from the south into the northern hemisphere for the first time in fifteen years (half of a Saturn year) and the rings were seen edge-on from earth.  Cassini will hopefully remain operating all the way to northern summer solstice in 2017, fully characterising half of a Saturnian year.

Linda Spilker, Cassini project scientist, gave a keynote talk describing some of the latest findings.  During equinox, Saturn's thin F ring remained glowing, revealing that it is slightly inclined compared to the other rings, which were only partially illuminated by Saturn shine (light reflected from the clouds below). Elevated features in the rings cast long shadows, and dusty streaks above the ring plane were evidence of recent impacts, their debris sheared out by keplerian motion and catching the sunlight.  Edge waves on the Keeler Gap, created by the tiny moon Daphnis, cast shadows due to a 4 km lifting out of the ring plane, whereas similar edge waves in the Encke Division, created by tiny Pan, had no similar vertical extension.  Bizarre propeller objects in the rings were also projected above the ring plane.  Thermal mapping showed that the rings were at their coldest at the equinox at 45K, in equilibrium with e thermal radiation from the planet itself.  Intriguingly, the corrugated nature of the rings (ripples observed at equinox) could be modelled and traced all the way back to an impact event in the rings in 1983.  A similar corrugated wave in the Jovian system could be traced back to the collision of comet Shoemaker-Levy 9 in 1994. 

Beyond the rings, the tidally-locked satellite Mimas has been shown to have an unusual interaction between the surface and the magnetospheric environment.  A dark lens shape on the leading face of Mimas coincides with a cold region, bounded by sharp warm edges that give the moon a "pacman" appearance in certain geometries.  Cassini scientists suggest that energetic electrons, travelling retrograde around Saturn, slam into the surface to cate material differences in colour and thermal inertia.  These electrons are mainly funnelled into the corner ansae, creating the lens-shaped appearance.  On the trailing hemisphere, cold plasma and E ring grains cause erosion.  Amanda Hendrix also talked about the UV darkening of water ice due to photolytic creation of hydrogen peroxide, produced in the summer but destroyed in winter by electron bombardment, leading to seasonal asymmetries in UV brightness of the moons.  They predict that the northern hemisphere will darken during southern summer as more and more of this H2O2 is produced.

Alice le Gall reported evidence of seasonal variations on Iapetus, the solar systems most distant tidally-locked moon.  Its inclined orbit makes Iapetus difficult to reach, but also means that the moon passed equinox back in 2007 before the rest of the Saturn system.  Passive radar scans of the Cassini Regio showed that the north was cooler than the south, despite more illumination in the north, and suggestive of heat buried in the upper few centimetres of the moon during the long summer season.  Beneath about 1 m, however, the temperature is just symmetric about the equator.

On Saturn itself, we have been gripped by observations of the gigantic springtime storm system since 2010.  The churning tropospheric storm persisted from December 2010 to July 2011, but produced after effects in the stratosphere that persist to this day, and will be the topic of future posts when the papers are all published.  But Spilker described one interesting development - it was thought that we'd never see lightning on Saturn because of light reflected from the rings.  But the northern storm was so powerful that Cassini could observe the flashes in blue light.  Away from the storm, the slow seasons march on, with Saturn's northern blue hues now completely replaced by the familiar yellow-ochre colours we are all familiar with.  My own work is showing that northern temperatures are warming, as expected, and that the hot south polar stratosphere we reported a few years ago is showing signs of diminishing.

Saturn's enigmatic moon, Titan, continues to undergo severe changes in its atmospheric circulation, as reported by Ralph Lorenz.  When Cassini arrived, the North Pole featured a dramatic vortex, enriched in chemicals due the downward branch of a global circulation system.  That dark north polar hood has now become a distinct dark lane with a detached haze layer.  The height of the detached layer of haze itself seems to change, rising and falling depending on the strength of the meridional circulation. A similar polar hood is now seen to be developing at the South Pole, with recent images showing a dramatic vortex structure reminiscent of Venus' dipole vortex discovered by Venus Express. The whole atmospheric circulation may be shifting direction as spring progresses.

One of the most spectacular Cassini results are the recent observations of specular reflections from Titan's northern lakes, observed by the VIMS instrument at 5 microns.  Glinting sunlight from a titanian sea is a wonderful thought.  

The Cassini spacecraft is still in excellent health, 8 years into the mission and 15 years since launch.  If all goes to plan, the proximal orbits in 2016 will see Cassini closer to Saturn than ever before, flying within the rings themselves as we approach northern summer solstice.  Then, around September 2017 Cassini will plunge to a fiery death, a fitting end to an incredible mission.