Tuesday, 7 May 2013

Saturn's Weather: A Guide in Pictures

The May 2013 episode of the Sky at Night is all about Saturn, and I was asked to collect together some of Cassini's nicest images to discuss our latest discoveries in Saturn's dynamic and evolving atmosphere. I thought it'd be nice to collect all these sources into one place, as I'm constantly having to track them down for presentations, so here they are!  The vast majority are available via NASA's Photojournal, and I've tried to tell a story with some of the images, including all the links and credits.  Enjoy!

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 Storms

Saturn's atmosphere is dominated by hazy material, either formed from cloud particles mixed from the deeper atmosphere, or from photochemically-produced materials raining down from above.  For that reason, a lot of the really interesting dynamics is hidden from view.  But we'd be mistaken for thinking that Saturn is a lot less active than Jupiter.  Small-scale storms do occur, and for much of Cassini's mission they were confined to a band at 35S known as storm alley.  This particular storm was imaged in March 2008, after it had been detected via the radio emission of its cracking lightning a few months earlier.  (Credit: NASA/JPL/Space Science Institute)  http://photojournal.jpl.nasa.gov/catalog/PIA08411

However, once every Saturnian year an enormous eruption of billowing white cloud material occurs on Saturn, generating structures that enthral amateur and professional astronomers alike.  This eruption was the sixth on record since 1876, and occurred in Saturn's northern hemisphere near the peak of a westward jet, which helped spread cloud material around the planet.  This image was obtained around 12 weeks after the eruption was first discovered in December 2010.  (Credit: NASA/JPL/Space Science Institute)   http://photojournal.jpl.nasa.gov/catalog/PIA12826

A close-up of the western storm head in February 2011, and details in the tail to the east.  Yellow-white clouds are thick and high; the blue colours represent the highest semi-transparent clouds lofted by the storm; and the reds are those that are deeper, so these false colour images give us an impression of the three-dimensional structure of the eruption.  Billowing material downstream also created a large anticyclonic vortex (blue oval, bottom right) which has persisted to this day (April 2013).  (Credit: NASA/JPL/Space Science Institute) http://photojournal.jpl.nasa.gov/catalog/PIA12825

This natural colour view of the storm band was obtained in March 2011, showing the snake-like appearance of the westward moving storm head, and the chaotic activity in the tails moving to the east.  The storm wrapped its way around the whole planet, the head encountering the tail which signalled the end of the convective activity and lightning. (Credit: NASA/JPL/Space Science Institute)   http://photojournal.jpl.nasa.gov/catalog/PIA14904

The long term evolution of the storm is captured in this montage of Cassini images between December 2010 and August 2011.  By the end of the sequence, the original storm head was no longer visible, lost in the chaotic jumble of the storm band.  The storm has had long-term repercussions for this region of the atmosphere, leaving a distinct cloud-free band in the northern hemisphere still visible today (2013).  (Credit:  NASA/JPL-Caltech/Space Science Institute)  http://photojournal.jpl.nasa.gov/catalog/PIA14905

Saturn's storm didn't just affect the visible atmosphere, it also had repercussions in the high atmosphere, sending waves of energy into the stratosphere to form an enormous, hot, circulating anticyclone.  This image was captured in July 2011 with the VISIR instrument on the Very Large Telescope in Chile, sensitive to stratospheric emission.  The stratospheric vortex persists to the present day, and is continuing to move west around the planet like an enormous glowing beacon.  See more details and a movie of the forming beacon in 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 Atmosphere

Cassini has spent much of its mission exploring the equatorial region of Saturn, but every so often it ramps up to higher-inclination orbits to provide an unprecedented glimpse of the polar atmosphere.  The poles are unlike any other region on Saturn, being the apex of a planet-wide circulation, and a site where the charged particle environment of the magnetosphere can actually interact with the atmosphere itself, via aurorae.  This movie from VIMS covers six hours in 2008, when the pole was still in winter darkness.  You're seeing a flipped image, so that clouds appear white against a dark background, whereas the real measurements at 5 µm saw clouds silhouetted against Saturn's internal glow.  You can see cloud motions within the polar vortex, and the bizarre hexagonal wave.  (Credit: NASA/JPL/University of Arizona) http://photojournal.jpl.nasa.gov/catalog/PIA11215

Staying with the VIMS instrument, this image from 2008 compares the northern and southern poles in infrared light, showing striking similarities between the small polar cyclones.  Both are located right at the pole, and may be long lived features permanently present irrespective of season.  The Cassini Composite Infrared Spectrometer (CIRS) had previously shown that these cyclones were glowing hot in infrared emission, having temperatures higher than their surroundings.  (credit: NASA/JPL/University of Arizona) http://photojournal.jpl.nasa.gov/catalog/PIA11216

Another view of the north polar hexagon, this time without inverting the 5-µm brightness, so that you're seeing dark clouds against a red glow.  This image is in fact a combination of an atmospheric image from 2008 and an auroral image from 2006 (auroral emission at 4 µm) (Credit: NASA/JPL/University of Arizona)  http://photojournal.jpl.nasa.gov/catalog/PIA11396

Shifting to even longer wavelengths, the Cassini Composite Infrared Spectrometer discovered that the hexagon was also visible in the thermal field, and that a compact hot polar cyclone was present at both north and south poles of Saturn, surrounded by rapid peripheral jets.  This image is a map of the atmospheric temperatures in the troposphere at a time when the north pole was shrouded in winter darkness.  The mean temperatures at this altitude are around -190 degrees Celsius. (Credit: NASA/JPL/GSFC/Oxford University) http://photojournal.jpl.nasa.gov/catalog/PIA10217

In 2008, the Cassini Imaging Sub System gazed right down into the heart of the south polar vortex, showing convective clouds within the swirling cyclonic vortex.  Other views of this vortex showed the outer edge to be a hurricane-like eyewall, casting shadows across the saturnian cloud tops.  (Credit: NASA/JPL/Space Science Institute) http://photojournal.jpl.nasa.gov/catalog/PIA11104

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

The combination of spring sunlight and a high orbital inclination in 2012 finally allowed Cassini to view the hexagon in reflected light, rather than in the infrared. Here it is in all its glory, as well as the compact north polar cyclone mirroring the one at the south pole. This colour composite is based on http://photojournal.jpl.nasa.gov/catalog/PIA14646 from November 2012, with a colour composition by Jason Major.  (Credit: NASA/JPL/SSI/Jason Major) and you can read more about it at Universe Today.

Also from November 2012, this is a raw Cassini/ISS image processed very lightly to remove some bad pixels (again by Jason Major, read more at 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)

Saturn's Shadow

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

Cassini repeated a shadowed view of Saturn in October 2012, this time from below the ring plane rather than above it (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).

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