Tuesday, 23 April 2013

Saturn's Storm Vortex Survives!

It seems to be the norm these days that amateur observers spot interesting features on the giant planets before us so-called professionals!  Their overwhelming advantage is long-baseline observations, working as a team to observe night after night to assemble a near-continuous record of meteorological activity at visible wavelengths.  I can't wait to see where this professional-amateur collaboration is going to take us in the next few years, but here's a very current example for Saturn, which is about to reach opposition (April 28th) to give us our best view of the ringed world.

False colour image of the storm on January 11th 2011,
splitting the longitude circle into three panels and
showing the anticyclone at the top.  (Credit:  NASA/JPL/SSI)
Back in 2011, shortly after Saturn's Great Northern Storm had erupted in December 2010, I was looking to compare thermal-infrared imagery of Saturn's storm temperatures with visible albedo patterns.  Colleagues put me in touch with Trevor Barry, a talented observer from down-under, who had captured some images at the same time as VLT, and we managed to publish them side by side in a paper on the Saturnian storm.  We spotted a new dark vortex downstream (i.e., to the east) of the main storm head, having formed among the churning, roiling turbulence of the storm system itself.  That vortex was 4000x5500 km in size in January 2011 (although it had apparently shrunk since it had first formed), colder than the surroundings (implying an anticyclone), and seemed to mark a transition between the western storm head and the eastern storm tail.  And there was something special about this vortex, the heart of the storm, as it seemed to coincide (at least to begin with) with the enormous glowing beacon of stratospheric air that we've been tracking ever since.  The Cassini cameras produced some stunning views of this anticyclone, seen at the top of the inset figure, and a paper was recently published by Kunio Sayanagi that traced its evolution from December 2010 until October 2011.  Kunio and the Cassini team watched as the vortex, originally at 39N, shifted northward and shrank in size, changing from westward motion to eastward motion as it moved into the region of an eastward jet near 47N.

Saturn on February 28th 2013,
Credit: NASA/JPL/Space Science Institute
Anthony Wesley's image of Saturn's dark oval,
March 18th 2013.
That could have been the end of it, with the anticyclone slowly disappearing from view as the storm came to its conclusion in the middle of 2011.  Through 2012 it was noted that the storm had left behind a new band in Saturn's northern hemisphere, free of cloud material and with slightly warmer temperatures, but in early 2013 we started to hear of a dark spot being tracked by the amateur community, Trevor Barry and Anthony Wesley among them.  Anthony had spotted it in images on March 18th and asked the Cassini folks what they made of it, and we pointed him to a raw Cassini image obtained on February 28th capturing the same feature.  This seemed to get everyone excited, and by combining all their various observations of the dark spot's longitude, they used the freely-available WinJUPOS software to measure the location.  From this, we come up with an eastward drift rate of 3.37 degrees per day at the present time, allowing us to predict when it'll be on the central meridian for all to see.  Trevor Barry's dataset went back even further to the last apparition in early 2012 (see chart below), and by co-plotting his dark spot longitudes with the Cassini/ISS observations from Kunio's paper, it looks likely that we're seeing the same anticyclone, two years after it was first born.  It may still be slowly drifting northward into the eastward jet, and accelerating slightly from 2012 to 2013. Note that this eastward-moving vortex is now completely disconnected from the westward moving stratospheric anticyclone.

Charting the drift of the dark oval from combining Cassini/ISS data from 2011 (Sayanagi et al., 2013) with observations from  Trevor Barry in 2012 and 2013.  Note that this is a work in progress, we'll continue to update this tracking as we go along.  To get the longitude on a particular date, just add multiples of 360 until you get a sensible number between 0 and 360!  You can see the original westward motion and the transition to eastward motion in July-August 2011.
What will be its fate in the coming months?  Have we witnessed the stormy birth of a long-lived oval in Saturn's northern hemisphere?  It's an exciting thought, and I'm sure that both Cassini and professional observers will be trying to figure out the properties of this vortex for some time to come...  Want to see this dark spot for yourself?  It should be approaching 93W longitude on April 28th, meaning that it'll transit the central meridian of Saturn at 23:00UT on April 27th, and 09:30 and 20:10 UT on April 28th.    All times are universal times.  Happy storm chasing!

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