Friday, 25 January 2013

Saturn's Beacon Two Years On

Last year we published a long paper which tracked the aftereffects of Saturn's gargantuan springtime storm that erupted in 2010.  The churning tropospheric storm created dramatic changes in visible light (a bright white storm system looping around the planet), but in infrared light we detected an enormous anticyclonic vortex high in the stratosphere, a large swirling airmass some 80 K warmer than its surroundings.  This stratospheric vortex, somehow generated by the churning storm below,  was moving slowly westward and cooling with time.  Nicknamed the beacon, the high temperatures within its peripheral jet presented a unique opportunity for the detection of species that are usually too faint to see.  The high temperatures made all these spectral fingerprints more prominent, allowing us to study gases that we don't normally see (e.g., ethylene).

Although our paper only covered dates through September 2011, Cassini has continued to monitor the location of the beacon as it slowly decays away, back to the normal background state. I'm heading out to Hawaii next week to use an extremely high-resolution infrared spectrometer (TEXES) to study the detailed chemistry in the heart of the beacon, so I thought it best to check that this swirling vortex is still there!  One particular observation by the Cassini Composite Infrared Spectrometer (CIRS) cut right through the centre of the feature at Saturn's northern mid-latitudes, and found the beacon at 170W longitude (System III) on January 5th 2013.  The beacon has continued to cool, now showing peak brightness temperatures of about 160 K in the methane emission band at 7.7 µm.  It's westward motion continues to be constant at around 3.0±0.05 degrees of longitude per day, a velocity of 31.3±0.5 m/s towards the west.  It's longitude in System III can be roughly obtained by the following formula, although it extrapolates over a large date range:
Lon = -248.23 + 3.01 * (Date - 2011-01-01)
... i.e., using the difference between today's date and January 1st 2011 (roughly when we first saw the beacon).

Cassini/CIRS observation on January 5th showing the location of the beacon.
Extended version of our beacon-tracking figure, showing how the westward motion is fairly constant with time.
In preparation for the TEXES observations from NASA's Infrared Telescope Facility, I calculated the times when the beacon will be within 20 degrees of the central meridian of the planet (and when Saturn is actually visible with airmass < 2.0).  This will occur as follows.  IRTF operates between approximately 18:00-06:00 HST (04:00-16:00 UT).  Saturn time is scheduled from 02:00-06:00 HST (1200-1600 UT), which means we'll currently only see the beacon on February 4th at around 14:00 UT (04:00 HST).  The beacon will be near 251 W on February 1st, moving to 277 W by February 11th 2013.

2013-Feb-01 10:40-11:40 UT (Could see tail end if starting at 12:00 UT)
2013-Feb-02 08:00-09:00 UT
2013-Feb-03 06:20-06:30 UT (High airmass)
2013-Feb-04 13:30-14:30 UT (Best Observation)
2013-Feb-05 11:00-12:00 UT  (Could see tail end if starting at 12:00 UT)
2013-Feb-06 08:20-09:20 UT
2013-Feb-07 06:00-06:40 UT (High airmass)
2013-Feb-08 13:50-14:10 UT (High airmass)
2013-Feb-09 11:10-12:10 UT  (Could see tail end if starting at 12:00 UT)
2013-Feb-10 08:30-09:40 UT
2013-Feb-11 06:00-07:00 UT
2013-Feb-12 14:00-14:10 UT (High airmass)

This updates the estimates of the beacon positions from the paper we published in 2012 (where x is the date minus January 1st 2011):

Phase 1 (B1):  2.74±0.11 deg/day (lon=2.74*x - 40.78)
Phase 1 (B2):  0.59±0.06 deg/day (lon=0.59*x +224.32)
Phase 2 (day 110-180):  1.61±0.20 deg/day (lon=1.61*x+92.12)
Phase 3 (day 180-300):  2.40±0.29 deg/day (lon=2.40*x -58.6)
Phase 4 (day 300+): 3.01±0.05 deg/day (lon=3.01*x - 248.23)

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