Notes from Titan talks at the 2012 Lunar and Planetary Science Conference (LPSC)
Posted by Emily Lakdawalla
20-03-2012 14:16 CDT
I took 8,000 words of notes yesterday, the first day of the 2012 Lunar and Planetary Science Conference -- clearly I won't get to write about everything from this meeting; I'll have to pick and choose. One of the topics I found most exciting yesterday was a series of talks on Titan's climate. Cassini has been at Titan for nearly 8 years, and in that time it's seen the season change from southern hemisphere summer, through the equinox in 2009. The slow march of Saturn's seasons has brought major change to the weather across Titan.
First, Bob West showed how Titan's detached haze has shifted with time. What's detached haze? If you look at global images of Titan, especially in short wavelengths, you see that the fuzzy globe is encircled by a fuzzy ring except at one pole, where the ring merges with the "polar hood." This ring is a photochemical haze. Ultraviolet rays from the Sun are ionizing gas molecules, particularly methane which recombine with each other, making longer-chain hydrocarbons. It only occurs at high altitudes because the ultraviolet light can't penetrate very deeply into the atmosphere.
Anyway, West said that when Cassini arrived, the photochemical haze peaked at an altitude of about 500 kilometers, which is very high -- Titan's diameter is 5150 kilometers. But as equinox approached, the haze dropped, and dropped very suddenly, bottoming out at 380 kilometers through and after equinox. Also, he noted that this altitude is identical to the altitude measured from Voyager 1 images -- which were taken exactly one Saturn year ago. Now, though, we're well after the equinox, and West showed some brand-new images of Titan that show that the detached haze has disappeared from Titan's south pole, and that Titan now has two polar hoods.
Zibi Turtle presented about how Titan's weather has changed with these seasonal changes. Near the solstices, Titan's atmospheric circulation is very simple: air warms and rises at the summer pole, dessicating the landscape, and flows toward the winter pole, where it sinks, dumping methane rain to fill polar lakes under cover of winter darkness; it flows back to the summer pole across the surface. But near equinox, this situation has to reverse. What happens is you get an "intertropical convergence zone," much as on Earth, where that region of downwelling, wet air moves from the spring pole to the autumn pole, transitioning across the entire globe over time. At this particular time, with spring just arriving to the northern hemisphere, that downwelling region of air has shifted southward. So it's not surprising that the biggest outburst of cloud activity near Titan's equator happened just after the equinox.
After this storm dissipated, Cassini observed significant changes on Titan's surface at the southern boundary of the dune field named Belet. Those changes covered an area of 500,000 square kilometers, or roughly the combined areas of Arizona and Utah. The part of the storm that is visible here measures 1,200 kilometers in length east-to-west. The wings of the storm that trail off to the northwest and southwest from the easternmost point of the storm are each 1,500 kilometers long.
Excitingly, the outburst of clouds was accompanied by a darkening of the surface across 500,000 square kilometers of previously brighter terrain -- an observation that Zibi attributed to wetting of the surface. The darkening faded with time, as you would expect from a wetted region drying out. But the area that experienced the rainfall actually overshot its previous mark, brightening relative to its previous appearance. With the passage of more than a year, that brightening is proving to be transient, fading back to its original appearance.
Here's a zoomed detail from those same three images:
Jason Barnes followed up Zibi's talk -- which was based on Cassini camera images -- with a study of the same regions using data from Cassini's imaging spectrometer, trying to figure out what 