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 was going on with that brightening. It's not hydrocarbon "snow," Jason said, because the haze can't fall out fast enough. So that bright material must represent the delayed deposition on the rain-wetted areas of some substance that can either sublimate or blow away. Jason said that it could be just about anything that has fine enough grains to appear white to the imaging spectrometer, which sees at a wavelength range of 1 to 5 microns. He advanced a plausible hypothesis: the rain wets the surface, darkening it, and ponding in some places. Rain falling on the uplands runs off to valleys, where it can saturate the soil. Wind blowing across these saturated surfaces will evaporate, and evaporation causes cooling. It might just cause enough cooling for methane frost to form on the surface, brightening the places that were darkened previously. But, he said, this is only a hypothesis, and asked the community to come up with quantitative versions of this model, or laboratory tests.
There was further discussion of rain on Titan today, in an interdisciplinary "planetary hydrology" session incorporating results from both Mars and Titan. Antoine Lucas showed evidence from Cassini RADAR images that there are "knickpoints" in Titan's polar rivers and terraces hidden beneath the waves of Titan's north polar seas that suggest that there have been recent changes in sea level beyond the cyclical, seasonal variation.
Ralph Lorenz talked about rainfall rates on Titan. Actually, though, the best part of his talk was right up front, when he addressed the criticism that some people don't like the adjective "Earth-like" being applied to Titan. Titan's surface geology bears strong superficial similarities to Earth's despite the huge differences in composition and temperature. So, Ralph presented a different way of looking at it, in the context of this interdisciplinary session on hydrology on Mars and Titan: "Mars is the second most Titan-like world in the solar system."
That particular session ended with a presentation by Jeff Moore, wh