Titan's thick atmosphere, sand dunes, and methane lakes make it a world whose geography could be very Earthlike - albeit cold and poisonous. And now astronomers believe this moon of Saturn has seasonal rainstorms and flooding, too. Here's the evidence.
What you're seeing in this picture is most likely the aftermath of an enormous rainstorm and flood around Titan's equator. The dark areas are regions that have been flooded after the white clouds dumped their methane rain. Right now it's spring on Titan, and physicist Elizabeth Turtle and her colleagues postulate that we may be seeing an exolunar version of "April showers."
We've known for a while that Titan's poles are dotted with methane lakes, and that its equatorial regions are arid regions full of sand dunes. The Cassini spacecraft observed what appeared to be dried-out riverbeds in these desert regions, but scientists weren't sure if they were evidence of a previous, wetter climate on Titan or occasional flooding. Now it seems that the moon may have seasonal floods that darken the sands.
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The authors write:
Methane precipitation could affect a huge area over a short period of time, explaining the rapid appearance (and disappearance) of the changes and their extensive and nonuniform nature. The cloud observed on 27 September was more than 1000 km in extent. Surface brightness could change by flooding or wetting, which renders materials darker by changing their optical properties.
The researchers are not ruling out the possibility that these dark spots were caused by volcanic activity or windstorms that blew away a lot of sand, revealing a darker substrate. But they believe the most likely explanation is rainstorms, as that would also explain the dried riverbeds in the desert areas.
Read their full report via Science.