A constant methane rain pelts Xanadu, the moon's brightest continent, each morning, the researchers discovered.
It's the first time researchers have seen differences between day and nighttime weather on Titan, Adamkovics pointed out.
"They were unexpected, because day-to-night temperature changes are expected to be small," he said.
Martin Tomasko, an astronomer at the University of Arizona in Tucson who designed Huygens' camera system, has long suspected methane smog and condensation occurred on Titan.
He has proposed that methane rains from Titan's sky as a thick, tarry gunk. The chemical then travels through channels until it reaches reservoirs visible as dark patches on the moon's surface.
The exact nature of the dark patches remains in dispute, however.
If methane rain is indeed widespread, it could be the main method that atmospheric methane returns to the surface, the study authors write.
But Adamkovics said there's still a long way to go to understand the process.
"Many predictions are being made, but we are far from conclusions," he said, "and I would wager that more surprises are going to be discovered before we have a textbook description of Titan's atmospheric cycling."
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