We’ve known for a while that Saturn’s moon Titan experiences a hydrocarbon cycle that parallels in many ways Earth’s own hydrological cycle. Compounds like methane evaporate from the surface, condense in the atmosphere, and precipitate out – helping fill the extensive landscape of lakes and rivers on Titan.
And just like Earth, Titan experiences seasons. The moon’s axial tilt is perpendicular to its orbit around Saturn, but that orbit closely matches Saturn’s own axial tilt with respect to its orbit around the Sun – about 27 degrees. So, in addition to day-night changes, Titan’s latitude dependent illumination by the Sun follows the 29.5 year long period of its mother-planet’s orbital motion.
Most recently, around 2010 Titan’s northern hemisphere was heading into its spring, and by 2017 it had reached its summer solstice – with the north pole at maximum exposure to the Sun.
Models of Titan’s climate and atmospheric seasonality had predicted a buildup of northern clouds and a corresponding increase in precipitation. In other words, northern summer on Titan would be quite familiar to (albeit significantly colder), for example, inhabitants of the U.K. – cloudy with a chance of rain.
Except examination of Cassini imaging data did not show the expected cloud build up.
Now though, after poring through, and re-analyzing the mission archive Dhingra et al. report in the Geophysical Research Letters evidence for a “wet-sidewalk effect” at Titan’s northern reaches in 2016. Literally the glinting reflection of light from a newly wetted, perhaps pebbly surface.
This bright surface feature covered about 120,000 square kilometers (seen using Cassini’s infrared imaging data that helps penetrate through layers of high haze).
The conclusion is (with apologies to Toto): we guess it does rain down on Titan in the northern summer. But without the use of this wet-sidewalk phenomenon, the thick obscuring atmosphere will hide the rainclouds from us.