Last July, radar images from NASA’s Cassini spacecraft captured a mysterious feature in a large hydrocarbon sea on Saturn’s moon Titan. Astronomers on the radar team has suggested a wide range of ideas -- surface waves, rising bubbles, floating solids, solids suspended just below the surface, or perhaps something more exotic -- though it’s still anyone’s guess.
"Science loves a mystery, and with this enigmatic feature, we have a thrilling example of ongoing change on Titan," says Stephen Wall from the Jet Propulsion Laboratory in a news release. "We're hopeful that we'll be able to continue watching the changes unfold and gain insights about what's going on in that alien sea."
The feature currently covers an area of about 160 square kilometers, and it’s been observed at least twice by Cassini during Titan flybys in 2013 and again in August this year. Previous observations showed no sign of bright features like this in that part of Ligeia Mare, one of Titan’s largest seas.
These three images, created from Cassini Synthetic Aperture Radar data, show the appearance and evolution of the feature, which looks bright here against the dark background of Ligeia Mare. Most of the white areas represent land surface that's above or just below the surface of the sea, which is thought to be mostly composed of methane and ethane.
After it was first spotted early last July (pictured up top), the feature appeared to vanish over the next several months. Astronomers didn’t see it again using low-resolution radar and Cassini's infrared imager later that month and in September and October of last year. But just last month, it was visible again -- and its appearance had changed. Not only was the shape different, but it also seemed to have doubled from 75 square kilometers.
The researchers suspect that its appearance could be related to the changing seasons on Titan: Summer is drawing near in the moon's northern hemisphere. Furthermore, they’re confident that the feature isn’t a result of an artifact or flaw in their data, or even simply evaporation in the sea. Monitoring changes like these is a major goal for Cassini's current extended mission.