The sudden disappearance of a bright spot on a methane sea on Titan could add more proof that Saturn's largest moon is a lot like Earth – just colder and gasier.
Astronomers at Cornell University were flipping between radar images of Ligeia Mare, a sea of frigid methane on Titan's northern hemisphere. They discovered a bright spot from a Cassini satellite fly-by in July 2013 that wasn't visible on images from a previous satellite pass, nor on a subsequent one.
They gave it the whimsical name "Magic Island," a moniker that embraced both their sense of wonder and scientific skepticism that the phenomenon might be little more than visual trickery.
"It was exciting, but our science reservations kick in and immediately we start thinking, 'What are the errors that could have caused this?' " said Jason Hofgartner, an astronomy graduate student at Cornell University and lead author of a paper published this week in Nature Geoscience. "It's sort of: 'Whoa, what is that? OK, let's go prove it's not there.' "
Radar imagery can play tricks on the observer, tossing out anomalies that are the equivalent of a fly on a lens.
"We went through all of the standard artifacts, these artificial things that come up in radar images, and it wasn't consistent with any of those," Hofgartner said.
Radar also can miss something as obvious as a mountain too. "If you change the illumination conditions, just the way you're looking at it with the radar, you can trick yourself into thinking something has changed, when it really hasn't."
But the research team, which included scientists from a dozen universities and space agencies, is nearly certain that those possibilities don't match the data. They believe the spot probably came from rising bubbles, suspended solids or liquids, or waves. All of these could be driven by thermal cycles that rely on changes in solar radiation, the study said.
Scientists have found ample evidence of a seasonally driven global methane cycle similar to the water cycle of Earth's surface and atmosphere. They've even detected evidence of methane storms.
But since Cassini arrived in 2004, in the midst of Titan's northern winter, there has been no evidence of thermally driven changes in a northern sea. The surface of Ligeia Mare has been flatter than a mill pond -- scientists estimate that it has varied only by about a millimeter during the time they've studied it.
Cassini will take more images of the area in August, and Hofgartner and others will be eager to see whether Magic Island reappears, drifts or multiplies.
If images sent back by the satellite show more magic islands in larger areas or in other seas, that would suggest that these are seasonal phenomena.