Scientists are investigating a mystery object that appeared and then vanished again from a giant lake on Titan, the largest moon of Saturn.
They spotted the object in an image taken by Nasa's Cassini probe last year as it swung around the alien moon, more than a billion kilometres from earth. Pictures of the same spot captured nothing before or some days later.
Little more than a white blob on a grainy image of Titan's northern hemisphere, the sighting could be an iceberg, rising bubbles or waves rolling across the normally placid lake's surface, scientists say.
Astronomers have named the blob the "magic island" until they have a better idea what they are looking at. "We can't be sure what it is yet because we only have the one image, but it's not something you would normally see on Titan," said Jason Hofgartner, a planetary scientist at Cornell University in New York.
Titan is one of the most extraordinary places in the solar system. The land is strewn with hydrocarbon dunes that rise above lakes fed by rivers of liquid methane and ethane. The atmosphere is so thick, and the gravity so weak, that a human could strap on wings and flap into the air. That air is laced with lethal hydrogen cyanide, though, and the hydrocarbon seas are a chilly minus 180 degrees Celsius.
The largest moon of Saturn is the only place beyond earth known to have stable liquids on its surface and rain falling from its skies. Spacecraft have mapped scores of lakes there. The three biggest are named after mythological beasts, the Kraken, Ligeia and Punga, and are large enough to qualify as seas, or mares.
The US team made their curious discovery while poring over radar images of Ligeia mare, a 150-metre-deep sea that stretches for hundreds of kilometres in Titan's northern hemisphere. Among the snapshots taken in 2007, 2009 and last year was one with the strange white feature, about 10km off the mountainous southern shore.
About 20km long and 10km wide, the bright spot appears in an image dated July 10 last year but is missing from pictures of the same spot taken previously and on July 26. Hofgartner said the team had ruled out any errors in the radar equipment that could result in the blob.
The scientists have whittled the number of potential explanations down to four. It could be one or more icebergs, or material in suspension beneath the surface. But Cassini might also have picked up a rush of bubbles from the depths, or the first signs of deep-sea waves.
Last year's Cassini fly-by found that Ligeia mare, Titan's second-largest lake, was as smooth as glass. The tranquil expanse of liquid methane and ethane had no surface ripples larger than 1mm. The profound stillness may be because the wind on Titan is so feeble. But that could be changing. Titan's seasons last for seven earth years. The northern hemisphere is warming as spring gives way to summer, which arrives in earnest in 2017. Warmer weather brings stronger winds, and stronger winds bring waves.
"The sun is shining brighter, and that energy can be powering the winds. All you would need is a light breeze, around half a metre per second," said Hofgartner. But the wind on Titan may never be strong enough to stir truly impressive waves. "If you had a large enough surfboard, you could certainly float there, but I don't think you'd really get the waves you'd want," Hofgartner said.
If waves are the cause of the curious white blob, then Cassini spotted them before they had spread more widely across the sea. Follow-up images in the next few months are expected to shed more light on the mystery.
In March, researchers who worked with Hofgartner reported what may have been tiny waves on another Titan sea, Punga mare. Nasa toyed with the idea of sending a boat to sail on the seas of Titan, but the proposal lost out to a mission to Mars.