A strange 'shape' is currently evolving before astronomers' eyes on Saturn's moon Titan.
Three images taken several years apart by the Cassini space probe show the shape arise from one of Titan's hydrocarbon seas, grow to an enormous size and then disappear.
The feature is said to cover an area about 100 square miles in size in the sea known as Ligeia Mare.
The object appears to be bright in photographs against the dark of the hydrocarbon lake, but scientists have no obvious answer for what it is.
Our own suggestion - that it is the remains of a gigantic alien mega squid rising from the depths of Titan's oceans to the surface - remain unexplored.
The Cassini team explains that it first saw the object on July 2013 when the space craft flew over Titan and pointed its Synthetic Aperture Radar at the moon, but that it had vanished when examined with low-res radar and infrared cameras.
But recently, during an August 21 flyby, the feature was back - though changed from its appearance 11-months previously.
The scientists on the Cassini team say the feature is not a result of flaws in their data, and do not think it is caused by the sea evaporating and revealing new land.
So far they think it might be "surface waves, rising bubbles, floating solids, solids suspended just below the surface, or perhaps something more exotic" according to Phys.org.
"Science loves a mystery, and with this enigmatic feature, we have a thrilling example of ongoing change on Titan," said Stephen Wall, the deputy team lead of Cassini's radar team, based at NASA's Jet Propulsion Laboratory in Pasadena, California. "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."Suggest a correction