A bright "transient feature" has appeared out of nowhere in radar images of a hydrocarbon sea on Saturn's giant moon,Titan, according to mystified scientists who have dubbed it a "magic island".
Their best explanations are that it could be the result of waves, bubbles, or buoyant solid matter.
Planetary scientist Jason Hofgartner, from Cornell University in New York, said: "This discovery tells us that the liquids in Titan's northern hemisphere are not simply stagnant and unchanging, but rather that changes do occur.
"We don't know precisely what caused this 'magic island' to appear, but we'd like to study it further."
The object was captured by the Cassini space probe, but it had disappeared in later images, said the researchers, writing in the journal Nature Geoscience.
Titan has large expanses of liquid on its surface, but unlike on Earth, they are formed by flowing rivers of liquid methane and ethane.