Saturn's largest moon Titan may have molten ice welling up to the surface from its warm interior, data from the Cassini spacecraft suggests.
A radar image taken during Cassini's flyby of Titan on 26 October shows a striking bright feature on the moon.
Mission scientists are speculating that this could be fluid oozing across the surface: a so-called cryovolcanic flow.
Water-rich ice may melt below Titan and flow out on to the moon's surface under pressure, scientists think.
"It may be something that flowed, or it could be something carved by erosion. It's too early to say," said Cassini radar team member Ralph Lorenz of the University of Arizona, US.
"But it looks very much like it's something that oozed across the surface. It may be some sort of cryovolcanic flow, an analogue to volcanism on Earth that is not molten rock but, at Titan's very cold temperatures, molten ice."
Out in the cold
Temperatures on the Saturnian moon are not thought to reach above -179C. Cryovolcanic flows are also hypothesised to exist on Jupiter's moon Ganymede.
The fact that the lower (southern) edges of the features in the synthetic aperture radar image are brighter is consistent with the structure being raised above the relatively featureless darker background.
Scientists have been puzzled by the relative lack of impact craters on the surface of Titan. A likely explanation for this seems to be that the surface of this moon is constantly being resurfaced by some sort of geological activity.
Some theories also propose that lakes or oceans of liquid hydrocarbons, such as methane and ethane, may lie on the moon's surface.
The synthetic aperture radar image was taken on 26 October, when Cassini conducted a close fly-by of Saturn's largest moon. Titan is shrouded in a thick orange smog, which has scuttled previous attempts to see through to its surface.
The image covers an area about 150km (90 miles) square in Titan's northern hemisphere. The area has not yet been imaged using Cassini's cameras.
The Cassini-Huygens mission is a co-operative project of Nasa, Esa and Asi, the Italian space agency.