By Jonathan Amos
Science reporter, BBC News, San Francisco
The tiger stripe fractures (bottom right) are places where the surface spreads
A US space agency (Nasa) probe has witnessed a moon of Saturn do something very unusual and Earth-like.
Pictures of the icy satellite Enceladus suggest its surface splits and spreads apart - just like the ocean floor on our planet splits to create new crust.
The information was released at a meeting of the American Geophysical Union in San Francisco.
The data from the Cassini spacecraft is said to strengthen the idea that Enceladus harbours a sub-surface sea.
"Bit by bit, we're accumulating the evidence that there is liquid water on Enceladus," said Carolyn Porco, team leader of the Cassini imaging group and one of the senior scientists on the mission.
The observation on Earth that the sea floor is splitting at mid-ocean ridges and moving apart was one of the great scientific discoveries of the 20th Century; and became a key feature in the theory of plate tectonics - the idea that massive slabs of the Earth's surface move around and are recycled.
Cassini sees something very similar on Enceladus.
The surface of this snow-white moon is riven with cracks - dubbed tiger stripes - at its south pole.
Dr Paul Helfenstein from Cornell University used digital maps of this region to reconstruct a history of the stripes, pushing the fractures around on a computer screen until they fitted together like pieces in a puzzle.
He found that sections of the cracks had clearly moved from their original locations.
Dr Helfenstein told BBC News that the resemblance to the Earth process was remarkable.
"What's different about them is that spreading ridges on the Earth typically spread symmetrically about a rift," he said.
"On Enceladus, what we see is a type of spreading but it is strongly asymmetric - it's like a conveyor belt, in which, if it's true it's coming up from a convection well, it seems to be only pushing in one direction. It does happen on Earth, but only in very peculiar situations."
On Earth, sea-floor spreading is fuelled by molten rock upwelling from deep inside the Earth.
Evidence is mounting that liquid water lies beneath the surface
On Enceladus, the scientists speculate the liquid may be water.
If that is the case, it makes this moon one of the most exciting targets for future exploration.
Enceladus is already known to have some of the fundamental chemistry required to make and sustain life. Liquid water currently is the major missing ingredient.
Dr Porco commented: "We first discovered this region in early 2005 and now it's nearly four years later, so it's still kind of brand new; but already there are some of us who really want to go back with a spacecraft that focuses on the south pole of Enceladus and investigates whether or not it is a site of either pre-biotic or biotic processes."
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