By precisely tracking Cassini's motion during its closest passes of Saturn's moon Titan, scientists have been given an insight into the large moon's internal structure, finding it to be a disorganised mix of rock and ice.

The Cassini spacecraft measurements were derived from four close flybys conducted between February 2006 and July 2008 that saw Cassini swoop to between 1,300 and 1,900 kilometres of the moon's surface. The results enabled scientists to construct a gravity map from the tiny tugs inflicted on Cassini's speed, as measured along a line of sight track from the Earth to the spacecraft.

"The ripples of Titan's gravity gently push and pull Cassini along its orbit as it passes by the moon and all these changes were accurately recorded by the ground antennas of the Deep Space Network within five thousandths of a millimeter per second even as the spacecraft was over a billion kilometers away," says Cassini radio scientist Luciano Iess and lead author of the research presented in today's issue of the journal Science. "It was a tricky experiment."

Scientists already knew that Titan was made of rock and ice, but the gravity measurements provided the crucial information on how this material is distributed, implying that Titan's interior is rather mixed up, unlike the Earth for example, which has differentiated into distinct layers comprising a core, mantle and crust. In contrast, Titan's interior probably never heated up enough to cause differentiation – only the outermost 500 kilometres of Titan's ice is devoid of any rock.

"To avoid separating the ice and the rock, you must avoid heating the ice too much," explains David J. Stevenson of the California Institute of Technology in Pasadena. "This means that Titan was built rather slowly for a moon, in perhaps around a million years or so, back soon after the formation of the Solar System."

Additional flybys will help answer outstanding questions such as whether Titan harbours a subsurface ocean at depth and how thick the crust is, which will paint a more detailed picture of how Titan's geological processes operate, or operated in the past. Titan's surface is heavily influenced by the action of methane carving out river channels and filling lakes, rather like the role water plays on Earth. An icy form of volcanism – cryovolcanism – is also thought to modify the surface.

"These results are fundamental to understanding the history of moons of the outer Solar System," says Cassini Project Scientist Bob Pappalardo. "We can now better understand Titan's place among the range of icy satellites in our Solar System."

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