Science: The interior structure and composition of solar system bodies are key to understanding their origin and evolution. Saturn‘s largest icy moon, Titan, and the jovian moons, Ganymede and Callisto, are of similar size, mean density, and primordial ice-rock fraction from which the satellites formed.Titan is distinct due to its dense nitrogen atmosphere, with methane as the next most abundant constituent, which precludes direct observations of the surface.Before the arrival of the Cassini-Huygens spacecraft to study the Saturn system in 2004, little was known about the nature of Titan’s interiorâmdash;information as to its origin, evolution, and the rate at which it degasses was limited.In Science, Luciano Iess of Sapienza University of Rome in Italy and associates report evidence based on the analysis of its gravitational field using radio waves that the interior was much colder than previously thought, and thereby impeded substantial melting and subsequent separation of the primordial ice-rock mixture."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 0.005 mm/s even as the spacecraft was over a billion km away,” said Iess. “It was a tricky experiment.”
The above image shows Titan’s likely interior structure. The surface is seen as yellow.In the cutaway, the outermost 500-km appear to be ice essentially devoid of any rock (light grey) with an internal ocean that is hypothesized from other Cassini data (blue) caught between the icy layers.The remaining interior is a cool mix of ice studded with rock (dark gray). This suggests Titan’s interior is cool and sluggish, and failed to allow the interior to separate into completely differentiated layers of ice and rock.
