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Saturn's largest moon, Titan, is perhaps best know for its unique, hazy atmosphere and lakes of liquid methane. But a new look at Titan's insides reveals even more oddities: Beneath the brittle crust of ice lies a layer of slush. Deeper still is an underground ocean over a solid core of rock and ice.
This new picture is based on measurements of Titan's gravity field. The measurements were made by clocking the speed of the NASA-ESA Cassini orbiter with extreme precision—gaguing how many five-thousands of a millimeter the craft traveled per second.
"The ripples of Titan's gravity gently push and pull the spacecraft. By studying the velocity changes we can calculate the gravity," explained study leader Luciano Iess, of Sapienza University of Rome.
Subtle differences in Titan's pull on Cassini suggest that the materials inside the moon are a slushy mix with no clearly defined rocky layers.
Titan's Mushy Insides
Until now, scientists had thought Titan's interior would look a lot like the inside of Jupiter's moon Ganymede: Both bodies are large, have similar densities, and are made of roughly the same materials.
Under Ganymede's thin, icy crust lies a well-defined upper mantle of warmer ice, an inner mantle of silicate, and a molten iron core. (Related blog: "Comets 'Melted' Jupiter's Biggest Moon.")
But the new gravity data suggest that Titan and Ganymede had very different evolutionary histories.
"It is really quite a surprise, and it tells us that [Titan] never got hot enough to separate out into a core, mantle, and crust," said Ulrich Köhler of the German Aerospace Center in Berlin, who wasn't on the study team.
Instead, Iess and colleagues think that Titan's ice and rock remained mixed together in a relatively lukewarm mush.
This mixture took a leisurely million years or so to settle toward Titan's center—"plenty of time for heat to escape" and for the moon to cool into its present state, Iess said.
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