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 mix of ice and rock with no clearly defined rocky layers.
Titan's Icy 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 together in a relatively lukewarm mixture.
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.
The team's calculations support the idea that Titan today has a subsurface liquid ocean from which methane bubbles up through an icy crust, constantly shrouding Titan in thick smog. (Related: "Methane Rain Formed New Lake on Saturn Moon.")
The study's notion of a relatively warm, spongy ice layer beneath a thin, hard outer shell would also explain Titan's lack of major mountains.
"Large mountains can't exist on Titan," Iess said. "They would simply sink into the ice."
Findings published in this week's issue of the journal Science.