Recent Knowledge: Interior of Titan
Typical model of the interior structure of Titan (Image credit: NASA/JPL)
Ideas about the Interior Structure
Titan is the second largest moon in the Solar System with a radius of 2575 km. For many years the mean density was the only physical quantity about Titan’s interior that was precisely known. The mean density told us that Titan should consist approximately in equal parts of ice and rocks. Due to gravitational separation the heavier rocks should be located underneath the ice layer, i.e. in the core of Titan. However, the ice layer of Titan is supposed to be more complex than that in many other icy moons. The near-surface icy crust could be methane clathrates (hydrates) in which methane molecules are enclosed. These methane clathrates may continuously or episodically supply the source of atmospheric methane.
Ice may be molten underneath the crust. This is possible because the extreme pressure of the overlying ice layer and the dissolved ammonia substantially depress the freezing point of ice. Therefore, a liquid subsurface ocean composed of water and ammonia is suspected.
In very deep layers water can no longer be liquid and also not in the ice phase Ih, but only in the form of high-pressure ice, i.e. ice phase with a much more compact crystal structure.
Depending on theoretical models the outer crust is believed to have a thickness of some tens to 100 km, while the ocean may be some 200 km deep. If these models are correct, Titan would possess 10 times more liquid water than the Earth in the world oceans.
Insight from Cassini/Huygens
There is so far no evidence from Cassini of a significant internal magnetic field of Titan. This implies that Titan does not harbour a liquid iron core in which a plantery dynamo could be maintained.
The moment of inertia of Titan determined by gravity measurements indicates that Titan is only partially differentiated. This means that the rocky core and high-pressure ice are not fully separated from each other. The possible explanation for this result is that Titan was initially to cold to enable full differentiation between the rocky core and icy mantle.
The moment of inertia itself does not reveal whether there is a subsurface ocean or not. Direct detection of a subsurface ocean is found to be quite formidable in the case of Titan. Nevertheless, there is some circumstantial evidence for such an ocean. Cassini found evidence for non-steady rotation of Titan, which could have been caused by a seasonal exchange of angular momentum between the atmosphere and surface. This effect would amplify if the outer crust (only the crust is visible for Cassini) is mechanically decoupled from the underlying ocean and mantle/core. Furthermore, Huygens detected extremely low-frequent electromagnetic waves, which are trapped in the electric cavity below the ionosphere (Schumann resonance). The vertical profile of the amplitude of the Schumann resonance implies the presence of an electrically conductive layer some tens of kilometres below the surface of Titan. The above mentioned subsurface ocean could be this conductive layer.