A recently formed ocean inside Saturn's moon Mimas.

Moons potentially harbouring a global ocean are tending to become relatively common objects in the Solar System1. The presence of these long-lived global oceans is generally betrayed by surface modification owing to internal dynamics2. Hence, Mimas would be the most unlikely place to look for the presence of a global ocean3. Here, from detailed analysis of Mimas's orbital motion based on Cassini data, with a particular focus on Mimas's periapsis drift, we show that its heavily cratered icy shell hides a global ocean, at a depth of 20-30 kilometres. Eccentricity damping implies that the ocean is likely to be less than 25 million years old and still evolving. Our simulations show that the ocean-ice interface reached a depth of less than 30 kilometres only recently (less than 2-3 million years ago), a time span too short for signs of activity at Mimas's surface to have appeared.

Evolution of Titan and implications for its hydrocarbon cycle.

Measurements of the carbon and nitrogen isotopic ratios as well as the detection of 40Ar and 36Ar by the gas chromatograph mass spectrometer (GCMS) instrument on board the Huygens probe have provided key constraints on the origin and evolution of Titan's atmosphere, and indirectly on the evolution of its interior. Those data combined with models of Titan's interior can be used to determine the story of volatile outgassing since Titan's formation. In the absence of an internal source, methane, which is irreversibly photodissociated in Titan's stratosphere, should be removed entirely from the atmosphere in a time-span of a few tens of millions of years. The episodic destabilization of methane clathrate reservoir stored within Titan's crust and subsequent methane outgassing could explain the present atmospheric abundance of methane, as well as the presence of argon in the atmosphere. The idea that methane is released from the interior through eruptive processes is also supported by the observations of several cryovolcanic-like features on Titan's surface by the mapping spectrometer (VIMS) and the radar on board Cassini. Thermal instabilities within the icy crust, possibly favoured by the presence of ammonia, may explain the observed features and provide the conditions for eruption of methane and other volatiles. Episodic resurfacing events associated with thermal and compositional instabilities in the icy crust can have major consequences on the hydrocarbon budget on Titan's surface and atmosphere.