Methane-ethane substitution in subsurface clathrate hydrates as a source of Titan's atmospheric methane
Résumé
Titan is a unique object in the Solar System: it is the only satellite bearing a dense atmosphere, which shrouds its surface to direct observation at most wavelengths. This atmosphere is subject to intense photochemistry by the solar UV, which constantly depletes its major constituents N2 and CH4. The current CH4 amounts would disappear over periods of 10-100 Myrs, thus requiring replenishment mechanisms to sustain its presence in the atmosphere. The favored scenarios involve cometary impacts and cryovolcanism, the emission of endogenic icy materials to the surface. However, Cassini observations to date show a sparse cratering record, and debated evidence for cryovolcanism in geologically recent times, suggesting the existence of other mechanisms to replenish the atmospheric methane. Cassini observations of the North Pole lakes between 2007 and 2010 show little change in surface area, suggesting that ethane (photolytic product of methane) is their primary constituent since it has a much lower saturation vapor pressure than methane. The current estimated amount of liquid ethane on Titan is much lower than that predicted by atmospheric models. Ethane is therefore to be trapped in the subsurface. Accretion and evolution scenarios predict that the subsurface and the upper crust of Titan most likely consist of methane clathrate hydrates. Interaction of liquid ethane with methane clathrate should result in the substitution of the encaged methane by ethane, and the subsequent release of methane to the atmosphere. Conditions for this substitution are investigated. A kinetic model shows that it can occur over periods much shorter than geologic timescales on Titan. Mass balance calculations then show that this mechanism can be an important source of Titan's atmospheric methane. Based on these results, we propose a new hydrocarbon cycle on Titan, with events of major outgassing (via impacts and cryovolcanism) that are followed by quiescent periods, during which the substitution of methane by ethane can sustain the abundance for longer periods of time than previously thought. Since ethane rainfalls are localized around the winter pole, we have calculated the change in polar radius induced by the change in density from methane to ethane clathrate. It is compared with Cassini altimetry data that demonstrated that the polar radius is smaller than the value predicted by the flattening. It allows us to place some bounds on the amounts of ethane trapped in the subsurface. This work has been conducted at the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA. Government sponsorship acknowledged.