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Episodic outgassing as the origin of atmospheric methane on Titan

Abstract

Saturn's largest satellite, Titan, has a massive nitrogen atmosphere containing up to 5 per cent methane near its surface. Photochemistry in the stratosphere would remove the present-day atmospheric methane in a few tens of millions of years1. Before the Cassini-Huygens mission arrived at Saturn, widespread liquid methane or mixed hydrocarbon seas hundreds of metres in thickness were proposed as reservoirs from which methane could be resupplied to the atmosphere over geologic time2. Titan fly-by observations3,4,5 and ground-based observations6 rule out the presence of extensive bodies of liquid hydrocarbons at present, which means that methane must be derived from another source over Titan's history. Here we show that episodic outgassing of methane stored as clathrate hydrates within an icy shell above an ammonia-enriched water ocean is the most likely explanation for Titan's atmospheric methane. The other possible explanations all fail because they cannot explain the absence of surface liquid reservoirs and/or the low dissipative state of the interior. On the basis of our models, we predict that future fly-bys should reveal the existence of both a subsurface water ocean and a rocky core, and should detect more cryovolcanic edifices.

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Figure 2: The evolution of the interior and the outgassing rate of methane over Titan's history.
Figure 1: Global heat power provided by the rocky core.
Figure 3: Thickness of the available methane clathrate reservoir when the final episode initiates.

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Acknowledgements

The first author benefits from post-doctoral fellowships from the Lavoisier programme of the ‘Ministères des Affaires étrangères’ and from the Centre National d'Etudes Spatiales (France). Support for the project and paper was provided by the Cassini Project and by the INSU ‘Programme National de Planétologie’.

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Correspondence to Gabriel Tobie.

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Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

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Supplementary Notes

This file is divided into two sections. Section 1 provides physical data on the density of different materials composing Titan’s interior and on the stability curves of methane clathrate in presence of ammonia. Section 2 provides a detailed description of our long-term evolution model, and it is subdivided in 4 subsections. This file also contains additional references. (DOC 898 kb)

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Tobie, G., Lunine, J. & Sotin, C. Episodic outgassing as the origin of atmospheric methane on Titan. Nature 440, 61–64 (2006). https://doi.org/10.1038/nature04497

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