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Atreya Sushil K., Adams Elena Y., Niemann Hasso B., Demick-Montelara Jaime E., Owen Tobias C., Fulchignoni Marcello, Ferri Francesca, Wilson Eric H.

Titan's methane cycle

Planetary and Space Science, 2006, vol. 54, pp. 1177-1187

Référence DOI : 10.1016/j.pss.2006.05.028
Référence ADS : 2006P&SS...54.1177A

Résumé :

Methane is key to sustaining Titan's thick nitrogen atmosphere. However, methane is destroyed and converted to heavier hydrocarbons irreversibly on a relatively short timescale of approximately 10-100 million years. Without the warming provided by CH <SUB>4</SUB>-generated hydrocarbon hazes in the stratosphere and the pressure induced opacity in the infrared, particularly by CH <SUB>4</SUB>-N <SUB>2</SUB> and H <SUB>2</SUB>-N <SUB>2</SUB> collisions in the troposphere, the atmosphere could be gradually reduced to as low as tens of millibar pressure. An understanding of the source-sink cycle of methane is thus crucial to the evolutionary history of Titan and its atmosphere. In this paper we propose that a complex photochemical-meteorological-hydrogeochemical cycle of methane operates on Titan. We further suggest that although photochemistry leads to the loss of methane from the atmosphere, conversion to a global ocean of ethane is unlikely. The behavior of methane in the troposphere and the surface, as measured by the Cassini-Huygens gas chromatograph mass spectrometer, together with evidence of cryovolcanism reported by the Cassini visual and infrared mapping spectrometer, represents a "methalogical" cycle on Titan, somewhat akin to the hydrological cycle on Earth. In the absence of net loss to the interior, it would represent a closed cycle. However, a source is still needed to replenish the methane lost to photolysis. A hydrogeochemical source deep in the interior of Titan holds promise. It is well known that in serpentinization, hydration of ultramafic silicates in terrestrial oceans produces H <SUB>2(aq)</SUB>, whose reaction with carbon grains or carbon dioxide in the crustal pores produces methane gas. Appropriate geological, thermal, and pressure conditions could have existed in and below Titan's purported water-ammonia ocean for "low-temperature" serpentinization to occur in Titan's accretionary heating phase. On the other hand, impacts could trigger the process at high temperatures. In either instance, storage of methane as a stable clathrate-hydrate in Titan's interior for later release to the atmosphere is quite plausible. There is also some likelihood that the production of methane on Titan by serpentinization is a gradual and continuous on-going process.

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