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Lara Luisa M., Lellouch Emmanuel, Martínez González M., Moreno Raphaël, Rengel Miriam

A time-dependent photochemical model for Titan s atmosphere and the origin of H2O

Astronomy and Astrophysics, 2014, vol. 566, pp. 143

Référence ADS : 2014A&A...566A.143L

Résumé :

Context. Titan's stratosphere contains oxygen compounds (CO, CO<SUB>2</SUB>, and H<SUB>2</SUB>O), implying an external source of oxygen whose nature is still uncertain. Recent observations from the Herschel Space Observatory using the HIFI and PACS instruments and the Cassini/CIRS, as well as steady-state photochemical modeling indicate that the amounts of CO<SUB>2</SUB> and H<SUB>2</SUB>O in Titan's stratosphere may imply inconsistent values of the OH/H<SUB>2</SUB>O input flux, and that the oxygen source is time-variable. <BR /> Aims: We attempt to reconcile the H<SUB>2</SUB>O and CO<SUB>2</SUB> observed profiles in Titan's atmosphere by using an updated photochemical scheme and developing several time-dependent scenarios for the influx/evolution of oxygen species. <BR /> Methods: We use a time-dependent photochemical model of Titan's atmosphere to calculate effective lifetimes and the response of Titan's oxygen compounds to changes in the oxygen input flux. Two variants for the C-H-O chemical network are considered. We investigate a time-variable Enceladus source and the evolution of material delivered by a cometary impact. <BR /> Results: We find that the effective lifetime of H<SUB>2</SUB>O in Titan's atmosphere is only a factor of six shorter than that of CO<SUB>2</SUB> and exceeds 10 yr below 200 km. A time-variable Enceladus source, involving a decrease by a factor of 5-20 in the OH/H<SUB>2</SUB>O flux over the last few centuries, shows promise in explaining the relative CO<SUB>2</SUB>/H<SUB>2</SUB>O profiles. However, if the previous measurements from the Herschel Space Observatory are representative of Titan's atmospheric water, an additional H<SUB>2</SUB>O loss to the haze term is needed to bring the model in full agreement with the data. In an alternate situation, CO<SUB>2</SUB> production following a cometary impact that occurred at least 220-300 yr ago can in principle explain the CO<SUB>2</SUB> "excess" in Titan's stratosphere, but this scenario is highly unlikely, given the estimates of the impact rate at Titan.

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