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

¹ Instituto de Astrofísica de Andalucía - CSIC, c/ Glorieta de la Astronomía s/n, 18008 Granada, Spain
e-mail: lara@iaa.csic.es
² LESIA, Observatoire de Paris-Meudon, 92195 Meudon Principal Cedex, France
³ Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany

Received: 19 November 2013
Accepted: 17 April 2014

Abstract

Context. Titan’s stratosphere contains oxygen compounds (CO, CO₂, and H₂O), 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 CO₂ and H₂O in Titan’s stratosphere may imply inconsistent values of the OH/H₂O input flux, and that the oxygen source is time-variable.

Aims. We attempt to reconcile the H₂O and CO₂ 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.

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.

Results. We find that the effective lifetime of H₂O in Titan’s atmosphere is only a factor of six shorter than that of CO₂ and exceeds 10 yr below 200 km. A time-variable Enceladus source, involving a decrease by a factor of 5–20 in the OH/H₂O flux over the last few centuries, shows promise in explaining the relative CO₂/H₂O profiles. However, if the previous measurements from the Herschel Space Observatory are representative of Titan’s atmospheric water, an additional H₂O loss to the haze term is needed to bring the model in full agreement with the data. In an alternate situation, CO₂ production following a cometary impact that occurred at least 220–300 yr ago can in principle explain the CO₂ “excess” in Titan’s stratosphere, but this scenario is highly unlikely, given the estimates of the impact rate at Titan.