Some speculations on Titan's past, present and future

The solar system's second largest natural satellite is shrouded by a thick nitrogen atmosphere, rich in methane, within which sunlight- and cosmic-ray -driven organic chemistry has gone on for some 4.5 billion years. The earli est history of Titan's atmosphere, and specifically its origin, remains unc lear until the Cassini-Huygens probe measures the ratio of argon to nitroge n and the abundances of other noble gases and isotopes. However, the abunda nce of deuterated;methane in the atmosphere today is consistent with an atm osphere that originated in the chemically-processed sub-nebula around Satur n, rather than in cometary material. Titan's overall atmospheric history is driven by the depletion of methane, and the mechanisms by which methane mi ght be resupplied from surface or external sources. Remote sensing data mit igate against a large reservoir of methane at Titan's surface, leaving open the possibility that methane is periodically depleted from Titan's atmosph ere on timescales of 10(7)- 10(8) years; under such conditions Titan might oscillate between thin and thick atmospheric epochs. Titan's surface may pr ovide a repository for complex organic molecules that were synthesized duri ng times when liquid water was temporarily available on the surface, such a s after impacts or cryo-volcanic eruptions. Such molecules might provide cl ues to the resolution of some difficult issues associated with the origin o f life. Identifying the presence and nature of such molecules is a difficul t exploration problem that must be left to missions which follow-on from th e Cassini-Huygens exploration of Titan. (C) 1998 Elsevier Science Ltd. All rights reserved.