title: Titan under a red giant sun: a new kind of "habitable" moon.
abstract: We explore the response of Titan's surface and massive atmosphere to the change in solar spectrum and intensity as the sun evolves into a red giant. Titan's surface temperature is insensitive to insolation increases as the haze-laden atmosphere "puffs up" and blocks more sunlight. However, we find a window of several hundred Myr exists, roughly 6 Gyr from now, when liquid water-ammonia can form oceans on the surface and react with the abundant organic compounds there. The window opens due to a drop in haze production as the ultraviolet flux from the reddening sun plummets. The duration of such a window exceeds the time necessary for life to have begun on Earth. Similar environments, with approximately 200K water-ammonia oceans warmed by methane greenhouses under red stars, are an alternative to the approximately 30OK water-CO2 environments considered the classic "habitable" planet.
authors.affiliation: Department of Planetary Sciences, University of Arizona, Tucson 85721-0092, USA. rlorenz@lpl.arizona.edu
authors.forename: R D
authors.initials: RD
authors.lastname: Lorenz
authors.forename: J I
authors.initials: JI
authors.lastname: Lunine
authors.forename: C P
authors.initials: CP
authors.lastname: McKay
mesh_terms: D000641:Ammonia / Q000032:analysis / Q000737:chemistry; D001272:Atmosphere / Q000737:chemistry*; D019419:Evolution, Planetary; D018559:Exobiology; D005118:Extraterrestrial Environment; D017752:Greenhouse Effect; D008956:Models, Chemical; D018539:Saturn; D016082:Solar System; D013472:Sunlight; D014466:Ultraviolet Rays; D014867:Water / Q000737:chemistry
chemical_list: D014867:Water; D000641:Ammonia