TITAN UNDER A RED GIANT SUN - A NEW KIND OF HABITABLE MOON

We explore the response of Titan's surface and massive atmosphere to t he change in solar spectrum and intensity as the sun evolves into a re d giant. Titan's surface temperature is insensitive to insolation incr eases as the haze-laden atmosphere ''puffs up'' and blocks more sunlig ht. However, we find a window of several hundred Myr exists, roughly 6 Gyr from now, when liquid water-ammonia can form oceans on the surfac e and react with the abundant organic compounds there. The window open s due to a drop in haze production as the ultraviolet flux from the re ddening sun plummets. The duration of such a window exceeds the time n ecessary for life to have begun on Earth. Similar environments, with s imilar to 200 K water-ammonia oceans warmed by methane greenhouses und er red stars, are an alternative to the similar to 300 K water-CO2 env ironments considered the classic ''habitable'' planet.