Projecting the evolution of Titan’s atmosphere under a reddening sun

Abstract

Titan is the only moon in our solar system known to retain a relatively dense atmosphere. This mainly consists of diatomic nitrogen and methane, with the concentration of methane varying considerably with altitude, with highest concentrations closest to Titan's surface. The similarities between Titan's reducing atmosphere and hypothesized primordial Earth conditions make Titan a candidate world of potential astrobiological significance, particularly when considering the possibility of tectonic events or meteor impacts eliciting the formation of liquid water bodies on Titan’s surface. This preliminary study aimed to investigate the evolution of Titan's atmosphere in parallel with the Sun’s evolution into the red giant phase. Using current conditions as a starting point, the simple model developed in this study explored different atmospheric processes occurring on Titan and assessed their interdependence. By applying varying solar inputs from a reddening Sun, Titan's atmospheric evolution was studied across a timeline of 7.6 Gyr, from present conditions (solar age 4.55 Gyr) up to before the Sun is expected to reach the tip of its red giant branch at solar age 12.15 Gyr. After 11.64 Gyr, temperatures at Titan's surface are projected to be high enough (>200 K) to allow the rapid formation of complex organic molecules in atmospheric conditions which are still reducing in nature. If the presence of methane at the surface is maintained, this could lead to a period, late in the Sun's lifetime, where Titan's surface conditions allow for the formation of these complex organic molecules in hydrocarbon or ammonia/water pools similar to those theorized for primordial Earth. Further studies building on this preliminary work will analyse the likelihood of possible methane reservoirs on Titan maintaining current surface level concentrations of methane, as well as an in-depth analysis of the rates of formation of complex organics in projected conditions. Such analysis on a moon like Titan can eventually be applied to other exoplanets or exomoons with similar atmospheric conditions.