The Air is Hot Lava! Titan's Atmosphere Makes Methane Ice Unstable.

Like many planetary bodies with dense atmospheres, ices on Titan likely play vital roles in its climate, weather, and cycling of materials about the climate system. Ice grains in the atmosphere can serve as nucleation sites forming raindrops and/or hailstones. On the surface, the presence of ice and frost can significantly increase a surface's bond albedo, thus reducing the equilibrium surface temperature. Under certain conditions, ices can become buoyant upon an underlying lake or sea, altering exchange between parts of the climate system. As the dominant volatile in Titan's hydrologic cycle, methane-dominated ices in particular have the potential to become abundant, and thus profoundly affect Titan's climate, weather, and surface evolution. Nevertheless, the CRYOCHEM numerical model suggests that methane ice is everywhere unstable under Titan-like conditions near the surface (equilibrium includes only liquid and vapor phases) but survives at altitudes higher than 15 km. Here we use laboratory studies and numerical simulations to explore the stability of methane ices and the kinetics of their melting on Titan across plausible conditions.

We first formed methane ice in the Astrophysical Ices Laboratory at NAU, before exposing it to a nitrogen atmosphere at 88 K; this is well below the observed surface temperature of 91-94 K. Nevertheless, we find that the N2 atmosphere melted the methane ice almost instantly. Using Molecular Dynamics simulations, we find that this is likely due to atmospheric N2 rapidly attacking methane molecules in the ice lattice, pulling methane molecules into a growing nitrogen-methane liquid layer that ultimately melts the methane ice. We further discuss methane ice's likely instability under conditions thought to exist during theoretical "Slushball Titan" climate regimes.