Evolution of Titan׳s major atmospheric gases and cooling since accretion

Abstract

This paper discusses two possible pathways of loss of the two main gases from Titan׳s post-accretional atmosphere, methane (CH4) and ammonia (NH3), by the mechanisms of thermal escape and emission from the interior coupled with thermal escape. The results give the decline of initial atmospheric gas masses to their present-day levels of 0.1bar CH4 and 1.4bar N2 (or equivalent 1.7bar NH3, as a precursor of N2). From the published data on planetary and Titan׳s accretion rates, the accretion temperature was estimated as Tac=355 to 300K. In the first 0.5–0.6Myr after accretion, Titan׳s surface cools to 150K and it takes about 5Myr to cool to near its present temperature of 94K. The present-day internal composition corresponds to the accreted Titan made of two solids, antigorite and brucite, that account for 59.5wt%, and an outer shell of an aqueous solution of NH3+(NH4)2SO4 accounting for 40.0wt%, and methane for a much smaller fraction of 0.6wt%. In thermal escape of CH4 and NH3, based on the Maxwell–Boltzmann distribution of gas-molecule velocities, the initial gas mass N0 in the atmosphere is lost by a first-order flux, Nt=N0 exp(−kt), where t is time (yr) and k (yr−1) is a rate parameter that depends on temperature, gas molecular mass, atmosphere thickness, and Titan׳s escape velocity.The computed initial Tac=355K is too high and the two gases would be lost from the primordial atmosphere in several hundred years. However, emissions of CH4 and NH3 from the interior, at reasonable rates that do not deplete the Titan gas inventory and function for periods of different length of time in combination with thermal escape, may result in stable CH4 and NH3 atmospheric masses, as they are at the present. The periods of emissions of different magnitudes of CH4 range from 6×104 to 6×105yr, and those of NH3 are 55,000–75,000yr.At the lower Tac=300K, thermal escape of gases alone allows their atmospheric masses to decrease from the primordial to the present-day levels in 50,000–70,000 years, when Titan׳s temperature has decreased to 245–255K. Below this temperature, the NH3 atmospheric mass is comparable to the present-day N2 mass. Thermal escape does not contradict the existence of the photolytic sink of CH4 in the cooled Titan atmosphere. The thermal escape mechanism does not require arbitrary assumptions about the timing of the start and duration of the gas emissions from the interior.