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

摘要

This paper discusses two possible pathways of loss of the two main gases from Titan's post-accretional atmosphere, methane (CH_4) and ammonia (NH_3), 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 0.1 bar CH_4 and 1.4 bar N_2 (or equivalent 1.7 bar NH_3, as a precursor of N_2). From the published data on planetary and Titan's accretion rates, the accretion temperature was estimated as T_(ac)=355 to 300 K. In the first 0.5-0.6 Myr after accretion, Titan's surface cools to 150 K and it takes about 5 Myr to cool to near its present temperature of 94 K. The present-day internal composition corresponds to the accreted Titan made of two solids, antigorite and brucite, that account for 59.5 wt, and an outer shell of an aqueous solution of NH_3 + (NH_4)_2SO_4 accounting for 40.0 wt, and methane for a much smaller fraction of 0.6 wt. In thermal escape of CH_4 and NH_3, based on the Maxwell-Boltzmann distribution of gas-molecule velocities, the initial gas mass N_o in the atmosphere is lost by a first-order flux, N_t=N_o 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 T_(ac)=355 K is too high and the two gases would be lost from the primordial atmosphere in several hundred years. However, emissions of CH_4 and NH_3 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 CH_4 and NH_3 atmospheric masses, as they are at the present. The periods of emissions of different magnitudes of CH_4 range from 6 × 10~4 to 6 ×10~5 yr, and those of NH_3 are 55,000-75,000 yr. At the lower T_(ac)=300 K, 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 245-255 K. Below this temperature, the NH_3 atmospheric mass is comparable to the present-day N_2 mass. Thermal escape does not contradict the existence of the photolytic sink of CH_4 in the cooled Titan atmosphere. The thermal escape mechanism does not require arbitrary assumptions about the timing of the start and duration of the eas emissions from the interior.