The tide in the seas of Titan

Abstract

The surface temperature of Titan¹ is about 95 K, between the melting point (90.6 K) and the boiling point (118 K) of methane at the total surface pressure of 1.6 bar. Voyager 1 IR observations^1,2 suggest a methane abundance in the lower atmosphere >0.19 of saturation. Comparison with the mean H₂O abundance in the Earth's atmosphere suggests¹ a CH₄ saturation level ≅0.5. Thus the volume mixing ratio of methane is probably about 6% and methane begins to condense out some 40 km above the surface. It is therefore a natural speculation that methane exists in the liquid state on the surface of Titan. The atmospheric CH₄ corresponds to a liquid layer ∼10 m deep. But it is unlikely that the buffer for atmospheric methane would have a capacity equal only to that of the atmosphere itself. On Earth, the quantity of water in the oceans is on average several times 10⁴ the quantity of water vapour in the atmosphere. Thus, if Titan has oceans of liquid CH₄, they are probably >100 m deep. We argue here that, if this ocean exists, the present high eccentricity of Titan requires its depth to be >400 m, and that such an ocean can be detected by radar techniques.