Titan: A Fiercely Frozen Echo of the Early Earth

As the only other world we know with a thick nitrogen atmosphere, Titan clearly invites comparisons with Earth. The key to the origin of Titan's nitrogen lies with the very low abundances of primordial noble gases in the atmosphere. For nitrogen to arrive as N2, one expects a solar ratio of N/36Ar, instead of the orders of magnitude larger value found by the Huygens GCMS. Possible carriers then include NH3 and other N-compounds, including organic compounds, all trapped in the icy planetesimals that accreted to form the satellite. The ratio of 15N/14N on Earth implies a similar origin for our nitrogen, but the much lower relative abundances of primordial noble gases on Titan suggest a decisive difference in the volatile carriers: those contributing to Titan would have been warmer. In this case, methane and CO would not have been trapped, and the methane we see on Titan today must be stored or produced in the satellite's interior and released episodically to the atmosphere. Methane can be produced through high-pressure hydrothermal transformations of organic compounds under hydrogen-rich conditions created by water-rock interactions. Alternatively, both methane and noble gases may have been brought in as clathrates and may now reside as clathrates hidden in the depths of a liquid water mantle. The need for replenishment of methane is established by the short photochemical lifetime (10--20 Myr) of the atmospheric complement, and confirmed by the much lower depletion of 12C compared with 14N. The nitrogen isotope ratio on Titan reveals massive loss of the early atmosphere, assuming a starting value similar to ours. These findings may be relevant for models of an early reducing atmosphere on Earth.