Modeling nitrogen-gas, -liquid, -solid chemistries at low temperatures (173–298K) with applications to Titan

Molecular diatomic nitrogen (henceforth, "nitrogen") is a major gas on Venus, Earth, Mars, Titan, Triton, and Pluto; a major condensed liquid component on Titan; and major condensed ices on Triton and Pluto. Nitrogen also occurs as a component of air gas hydrates in Earth's polar ice sheets. The Solar System's nitrogen originally might have been produced by condensation of nitrogen ice in the outer Solar Nebula disk, or it might have first condensed as and then decomposed from nitrogen gas hydrate or ammonia. The specific objectives of this study were to add nitrogen into the FREZCHEM model (which already included ammonia) and explore some roles of nitrogen on planetary satellites, especially Titan, where nitrogen is a major atmospheric gas and a component of Titan's hydrocarbon-rich lakes and rain. Nitrogen gases, aqueous phases, and gas hydrates were added to FREZCHEM. Nitrogen, methane, and carbon dioxide gas hydrates were parallel with respect to temperature, which allowed extensions to 173 K, including for mixed N2–CH4 and N2–CO2 gas hydrates. Simulations from 273 to 173 K used a surface Titan pressure of 1.467 bars with a major mixing gas of nitrogen at 1.394 bars (95%) and methane at 0.073 bars (5%). These gas concentrations eventually led to formation of N2–CH4 gas hydrate at 178 K, which suggests that these gas hydrates could be forming on the surface of Titan today (at 94 K) and in sub-surfaces with higher temperatures. On Earth, air gas hydrates are common deep within polar ice sheets and are dominated by atmospheric N2/O2 gas compositions; and as a consequence, gas hydrates do not control, but are controlled by, atmospheric N2/O2 gas compositions. Given that Earth and Titan have similar atmospheric nitrogen (0.78 vs. 1.39 bars) and icy environments, on neither body are gas hydrates likely the main source of atmospheric nitrogen; but rather, atmospheric gases are likely the source of minor amounts of nitrogen in gas hydrates on Earth and Titan.