Modeling nitrogen and methane with ethane and propane gas hydrates at low temperatures (173–290 K) with applications to Titan

Abstract

The FREZCHEM model was primarily designed for cold temperatures (173–298K) and high pressures (1–1000bars). Nitrogen gas (95.0%) and methane gas (5.0%) are major gases on the surface of Titan. Recently, we added nitrogen and methane gas hydrates to FREZCHEM model on Titan; and nitrogen–methane gas hydrates formed on Titan at 178K. The other common but less abundant gases on Titan are ethane (C2H6) and propane (C3H8) that can also form gas hydrates with nitrogen and methane. The specific objectives of this study were to (1) add ethane and propane to gas hydrates, including mixtures with nitrogen, methane, and carbon dioxide, and (2) explore the potential roles of gas hydrates on Titan. At 273K, the Ln(gas hydrates) were 5.095 for N2, 3.217 for CH4, 2.327 for CO2, 1.288 for C2H6, and 0.281 for C3H8. At 173K, the Ln(gas hydrates) were −4.968 for N2, −6.102 for CH4, −7.803 for CO2, −5.125 for C2H6, and −5.512 for C3H8. Apparently C2H6 and C3H8 gas hydrates change less at lower temperatures than N2, CH4, and CO2 gas hydrates. In previous papers, we added three mixed CH4–CO2, N2–CH4, and N2–CO2 binary gas hydrates. In this paper, we added ethane and propane to include new binary gas hydrate mixtures of N2–C2H6, N2–C3H8, C2H6–C3H8, CH4–C2H6, CH4–C3H8, CO2–C2H6, and CO2–C3H8. Today, there are ten binary gas hydrates in the FREZCHEM model. In the text, how to cope with more than two species is described. Simulations from 273K to 173K used a surface Titan pressure of 1.467 bars with a major gas of nitrogen (94.24%), a minor gas of methane (5.65%), and extremely minor gases of ethane (0.0038%), and propane (0.000343%). Eventually at 178K, N2·6H2O formed with 0.17694mol, CH4·6H2O formed with 0.04101mol, C2H6·6H2O formed with 6.48e−6mol, and C3H8·6H2O formed with 9.36e−7mol. Based on the atmospheric conditions of Titan, the trace gases of ethane and propane led to low gas hydrate precipitations of ethane and propane with nitrogen and methane. However, the gas concentrations could be very different in an ocean of Titan that could be 50 to 175km beneath the surface where the temperatures may increase as high as 260K. Methane gas hydrates are likely dominant beneath the surface of Titan with some minor contributions from nitrogen, ethane, and propane. Below the surface of Titan in the ocean and icy shell, gas hydrates clearly depend on the gas concentrations of methane, nitrogen, ethane, and propane as well as temperature and pressure.