Stability of Condensed Hydrocarbons in the Solar Nebula

Abstract

Thermodynamic calculations of metastable equilibria in the H–C–O system are used to evaluate the stability of condensed polycyclic aromatic hydrocarbons (PAHs) and normal alkanes in the solar nebula. The effects of temperature, total pressure (governed by H 2), the abundances of gaseous CO and H 2O, as well mass accretion rate into the Sun and viscous efficiency at the nebula midplane are explored. We show that the inhibited formation of graphite and methane permits metastable existence of hydrocarbons with respect to the inorganic gases H 2, CO, and H 2O. Low temperatures, high pressures, high abundances of CO, low abundances of H 2O, low accretion rates, and low viscous efficiencies favor stability of hydrocarbons. Condensed PAHs are stable relative to nominal abundances of the inorganic gases at temperatures below ∼450 K depending on the physical parameters adopted for the nebula. Normal alkanes with carbon numbers <10 are stable at temperatures 30–60 degrees lower. During the evolution of the nebula, hydrocarbons have a thermodynamic potential to form in a narrow zone, which moved toward the Sun as the accretion rate decreased. At radial distances of 2–4 AU, hydrocarbons had a potential to form at the time when the accretion rate was 10 −6.3–10 −7.7 solar mass yr −1, depending on the viscous efficiency. High temperature, low pressure, and a high CO/H 2O ratio in the nebula increase the stability of PAHs compared with their alkylated versions and relative to their aliphatic counterparts with the same carbon number. The calculations reveal the thermodynamic possibility for nebular Fischer–Tropsch type (FTT) synthesis of condensed hydrocarbons on the surface of mineral grains from CO and H 2 in an H 2O-depleted and/or CO-rich environment.