Low Temperature Condensation from the Solar Nebula

Two models of the low temperature condensation accretion process for different extreme assumptions are investigated. In one extreme the time scale for accretion is taken to be long compared to the time scale for cooling of the nebula: in this case detailed chemical equilibrium is maintained at all temperatures, and the low temperature condensation process is characterized by solid hydrates such as NH ₃·H ₂O and CH ₄·8H ₂O. In the opposite extreme, accretion is assumed to be rapid compared to the time scale for cooling: here condensates, once formed, are assumed to be isolated from further chemical interaction with the gas. This inhomogeneous accretion model is closely analogous to condensation sequences in solar-composition planetary atmospheres, and is characterized by formation of aqueous NH ₃ solution, solid NH ₄SH, and solid NH ₃. The results of these models are compared and their relevance to processes in the solar nebula is discussed.