Thermodynamics and mass transport in multicomponent, multiphase H ₂O systems of planetary interest

Heat and mass transport in low-temperature, low-pressure H ₂O systems are important processes on Earth, and on a number of planets and moons in the Solar System. In most occurrences, these systems will contain other components, the so-called noncondensible gases, such as CO ₂, CO, SO ₂, CH ₄, and N ₂. The presence of the noncondensible components can greatly alter the thermodynamic properties of the phases and their flow properties as they move in and on the planets. We review various forms of phase diagrams that give information about pressure- temperature-volume-entropyenthalpy-composition conditions in these complex systems. Fluid dynamic models must be coupled to the thermodynamics to accurately describe flow in gas-driven liquid and solid systems. The concepts are illustrated in detail by considering flow and flow instabilities such as geysering in modern geothermal systems on Earth, paleofluid systems on Mars, and cryogenic ice-gas systems on Mars and Enceladus. We emphasize that consideration of single-component end-member systems often leads to conclusions that exclude many qualitatively and quantitatively important phenomena.