FORMATION, OCCURRENCE, AND COMPOSITION OF GAS HYDRATES IN THE SOLAR SYSTEM

Gas hydrates may be the most abundant form of volatile materials in the Solar System. Clathrate hydrates probably are more abundant than water ice phases in the icy satellites of the gas giant planets. In the gas giants themselves, gas hydrates of unknown mineral structure (or possibly liquid solutions) are probably more abundant than rocky materials and all polymorphs of pure ice; it is possible that only hydrogen and helium phases are more abundant in our Solar System. In comets, amorphous associations of H2O and hydrate-forming gases are important. In contrast, gas hydrates are comparatively rare on the terrestrial planets, but clathrates serve important geological roles on Earth and probably Mars. Modes of formation of gas hydrates in the Solar System are highly varied, and resulting hydrate compositions are also. Likely possibilities include: (1) direct condensation of amorphous CO-CO2-N2-rich hydrates from the Solar Nebula by reaction of hydrate-forming gases with water ice (comets and Kuiper Belt Objects), (2) thermal reprocessing of Solar Nebula condensates to form N2-CO-rich and CH4-rich clathrate hydrates (comets, Pluto, Charon, Triton, and Plutinos), (3) direct condensation of N2-CO-rich and CH4-rich clathrates from circumplanetary nebulae of the outer planets (Callisto and regular satellites of Saturn, Uranus, and Neptune), (4) formation of SO2-, CO2-, and N2-rich clathrates in primordial hydrospheres (Hadean Earth, Mars, Europa, Ganymede, and Callisto), (5) formation of SO2-CO2 clathrates or CO2-dominated clathrates in evolved hydrospheres (Martian permafrost and Europa's icy crust and ocean), biogenic origin of CH4 clathrate (Earth's permafrost and seafloor; Mars? Europa?), (6) thermogenic origin of CH4-C2H6-C3H8-rich clathrates (hydrates related to petroleum and gas deposits on Earth). In addition, Mars and Earth have a variety of other minor clathrate-forming mechanisms, including occlusion of air in polar ices (forming N2-O2-rich clathrates in the case of Earth and CO2-rich clathrates in the case of Mars) and direct formation of extremely minor quantities of clathrate snow and frost on Mars.