4. Degassing Processes and Chemistry

Degassing of volatile-bearing solids requires addition of energy in order to liberate the volatile elements from various compounds contained within the (initially) meteoritic rocks. There are two main sources of this energy: 1) that associated with the kinetic energy of impacting bodies and 2) heat from within the Earth, either in shallow, hydrothermal settings or in more deep-seated magmatic settings. The former energy source is most important for larger objects which arrive at the solid surface retaining most of their velocity. The latter become important for solid materials that survive passage through the atmosphere and impact at the surface. These surviving fragments may be heated during the Hadean equivalent of subduction (whatever that might entail), probably under hydrothermal conditions at shallow depth, or during magmagenesis in the upper mantle. The conditions of impact for larger objects, and the probable compositions of the impacting material, suggest production and maintenance of a strongly reducing atmosphere from an initial but short-lived high temperature phase, followed by a longer period of relatively low temperature atmospheric processing. The overall result is the formation of reduced carbon and nitrogen compounds that rain out into the primordial (recondensed) ocean not long after the impact. The hydrothermal processing of meteoritic “survivors” takes place under conditions of temperature, hydration and pressure conducive to the initial formation of methane and ammonia, which are then processed in the atmosphere into additional higher molecular weight organics, which also rain out into the ocean. The material that ends up in the magmagenic zone, where silicate melts are produced in the upper mantle, will initially be processed with some amount of meteoritic iron present, leading to the formation and release of additional reduced gases as well as some more oxidized gases. These will enter the atmosphere resulting from the first two processes, leading to still more production of reduced CHNO compounds. The result is an ocean enriched in reduced organic compounds similar to what would be expected from the Miller-Urey experiment. The atmosphere would be maintained as a largely reduced entity, although the reduced gases would be unstable to photochemical processing. Undoubtedly some carbon dioxide would have been produced and be present in the atmosphere as well.