Modelling the composition of volcanic gases on young terrestrial planets in the C-H-O-N-S system

Kurzfassung

Subsequently to the magma ocean state, secondary atmospheres build up via early volcanic degassing of planetary interiors. The terrestrial planets Venus, Earth and Mars are believed to have originated from similar source material but reveal different present-day atmospheric compositions, pressures and temperatures. To investigate how such diverse atmospheres emerge, we have developed a model that allows us to compute the molar fractions of gases involved in volcanic degassing. We simulate the gas chemical speciation in the C-H-O-N-S system since C-H-O-S volatiles can be stored in significant amounts in basaltic magmas and therefore make up about 95 % of the magmatic gases released by volcanoes on earth. Because of its important role in the terrestrial atmosphere Nitrogen was added to our calculations as well. We consider the influence of temperature, pressure, oxygen fugacity and melt composition on the solubility of each phase as well as the speciation of all phases and fitted these parameters to conditions present on early Earth and Mars. We show that oxygen fugacity has a major influence on the gas composition during degassing due to its important influence not just on the speciation of all volatiles but also on the solubility of Nitrogen and Sulfur in the magma. According to the volatile content of the melt, under reducing conditions H2, CO, H2S, S2 and NH3 are the main outgassed species, while H2O, CO2 and SO2 and N2 dominate in the oxidizing case. We furthermore show that the temperature and pressure conditions of the source region in which the magma is formed and the chemical composition of the magma itself also play a key role in the chemical composition of volcanic gasses and that hence, varying parameters like planet mass, atmospheric pressure, tectonic regime, volatile budget or the abundance of heat sources can lead to significantly different atmospheric compositions and degassing rates.