Modelling the effect of volcanic outgassing of sulphur on early Martian surface temperatures using a 3-D Global Climate Model

Around the time of the transition from the Late Noachian to the Early Hesperian eras ($\sim$3.6 Gya), Mars was predicted to have been both volcanically active, and have sustained a climate warm enough to melt liquid water on its surface episodically despite a faint young sun. The effect of volcanic outgassing on the climate of early Mars and its ability to raise temperatures above the melting point of water has, however, been disputed, with a major uncertainty being the timescales over which the greenhouse effect of outgassed sulphur dioxide (SO₂) and hydrogen sulphide (H₂S) can warm the atmosphere of Mars before they react to form H₂SO₄ and S₈ aerosols which act to cool the surface of Mars. We have developed the first 3-D model of the Martian sulphur cycle from source to sink that includes outgassing of SO₂, H₂S and S₂ from the surface, the formation of H₂SO₄ and S₈ through atmospheric chemistry, and the condensation and deposition of H₂SO₄ and S₈ to the surface. We simulate the effect of a single large, day-long volcanic outgassing event on global surface temperatures, identifying the magnitude and duration of any net warming and cooling as a function of outgassing magnitude, atmospheric pressure, obliquity and aerosol particle size distribution. We confirm the results of Tian et al. (2010, EPSL 295, 412-418) and find that the persistence of the warming effect of volcanically outgassed SO₂ and H₂S on the Martian atmosphere is only of the order of days to weeks for a surface pressure range of between 0.5 and 1.5 bars. Typical outgassing magnitudes result in a net cooling of the Martian surface over timescales of 3–4 years near the equator and several decades at the poles for plume neutral buoyancy heights within the troposphere. For very high magnitudes of outgassing, the rate of H₂SO₄ cloud formation slows down due to the depletion of water vapour in the atmosphere, thereby slowing down the rate of cooling and providing a buffer against atmospheric collapse.