Photochemical and climate consequences of sulfur outgassing on early Mars

Ancient Mars might have been warm and wet compared to today, but climate models have trouble reproducing this warmth, partly because of the faintness of the young Sun and partly because of inherent limitations to CO2–H2O greenhouse warming. In particular, Rayleigh scattering of incoming sunlight by a dense, CO2-rich atmosphere leads to a high planetary albedo, thereby reducing the amount of sunlight absorbed by the planet. It has been recently suggested that the presence of 1–100 ppmv SO2 in Mars' early atmosphere might have provided enough additional greenhouse warming to maintain a warm, wet early Mars. We show, however, that this warming should have been more than offset by cooling from sulfate and sulfur aerosols in early martian atmosphere. Hence, the paradox of Mars' early climate remains unresolved.