Mars CO2 cycle: Effects of airborne dust and polar cap ice emissivity

Mars General Circulation Model (GCM) simulations are presented to illustrate the importance of the ice emissivity of the seasonal CO2 polar caps in regulating the effects of airborne dust on the martian CO2 cycle. Simulated results show that atmospheric dust suppresses CO2 condensation when the CO2 ice emissivity is high but enhances it when the CO2 ice emissivity is low. This raises the possibility that the reason for the repeatable nature of the CO2 cycle in the presence of a highly variable dust cycle is that the CO2 ice emissivity is "neutral" - the value that leads to no change in CO2 condensation with changing atmospheric dust. For this GCM, the "neutral" emissivity is approximately 0.55, which is low compared to observed cap emissivities. This inconsistency poses a problem for this hypothesis. However, it is clear that the CO2 ice emissivity is a critical physical parameter in determining how atmospheric dust affects the CO2 cycle on Mars.