The most likely candidate for the atmosphere is melted co2. Mara would have liquid oceans if the co2 ice caps were melted away. From there, a biosphere would ne

The most likely candidate for the atmosphere is melted co2. Mara would have liquid oceans if the co2 ice caps were melted away. From there, a biosphere would need to be introduced for oxygen production. The main limiter on earth based animal life would be a lack of buffer gas such as nitrogen, and the excessive co2 concentrations.