Carbon dioxide to generate the atmosphere originates in the planet's mantle and is released through volcanoes or trapped in rocks crystallized from magmas. Once in the atmosphere, the CO2 can exchange with the polar caps, passing from gas to ice and back to gas again. The CO2 can also dissolve into waters, which can then precipitate out solid carbonates, either in lakes at the surface or in shallow aquifers. Importantly, CO2 gas in the atmosphere is continually lost to space at a rate controlled in part by the sun's activity. The ultraviolet (UV) photodissociation mechanism that we highlight occurs when UV radiation encounters a CO2 molecule, breaking the bonds to first form CO and then C atoms. Isotope fractionation occurs when the C atoms are lost to space because the lighter carbon-12 isotopes are more easily removed than the heavier carbon-13 isotopes. This fractionation, the preferential loss of carbon-12 to space, leaves a fingerprint: enrichment of the heavy carbon-13 isotope, measured in the atmosphere of Mars today.