Ask an Explainer
Why does carbon dioxide does not get recycled on mars?
There if no life on Mars - that we know of - which means there is nothing there to process the air. On Earth, we humans breathe in oxygen, and breathe out CO2, while plant life "breathes" in this CO2 and recycle it into more oxygen.
On Mars, the planet doesn't have that system. It is stuck with the CO2 it has.