This isn't about putting a whole new atmosphere on Mars. But if we did, it would have to be replenished but it would be lost on geological not human timescales

This isn't about putting a whole new atmosphere on Mars. But if we did, it would have to be replenished but it would be lost on geological not human timescales. Think significant amounts of atmosphere lost after 100,000 years. You could do the same with the moon.