Mars's atmosphere is also predominantly CO2, though far thinner. That's ... somewhat to be expected of any non-life-bearing rocky planet. Free oxygen is unsta

Mars's atmosphere is also predominantly CO2, though far thinner. That's ... somewhat to be expected of any non-life-bearing rocky planet. Free oxygen is unstable, lighter elements (hydrogen, ammonia) would tend to boil off with either low gravity or (relatively) high temperatures (as opposed to the ice-moons of Jupiter and Saturn, especially Titan).