For all the drama around getting to Mars, the harder question starts after landing. A person can step onto the planet with food, water and shelter plans in place, but none of that matters for long without something far more basic: oxygen. On Earth, breathing feels automatic because the atmosphere does the work for you. On Mars, it does not. That difference starts with the air itself. Here on Earth, oxygen makes up about 21% of the atmosphere. Nitrogen takes up most of the rest, about 78%. When you breathe in, you take in all of it, but your body uses the oxygen and gets rid of the rest when you exhale. Mars offers a much harsher version of air. Its atmosphere is thin, with only about 1% of the volume of Earth’s atmosphere. Put another way, Mars has 99% less air than Earth. The planet is about half Earth’s size, and its weaker gravity cannot hold atmospheric gases as effectively, allowing them to escape into space. A revolutionary electrochemical device, MOXIE, efficiently converts carbon …