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The challenge now, he says, is assessing “the relative importance of this process versus the other things we know about the global oxygen cycle.”

“There’s a lot we don’t know about the oxygen cycle on Mars. That’s become apparent.”

The former endangers health, and many ecologists say that increased carbon dioxide threatens the earth's oxygen cycle.

He could listen to his own lungs and pulse, loud in the spacesuit; he could even notice its interior smell, blend of plastic and oxygen cycle chemicals, flesh and sweat.