Two thirds of Earth’s surface are covered by water—and two thirds of Earth’s atmosphere reside over the oceans, far from land and the traditional ways that people measure the gases and pollutants that cycle through the air and around the globe. While satellites in space measuring the major gases can close some of that gap, it takes an aircraft to find out what’s really happening in the chemistry of the air above the oceans. That’s where NASA’s Atmospheric Tomography (ATom) mission comes in.