Clues to the Martian Atmosphere

Some 30 years ago, it was discovered that the main constituent of the thin martian atmosphere is carbon dioxide (CO) but that it only contained trace amounts of carbon monoxide and molecular oxygen, the photodissociation products of CO. Chemical reaction cycles involving water and hydrogen species have been developed to explain these observations, but as Hunten explains in his Perspective, observational evidence has been slow to emerge. He highlights the importance of Krasnopolsky and Feldman9s discovery of molecular hydrogen in the martian atmosphere.