NASA recently announced its Curiosity rover had discovered complex organic molecules - key raw materials for life as we know it - on the surface of Mars. But now it seems a previous NASA probe may have made the same discovery more than 40 years ago, then accidentally burned it up. In 1976, NASA's twin Viking landers conducted the first experiments that searched for organic matter on the Red Planet. Researchers had long known that all planets receive a steady rain of carbon-rich micrometeorites and dust from space, meaning that Mars should be smothered in organic molecules. But the Viking landers found nothing, leaving researchers dumbstruck. "It was just completely unexpected and inconsistent with what we knew," says Chris McKay at NASA's Ames Research Centre in Mountain View, California. Haunted by Mars's missing molecules, researchers proposed one explanation after another, but none seemed to fit - until yet another probe came into play.