When NASA announced last week it would spend $1 billion developing two new missions to Venus—the agency’s first visits in decades to Earth’s hothouse twin—planetary scientists were elated, and not just because a long wait had ended. A dramatic shift in thinking about the planet over the past few years has made a visit even more enticing. Venus was once thought to have boiled off all its water almost as soon as it was born 4.5 billion years ago, turning into the parched, hostile world of today. But many scientists now think Venus might have kept expansive oceans for billions of years—a nearly perfect setting for life.
The missions, to arrive late this decade, are equipped to look for signs of that water—and clues to why Venus ultimately declined into an inferno. If their findings support the new picture, Mars, the longtime hope for discovering signs of ancient extraterrestrial life, will have a rival. “Why look at Mars, which had water for 300 million years, when Venus had water for 3 billion years?” asks Darby Dyar, a planetary scientist at Mount Holyoke College who is deputy principal investigator for one of the new missions, VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy).
Early visits to Venus cemented the picture of a dead, waterless planet when they detected no sign of oxygen in its thick carbon dioxide (CO2) atmosphere, which keeps its surface at a lead-melting 460°C. Venus likely started out with plenty of water, as it formed from roughly the same building blocks as Earth. The thinking was that as the water boiled off, ultraviolet light broke down water vapor molecules. Hydrogen would have escaped to space while oxygen, being heavier, would have lingered in the air. Its absence led scientists to assume Venus lost its water very early on.