Evidence mounts for an ocean on early Venus

Venus may have been all wet early on.

New simulations suggest that if the now-hellish planet had just the right amount of cloud cover, carbon dioxide and water to start with, Venus could have formed an ocean. The result, published online July 18 in the Journal of Geophysical Research: Planets, provides a new clue to whether Venus could have ever supported life.

The finding could also help planetary scientists in their search for habitable planets far beyond the solar system by revealing what conditions in a planet’s atmosphere and on its surface may make it suitable for life.

“This work plays into a much bigger puzzle of understanding the habitability of exoplanets,” says Michael Way, an astrophysicist at the NASA Goddard Institute for Space Studies in New York City who was not involved in the study.

Last year, Way and colleagues reported that Venus’ slow rotation — one day lasts roughly 116 Earth days — could have led to a buildup of continual cloud cover that allowed for average temperatures of around 15° Celsius as recently as 715 million years ago. Those cooler conditions, compared with Venus’ inferno today at 460°, could also have made it possible for the planet to have a shallow ocean (SN Online: 8/26/16). The new work supports that study, revealing the delicate interplay of cloud cover, carbon dioxide and water that would produce an ocean.

In the new study, planetary scientist Emmanuel Marcq of the Université de Versailles Saint-Quentin-en-Yvelines in Guyancourt, France, and colleagues ran computer simulations that calculated how the cooling of a young rocky planet’s molten surface would interact with its developing atmosphere and incoming heat from the young sun. With carbon dioxide similar to current levels, early Venus would need only 10 percent of the mass of water in Earth’s oceans to form its own watery surface. Changing the reflectiveness of the clouds and a few other factors shows Venus would need, at most, 30 percent of the mass of water in Earth’s ocean to form its own ocean.

It’s not surprising that Venus could have theoretically had an ocean, Marcq says. But whether it actually did is “very much a hotly debated, open question.” The new work might raise the odds that water vapor could have condensed into an ocean on Venus, Marcq says.

“Life as we know it requires stable liquid water on a solid surface,” he says. Keeping water on a planet requires a specific range of temperatures and pressures, which depends on a complex interplay of the planet’s atmospheric composition, the reflectivity of its clouds, how much heat the atmosphere and surface absorb from its star, how much the atmosphere leaks into space and much more.

The simulations didn’t consider how long favorable conditions for an ocean on Venus lasted. So it isn’t clear what might have happened to the ocean, if it existed at all. Other work suggests any ocean might have eventually boiled away or got reinjected into part of the planet’s interior. Either scenario could explain why the planet is extremely dry today.