Why scientists say Venus was once a habitable world

Venus’ carbon dioxide-filled atmosphere today is 90 times as thick as Earth’s, with temperatures that reach 864 degrees F., at its surface.

But NASA researchers say Venus may have once been habitable, with an ocean and an Earth-like, thinner atmosphere.

The findings, which used a computer model of the planet’s ancient climate, go against prior assumptions that Venus’ slower rotation on its axis depended on its having a thicker atmosphere.

“Many of the same tools we use to model climate change on Earth can be adapted to study climates on other planets, both past and present,” Michael Way, a researcher at NASA’s Goddard Institute for Space Studies, said in a statement.

“These results show ancient Venus may have been a very different place than it is today,” said Dr. Way, the lead author of a paper the team published this week in the journal Geophysical Research Letters.

Scientists have long believed that Venus was formed from similar ingredients as Earth, but theorized that those ingredients evolved differently. In the 1980s, measurements from NASA’s Pioneer mission first suggested that Venus may have once had an ocean.

Because Venus is closer to the sun than Earth, it receives much more sunlight. As the planet’s early ocean evaporated, the researchers say, water-vapor molecules were broken up by ultraviolet radiation, causing hydrogen to escape to space. In the absence of water, carbon dioxide built up in the planet’s atmosphere, fostering what the researchers call a “runaway greenhouse gas effect” that led to the planet's present conditions.

But what about its past? Hypothesizing that ancient Venus had much more dry land than Earth, they simulated conditions on Venus with a shallow ocean, added topographic information from NASA’s Magellan mission and filled lowland areas with water. If Venus had more dry land that Earth, they say, the amount of water evaporated from the oceans – and the resulting greenhouse effect from water vapor – was likely limited, making it possible for life to thrive.

As a result, that surface, with enough water and sufficient land to reduce the planet’s sensitivity to the sun’s power, could have sustained life.

By using a climate model, the Goddard Institute researchers found that a slower rotation, producing a day as long as Venus’ current day – or 117 Earth days – could still produce the same conditions over time as Venus’ current, thick atmosphere.

After leaving the planet’s highlands exposed and factoring in a sun that was about 30 percent dimmer than today, they still found that ancient Venus still gained about 40 percent more sunlight than Earth does today. 

In their model, “Venus’ slow spin exposes its dayside to the sun for almost two months at a time,” Anthony Del Genio, a co-author and Goddard Institute researcher, said in the statement.

“This warms the surface and produces rain that creates a thick layer of clouds, which acts like an umbrella to shield the surface from much of the solar heating. The result is mean climate temperatures that are actually a few degrees cooler than Earth’s today,” he added.

The research comes in the wake of research in June that found that a surprisingly strong electrical field wrapped around Venus could have whisked away the oxygen ions that made up the steam generated as the planet’s water boiled off.

The NASA researchers say more information about Venus’ ancient atmosphere could impact future searches for habitable planets, including exoplanets, those which orbit other stars.

The space agency is currently preparing to launch the Transiting Exoplanet Survey Satellite and the James Webb Space Telescope, which will replace the Hubble Space Telescope.