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Venus鈥� carbon dioxide-filled atmosphere today is 90 times as thick as Earth鈥檚, 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鈥檚 ancient climate, go against prior assumptions that Venus鈥� slower rotation on its axis depended on its having a thicker atmosphere.

鈥淢any 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,鈥� Michael Way, a researcher at NASA鈥檚 Goddard Institute for Space Studies, said in a statement.

鈥淭hese results show ancient Venus may have been a very different place than it is today,鈥� 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鈥檚 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鈥檚 early ocean evaporated, the researchers say, water-vapor molecules were broken up by ultraviolet radiation, causing hydrogen to escape to space.聽In the absence of water, carbon dioxide built up in the planet鈥檚 atmosphere, fostering what the researchers call a 鈥渞unaway greenhouse gas effect鈥� that led to the planet's present conditions.

But what about its past?聽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鈥檚 Magellan mission and filled lowland areas with water. If聽Venus had more dry land that Earth, they say, the amount of water evaporated from the oceans 鈥� and the resulting greenhouse effect from water vapor 鈥� 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鈥檚 sensitivity to the sun鈥檚 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鈥� current day 鈥� or 117 Earth days 鈥� could still produce the same conditions over time as Venus鈥� current, thick atmosphere.

After leaving the planet鈥檚 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.聽

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

鈥淭his 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鈥檚 today,鈥� 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鈥檚 water boiled off.

The NASA researchers say more information about Venus鈥� 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.