Was Venus Habitable for 2 Billion Years?

Venus may have had a shallow liquid-water ocean and habitable surface temperatures for up to 2 billion years of its early history, according to computer modeling of the planet's ancient climate by scientists at NASA's Goddard Institute for Space Studies (GISS) in New York.

The findings, published this week in the journal Geophysical Research Letters, were obtained with a model similar to the type used to predict future climate change on Earth.

"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," said Michael Way, a researcher at GISS and the paper's lead author. "These results show ancient Venus may have been a very different place than it is today."

Venus today is a hellish world. It has a crushing carbon dioxide atmosphere 90 times as thick as Earth's. There is almost no water vapor. Temperatures reach 864 degrees Fahrenheit (462 degrees Celsius) at its surface.

Scientists long have theorized that Venus formed out of ingredients similar to Earth's, but followed a different evolutionary path. Measurements by NASA's Pioneer mission to Venus in the 1980s first suggested Venus originally may have had an ocean. However, Venus is closer to the sun than Earth and receives far more sunlight. As a result, the planet's early ocean evaporated, water-vapor molecules were broken apart by ultraviolet radiation, and hydrogen escaped to space. With no water left on the surface, carbon dioxide built up in the atmosphere, leading to a so-called runaway greenhouse effect that created present conditions.

Previous studies have shown that how fast a planet spins on its axis affects whether it has a habitable climate. A day on Venus is 117 Earth days. Until recently, it was assumed that a thick atmosphere like that of modern Venus was required for the planet to have today's slow rotation rate. However, newer research has shown that a thin atmosphere like that of modern Earth could have produced the same result. That means an ancient Venus with an Earth-like atmosphere could have had the same rotation rate it has today.

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