Venus today is an inhospitable place with surface temperatures approaching 864 degrees Fahrenheit (462 degrees Celsius) and an atmosphere 90 times as thick as Earth’s. Several billion years ago the picture may have been different, says a team of planetary researchers at NASA’s Goddard Institute for Space Studies.

Planetary researchers long have theorized that Venus formed out of ingredients similar to Earth’s, but followed a different evolutionary path.

Measurements by NASA’s Pioneer-Venus mission in the 1980s first suggested Earth’s twin originally may have had an ocean.

However, Venus is closer to the Sun than our planet and receives far more sunlight. As a result, the planet’s early ocean evaporated, water-vapor molecules were broken apart by UV 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.

Another factor that impacts a planet’s climate is topography.

Lead author Michael Way and his colleagues postulated ancient Venus had more dry land overall than Earth, especially in the tropics. That limits the amount of water evaporated from the oceans and, as a result, the greenhouse effect by water vapor.

This type of surface appears ideal for making a planet habitable; there seems to have been enough water to support abundant life, with sufficient land to reduce the planet’s sensitivity to changes from incoming sunlight.

The team simulated conditions of a hypothetical early Venus with an atmosphere similar to Earth’s, a day as long as Venus’ current day, and a shallow ocean consistent with early data from the Pioneer spacecraft.

The scientists added information about Venus’ topography from radar measurements taken by NASA’s Magellan mission in the 1990s, and filled the lowlands with water, leaving the highlands exposed as Venusian continents.

The study also factored in an ancient Sun that was up to 30% dimmer. Even so, ancient Venus still received about 40% more sunlight than Earth does today.

“We have created a suite of 3D climate simulations using topographic data from the Magellan mission, solar spectral irradiance estimates for 2.9 and 0.715 billion years ago, present-day Venus orbital parameters, an ocean volume consistent with current theory, and an atmospheric composition estimated for early Venus,” Dr. Way and co-authors explained.

“Using these parameters we find that such a world could have had moderate temperatures if Venus had a rotation period slower than about 16 Earth days, despite an incident solar flux 46 − 70% higher than Earth receives.”

“At its current rotation period, Venus’s climate could have remained habitable until at least 715 million years ago.”

The team’s results were published August 11 in the journal Geophysical Research Letters (arXiv.org preprint).

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Michael J. Way et al. Was Venus the First Habitable World of our Solar System? Geophysical Research Letters, published August 11, 2016; doi: 10.1002/2016GL069790