Venus may have been habitable, says NASA

Venus may have hosted liquid water for 2-3 billion years, until a dramatic transformation starting over 700 million years ago resurfaced around 80 per cent of the planet, according to a NASA study. The research gives a new view of Venus's climatic history and may have implications for the habitability of exoplanets in similar orbits. Forty years ago, NASA's Pioneer Venus mission found tantalising hints that Earth's 'twisted sister' planet may once have had a shallow ocean's worth of water.

To see if Venus might ever have had a stable climate capable of supporting liquid water, researchers from NASA Goddard Institute for Space Studies (GISS) in the US created a series of five simulations assuming different levels of water coverage. In all five scenarios, they found that Venus was able to maintain stable temperatures between a maximum of about 50 degrees Celsius and a minimum of about 20 degrees Celsius for around three billion years. A temperate climate might even have been maintained on Venus today had there not been a series of events that caused a release, or 'outgassing', of carbon dioxide stored in the rocks of the planet about 700-750 million years ago.

"Our hypothesis is that Venus may have had a stable climate for billions of years. It is possible that the near-global resurfacing event is responsible for its transformation from an Earth-like climate to the hellish hot-house we see today," said Way. Three of the five scenarios studied by researchers assumed the topography of Venus as we see it today and considered a deep ocean averaging 310 metres, a shallow layer of water averaging 10 metres and a small amount of water locked in the soil.

For comparison, they also included a scenario with Earth's topography and a 310-metre ocean and, finally, a world completely covered by an ocean of 158 metres depth. To simulate the environmental conditions at 4.2 billion years ago, 715 million years ago and today, the researchers adapted a 3D general circulation model to account for the increase in solar radiation as our Sun has warmed up over its lifetime, as well as for changing atmospheric compositions.