Venus May Have Been Habitable for Three Billion Years

The Venusian climate remained stable and temperate for about three billion years, until a mysterious global event resurfaced around 80% of the planet, according to new research by NASA planetary scientists.

Observations suggest Venus may have had water oceans in its distant past; a land-ocean pattern like that above was used in a climate model to show how storm clouds could have shielded ancient Venus from strong sunlight and made the planet habitable. Image credit: NASA.

“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 Dr. Michael Way, from NASA’s Goddard Institute for Space Science.

In the 1970s, NASA’s Pioneer Venus spacecraft found hints that the planet may once have had a shallow ocean.

To see if the planet might ever have had a stable climate capable of supporting liquid water, Dr. Way and his colleague, Dr. Anthony Del Genio, 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 20 and 50 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 of carbon dioxide stored in the rocks of the planet approximately 700-750 million years ago.

Three of the five scenarios assumed the topography of Venus as we see it today and considered a deep ocean averaging 310 m, a shallow layer of water averaging 10 m and a small amount of water locked in the soil.

For comparison, they also included a scenario with Earth’s topography and a 310-m ocean and, finally, a world completely covered by an ocean of 158 m 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.

At 4.2 billion years ago, soon after its formation, Venus would have completed a period of rapid cooling and its atmosphere would have been dominated by carbon dioxide.

If the planet evolved in an Earth-like way over the next 3 billion years, carbon dioxide would have been drawn down by silicate rocks and locked into the surface.

By the second epoch modelled at 715 million years ago, the atmosphere would likely have been dominated by nitrogen with trace amounts of carbon dioxide and methane and these conditions could have remained stable up until present times.

The cause of the outgassing that led to the dramatic transformation of Venus is a mystery, although probably linked to the planet’s volcanic activity.

One possibility is that large amounts of magma bubbled up, releasing carbon dioxide from molten rocks into the atmosphere.

The magma solidified before reaching the surface and this created a barrier that meant that the gas could not be reabsorbed.

The presence of large amounts of carbon dioxide triggered a runaway greenhouse effect, which has resulted in the scorching 462 degree average temperatures found on Venus today.

“Something happened on Venus where a huge amount of gas was released into the atmosphere and couldn’t be re-absorbed by the rocks,” Dr. Way said.

“On Earth, we have some examples of large-scale outgassing, for instance the creation of the Siberian Traps 500 million years ago which is linked to a mass extinction, but nothing on this scale. It completely transformed Venus.”

Dr. Way and Dr. Del Genio presented their results September 20 at the EPSC-DPS Joint Meeting 2019 in Geneva, Switzerland.

_____

Michael J. Way Anthony D. Del Genio. 2019. A view to the possible habitability of ancient Venus over three billion years. EPSC Abstracts 13, EPSC-DPS2019-1846-1