Venus likely maintained stable temperatures and hosted liquid water for billions of years before an event triggered drastic changes in the planet, according to a new study.
Now, Venus is a mostly dead planet with a toxic atmosphere 90 times thicker than ours and surface temperatures that reach 864 degrees, hot enough to melt lead. It's often called Earth's twin because the planets are similar in size. But the modern comparisons stop there.
However, a recent study compared five climate simulations of Venus' past and every scenario suggested that the planet could support liquid water and a temperate climate on its surface for at least three billion years. Like the other planets in our solar system, Venus formed 4.5 billion years ago.
Those temperatures could have included a maximum of 122 degrees Fahrenheit and a minimum of 68 degrees Fahrenheit.
But between 700 and 750 million years ago, something triggered the release of carbon dioxide from rocks on the planet, transforming its climate.
"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 hothouse we see today," said Michael Way, study author at The Goddard Institute of Space Science. Way presented his study this week at the European Planetary Science Congress - Division for Planetary Sciences Joint Meeting 2019 in Geneva.
Previously, Way authored a 2016 study about climate and oceans on Venus in its past.
The ocean was first suggested by NASA's Pioneer mission in the 1980s. But given its placement as the second planet from the sun, Venus wasn't considered conducive to sustaining an ocean.
Venus receives more sunlight than Earth, which would evaporate liquid water, sending hydrogen into space and trapping a buildup of carbon dioxide. That would lead to a nonstop greenhouse effect that would create its current toxic atmosphere. Venus' topography was completely altered by volcanic eruptions that most likely filled in lowland regions and potential ocean basins over the past billion years.
Out of the five simulations, three of them included Venus' current topography and added a deep 1,017-foot ocean, a shallow 32-foot ocean and trace amounts of water in the soil. Researchers compared this to two other simulations, one using Earth's topography with a deep ocean and an ocean world.
To recreate likely conditions on Venus that happened 4.2 billion years ago and changed over time, they gradually increased solar radiation to reflect the sun as it warmed. This also shifted atmospheric conditions over time.
Previously, researchers believed that Venus is too close to the sun to sustain liquid water on its surface, beyond the inner limit of the sun's habitable zone. The new simulations shift that belief for Way and his colleagues.
"Venus currently has almost twice the solar radiation that we have at Earth. However, in all the scenarios we have modeled, we have found that Venus could still support surface temperatures amenable for liquid water," Way said.
The simulations suggest that Venus went through a rapid cooling phase a few billion years after it formed. Then, the atmosphere would have been full of carbon dioxide. If Venus evolved similarly to Earth, that carbon dioxide would have come down from the atmosphere, drawn by silicates, and become trapped in the surface. This would allow the atmosphere to be full of nitrogen with tiny amounts of carbon dioxide and methane, providing stability.
But something happened around 700 million years ago that remains a mystery, although the researchers think its connected to volcanic activity. Magma would have released carbon dioxide into the atmosphere, and as the magma cooled, the gas couldn't be reabsorbed in the surface.
Whatever happened, so much carbon dioxide in the atmosphere caused a runaway greenhouse effect, evidenced in the scorching hot temperatures on the planet now.
"Something happened on Venus where a huge amount of gas was released into the atmosphere and couldn't be re-absorbed by the rocks. 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," Way said.
Besides the mystery of the event's cause, the researchers need to know how quickly Venus cooled after its formation and if water could have existed on the surface, as well as if the outgassing event only happened once or was merely one in a series of events.
"We need more missions to study Venus and get a more detailed understanding of its history and evolution," Way said. "However, our models show that there is a real possibility that Venus could have been habitable and radically different from the Venus we see today. This opens up all kinds of implications for exoplanets found in what is called the 'Venus Zone', which may in fact host liquid water and temperate climates."