Did Venus ever have an ocean?


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Venus, long forgotten, has once again captured the attention of scientists and the general public, especially after the highly controversial study of phosphine, which suggested the possibility of microbial life floating freely in its atmosphere. In addition, NASA and the European Space Agency (ESA) have decided to send to this world, the second closest to the Sun, at least three exploration missions over the next decade. One of the questions that these missions seek to clarify is whether Venus, today a hell, sheltered primitive oceans, a possibility that some scientists defend. However, a team of Swiss astrophysicists believe that this primitive Venus with its temperate climate, blue skies and large amounts of water forming rivers and seas on its surface, never existed.

At first glance, Venus has many similarities to Earth. Its mass and size are comparable, it is composed of rocky material, it contains some water, and it has an atmosphere. However, a closer look reveals big differences: a thick CO2 atmosphere, extreme surface temperature and pressure, and clouds of sulfuric acid that have nothing to do with the conditions that make Earth habitable.

However, this may not always have been the case. Previous studies have suggested that Venus may have been a much more hospitable place in the past, with its own oceans of liquid water. A team of astrophysicists, led by the University of Geneva (UNIGE) and the National Center for Research Competence NCCR PlanetS, have tried to answer this question with the tools available on Earth.

“We simulated the climate of the Earth and Venus at the beginning of their evolution, more than four billion years ago, when the surface of the planets was still molten”, explains Martin Turbet, head of the study published in
‘Nature’.
“The associated high temperatures meant that any water would have been present in the form of vapor, like in a gigantic pressure cooker,” he notes.

An artist's view of the surface and atmosphere of Venus early, more than 4 billion years ago.  In the foreground, a mysterious explorer is surprised to see the oceans completely vaporized in the sky
An artist’s view of the surface and atmosphere of Venus early, more than 4 billion years ago. In the foreground, a mysterious explorer is surprised to see the oceans completely vaporized in the sky – Manchu

Using sophisticated three-dimensional models of the atmosphere, similar to those scientists use to simulate Earth’s current climate and future evolution, the team studied how the atmospheres of the two planets evolved over time and whether oceans could form in the process.

“Thanks to our simulations, we were able to show that weather conditions did not allow water vapor to condense in the atmosphere of Venus,” says Martin Turbet. This means that temperatures never dropped low enough for the water in its atmosphere to form raindrops that could fall on its surface. Instead, the water remained as a gas in the atmosphere and the oceans never formed. ‘One of the main reasons for this is the clouds that preferentially form on the night side of the planet. These clouds cause a very powerful greenhouse effect that prevented Venus from cooling down as fast as previously thought, ”continues the Geneva-based researcher.

The weak Sun paradox

Surprisingly, astrophysicists’ simulations also reveal that Earth could easily have suffered the same fate as Venus. If the Earth had been a little closer to the Sun, or if the Sun had shone as brightly in its ‘youth’ as ​​it does today, our home planet would look very different today. It is likely that the relatively weak radiation from the young Sun allowed the Earth to cool enough to condense the water that makes up our oceans.

For Emeline Bolmont, professor at UNIGE, member of PlaneS and co-author of the study, “this is a total investment in the way we look at what has long been called the ‘Paradox of the Young and Weak Sun’. It has always been considered a great obstacle to the appearance of life on Earth! ” The argument was that if the Sun’s radiation was much weaker than it is today, it would have turned the Earth into a ball of ice hostile to life. “But it turns out that for the young and very hot Earth, this weak Sun may in fact have been an unexpected opportunity,” continues the researcher.

“Our results are based on theoretical models and are an important pillar in answering the question of the history of Venus,” says study co-author David Ehrenreich, a professor in the Department of Astronomy at UNIGE and a member of NCCR PlanetS. “But we will not be able to rule definitively on the matter on our computers,” he acknowledges. Observations from the three future Venus space missions will be essential to confirm, or disprove, our work. ‘

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