Astrophysicists have investigated the past of Venus to find out whether Earth’s sister planet once had oceans

In many ways, our neighbouring planet is very similar to Earth. It’s a rocky planet of about the same size, and it has an atmosphere and some water.

Yet, a closer look reveals striking differences between them: Venus’ thick CO2 atmosphere, extreme surface temperature and pressure, and sulphuric acid clouds are indeed a stark contrast to the conditions needed for life on Earth.

This may, however, have not always been the case. Previous studies have suggested that Venus may have been a much more hospitable place in the past, with its own liquid water oceans.

But new research published in Nature suggests that it didn’t. The team used 3D models of the atmosphere – similar to those currently used to understand the Earth’s climate – to wind back the clock and see how conditions evolved.

Venus has recently become an important research topic for astrophysicists. ESA and NASA have decided this year to send no less than three space exploration missions over the next decade to the second closest planet to the Sun.

Did Venus ever hosted early oceans ?

Astrophysicists led by Martin Turbet, researcher at the Department of Astronomy of the Faculty of Science of the UNIGE and member of the 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 very beginning of their evolution, more than four billion years ago, when the surface of the planets was still molten,” explains Martin Turbet. “The associated high temperatures meant that any water would have been present in the form of steam, as in a gigantic pressure cooker.”

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

“Thanks to our simulations, we were able to show that the climatic conditions did not allow water vapour to condense in the atmosphere of Venus,” says Martin Turbet. This means that the temperatures never got low enough for the water in its atmosphere to form raindrops that could fall on its surface.

Instead, water remained as a gas in the atmosphere and oceans never formed. “One of the main reasons for this is the clouds that form preferentially on the night side of the planet. These clouds cause a very powerful greenhouse effect that prevented Venus from cooling as quickly as previously thought,” continues the Geneva researcher.

Small differences with serious consequences

Surprisingly, the astrophysicists’ simulations also reveal that the Earth could easily have suffered the same fate as Venus.

If the Earth had been just a little closer to the Sun, or if the Sun had shone as brightly in its ‘youth’ as it does nowadays, our home planet would look very different today.

It is likely the relatively weak radiation of the young Sun that allowed the Earth to cool down enough to condense the water that forms our oceans.

For Emeline Bolmont, professor at UNIGE, member of PlaneS and co-author of the study, “this is a complete reversal in the way we look at what has long been called the ‘Faint Young Sun paradox’. It has always been considered as a major obstacle to the appearance of life on Earth!”

The argument was that if the Sun’s radiation was much weaker than today, it would have turned the Earth into a ball of ice hostile to life. “But it turns out that for the young, very hot Earth, this weak Sun may have in fact been an unhoped-for opportunity,” continues the researcher.

Sources:

https://www.nature.com/articles/s41586-021-03873-w

https://www.unige.ch/communication/communiques/en/2021/y-a-t-il-eu-des-oceans-sur-venus/