New research led by the Observatoire astronomique de l’Université de Genève suggests that water never condensed and that, consequently, oceans never formed on the surface of Venus.
Earth has had oceans for nearly 4 billion years and Mars had lakes and rivers 3.5-3.8 billion years ago.
However, it is still unknown whether water has ever condensed on the surface of Venus, because the planet — now completely dry — as undergone global resurfacing events that obscure most of its history.
“We simulated the climate of the Earth and Venus at the very beginning of their evolution, more than 4 billion years ago, when the surface of the planets was still molten,” said lead author Dr. Martin Turbet, a researcher at the Observatoire astronomique de l’Université de Genève.
“The associated high temperatures meant that any water would have been present in the form of steam, as in a gigantic pressure cooker.”
In the research, Dr. Turbet and colleagues performed 3D global climate model simulations designed to simulate the set of conditions required for water condensation and, consequently, ocean formation to take place on terrestrial planets, and in particular on early Venus and Earth.
For this, they adapted their climate model to cope with the extreme environments of hot, water-dominated primitive atmospheres, as well as their extremely long convergence time.
“Thanks to our simulations, we were able to show that the climatic conditions did not allow water vapor to condense in the atmosphere of Venus,” Dr. Turbet said.
“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.”
Surprisingly, the simulations revealed that Earth could easily have suffered the same fate as Venus.
If our planet had been just a little closer to the Sun, or if the Sun had shone as brightly in its ‘youth’ as it does nowadays, Earth would look very different today.
It is likely the relatively weak radiation of the young Sun that allowed Earth to cool down enough to condense the water that forms oceans.
“This is a complete reversal in the way we look at what has long been called the ‘Faint Young Sun paradox’,” said co-author Professor Emeline Bolmont, also from the Observatoire astronomique de l’Université de Genève.
“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.”
“Our results are based on theoretical models and are an important building block in answering the question of the history of Venus,” said co-author Professor David Ehrenreich, also from the Observatoire astronomique de l’Université de Genève.
“But we will not be able to rule on the matter definitively on our computers.”
“The observations of the three future Venusian space missions will be essential to confirm — or refute — our work.”
The results were published in the journal Nature.
M. Turbet et al. 2021. Day-night cloud asymmetry prevents early oceans on Venus but not on Earth. Nature 598, 276-280; doi: 10.1038/s41586-021-03873-w