With Venus, discoveries are often combined in the conditional.
Contrary to a scenario envisaged a few years ago, the planet that we like to call Earth's "twin" has never been able to accommodate oceans, according to astrophysicist and climatologist Martin Turbet, of the Astronomical observatory of the University of Geneva, associated with a team of scientists from its university and from the astrophysical laboratories of Bordeaux and Latmos, specializing in the atmosphere.
"We have probably underestimated the difficulty necessary to make oceans appear on planets such as Earth or Venus or even exoplanets" according to him.
Here are some benchmarks to understand what we are talking about, and what such a statement implies.
What do we know about Venus?
Not much, and that's the problem.
Thanks to probes and explorations, we now know that Mars sheltered large bodies of water.
But on Venus, the colossal pressure and the infernal temperature of more than 470 degrees Celsius which reign there quickly got the better of the rare probes which managed to land there.
Hidden by thick clouds of sulfuric acid droplets, the surface of Venus remains largely a mystery.
Yet a landmark 2016 study questioned whether Venus could have been habitable, assuming that its cloud cover would have long protected hypothetical bodies of water.
A scenario entirely called into question by this new study published in Nature.
Why does the oceans hypothesis no longer hold true?
“Before asking how an ocean can be stable on the surface of Venus, we must ask how it could have been formed,” says Martin Turbet. We must therefore study how, a few billion years ago, we went from a "very young, very hot" planet, where all the available water was "in the atmosphere in the form of vapor", to a planet, where by condensing by cooling, steam could have formed oceans. His team used a sophisticated climate model, taking into account cloud formation and atmospheric circulation. Its conclusions are final.
The sun heated the water vapor in Venus' atmosphere to a temperature too high to allow clouds to form by condensation.
Clouds which, by protecting the sunny side of the planet, would have allowed its atmosphere to cool enough to cause the condensation of its water vapor and thus form oceans.
Aggravating circumstance, the air masses heated by the sun, on the “day” side, have moved to the night side of the planet.
There they formed clouds at high altitude, causing a greenhouse effect that prevented the cooling of the atmosphere of Venus.
How did the Earth escape this sad fate?
Because, "when the sun was younger, four billion years ago, it was 25 to 30% less luminous compared to today," explains Martin Turbet.
The heat with which it flooded our planet was then sufficiently low to allow the condensation of water vapor and the formation of oceans.
Venus, much closer to the sun, suffered at the time an insolation of about double, too high to allow such a phenomenon.
A sun just a little warmer a few billion years ago would have prevented the appearance of oceans, and arguably life.
What does that change in our knowledge of Venus?
The climate model thus developed will be used for the study of exoplanets, belonging to other solar systems.
Meanwhile, for Venus, it complicates the scenario of an appearance of life.
A study noticed last year reported the detection, in the clouds, of a gas possibly linked to living organisms.
She imagined possible that an evaporation of the oceans on the planet would have caused a form of life to migrate into these clouds.
Several studies refuted these observations, before another, last June, judged impossible any form of life in these clouds, for lack of enough water to support it.