According to a model, Venus would never have had an ocean

On paper, we could take the Earth and Venus for binoculars: almost identical size, orbits around the Sun quite similar… However, rarely do we see more dissimilar sisters, Eden and Gehenna. Our planet looks like a paradise with its mild temperatures, its atmosphere well supplied with oxygen, its abundant water and a whole range of living beings, while Venus, closer to the Sun, evokes hell: a heat such as lead There would be liquid, an overwhelming atmospheric pressure, an air stuffed with carbon dioxide and no trace of life or water on its surface. However, planetologists have long imagined that, once, the story started well. That, as on Earth, water had flowed abundantly over the white star nicknamed the “Shepherd’s Star”, before a monstrous greenhouse effect made the Venusian oceans evaporate. A study published Thursday, October 14 in Nature contradicts this scenario by concluding that Venus probably never saw the sea.

The French and Swiss authors of this article underline that previous work on the “paleo-Venus” all started from the idea that the water molecules present from the start in the atmosphere had condensed and formed oceans. Without asking the question of knowing if this hypothetical deluge could have happened one day. The fault of models not mature enough to determine it. In addition, these models were one-dimensional: we were only studying air columns. The study published in Nature took advantage of a three-dimensional model, developed for several decades in France at the Laboratory of Dynamic Meteorology to simulate the Earth’s climate.

“The physical equations are sufficiently universal so that we can, with a few adaptations, use this model for all planets, says Martin Turbet, a young French postdoctoral researcher at the University of Geneva and the first author of the study. The plus, compared to one-dimensional models, is that one can see the dynamic circulation in the atmosphere and the formation of clouds. ” Therein indeed lies the key to the problem.

Flight into space

The atmosphere of young Venus was stuffed with water, a molecule that absorbs solar energy very well. “The Sun very strongly heated the upper atmosphere of the planet on the day side and prevented water from condensing”, explains Martin Turbet. Clouds therefore formed preferentially on the night side, in the stratosphere. These high clouds acted like a cover and, by greenhouse effect, prevented the cooling of the atmosphere, which would have opened the floodgates to torrential rains. Over time, the molecules of H2O’s of the atmosphere eventually broke and their hydrogen atoms leaked into space. Final result, Venus is dry and has never been “habitable” in the sense that astronomers understand it: too close to our star and too heated by it, the planet has not known water in liquid form at its area. Water of which “It is considered to be the most important ingredient for the emergence of living things as they are understood on Earth”, rappelle Martin Turbet.

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