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The planet Venus is not the best world in search of extraterrestrial life. Its toxic atmosphere is 90 times thicker than Earth's and its surface temperatures are warm enough to melt lead.
A study presented today at the 2019 EPSC-DPS joint meeting by Michael Way of the Goddard Institute of Space Science gives a new vision of Venus' climatic history and could have consequences for the future. 39, habitability of exoplanets in similar orbits.
Forty years ago, NASA's Pioneer Venus mission uncovered tantalizing clues that the Earth's "twisted-sister" planet may have already had very shallow water. To see if Venus could have had a stable climate able to withstand liquid water, Dr. Way and his colleague Anthony Del Genio created a series of five simulations assuming different levels of water coverage.
In the five scenarios, they found that Venus was able to maintain stable temperatures between a maximum of about 50 degrees Celsius and a minimum of about 20 degrees Celsius for about three billion years. A temperate climate could even have been maintained on Venus today if there had not been a series of events that caused a release, or "degassing", of carbon dioxide stored in the rocks of the planet. about 700 to 750 million years ago.
"Our hypothesis is that Venus could have a stable climate for billions of years. It is possible that the near-global resurfacing event is responsible for its transformation from a climate similar to Earth to the greenhouse we see today, "said Way.
Three of the five scenarios studied by Way and Del Genio assumed the topography of Venus as we see it today and considered a deep ocean an average of 310 meters, a shallow layer of water an average of 10 meters and a small amount of water trapped in the soil. For comparison, they also included a scenario with the topography of the Earth and an ocean of 310 meters and, finally, a world entirely covered by an ocean of 158 meters deep.
To simulate the environmental conditions of 4.2 billion years ago, 715 million years ago and today, researchers have adapted a general circulation model in 3D in order to take into account the increase of solar radiation due to the warming of our Sun during its life, as well as the atmospheric changes. compositions.
Although many researchers believe that Venus is beyond the inner limit of the habitable zone of our solar system and that it is too close to the Sun to support liquid water, the new study suggests that this may not be possible. not be the case.
"Venus currently has almost twice the solar radiation we have on Earth. However, in all the scenarios we modeled, we found that Venus could still withstand acceptable surface temperatures for liquid water, "said Way.
4.2 billion years ago, shortly after its formation, Venus would have completed a period of rapid cooling and its atmosphere would have been dominated by carbon dioxide. If the planet had evolved in the same way as the Earth over the next three billion years, the carbon dioxide would have been sucked up by silicate rocks and stuck to the surface. In the second model period 715 million years ago, the atmosphere would probably have been dominated by nitrogen containing trace amounts of carbon dioxide and methane – similar to those of the Earth – and these conditions could have been stay stable until today.
The cause of the degassing that led to the dramatic transformation of Venus is a mystery, although it is probably related to the volcanic activity of the planet. One possibility is that large amounts of magma bubble up, releasing carbon dioxide from molten rocks into the atmosphere. The magma solidified before reaching the surface, which created a barrier preventing reabsorption of gas. The presence of large amounts of carbon dioxide triggered an uncontrollable greenhouse effect, which caused the torrid temperature of 462 degrees that we find today on Venus.
"Something happened to Venus where a huge amount of gas was released into the atmosphere and could not be reabsorbed by the rocks. On Earth, we have some examples of large scale degassing, for example the creation of Siberian traps 500 million years ago that is related to mass extinction, but nothing on this scale. It completely transformed Venus, "said Way.
There are still two major unknowns to resolve before we can fully answer the question of whether Venus could have been habitable or not. The first relates to the initial cooling rate of Venus and its ability to condense liquid water to the surface. The second unknown is whether the global resurfacing event was an isolated event or simply the last in a series of events dating back billions of years in the history of Venus.
"We need more missions to study Venus and get a more detailed understanding of its history and evolution," said Way. "However, our models show that it is quite possible that Venus was habitable and radically different from the Venus we see today. This opens up all sorts of consequences for the exoplanets present in what is called the "zone of Venus", which can in fact host liquid water and temperate climates.
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