Venus: steam hell instead of second earth? – Young Venus may never have been cool enough for liquid water
Steam instead of water: Even in its early days, our neighboring planet Venus could have been too hot for liquid water or even oceans, as a model simulation has now revealed. Unlike Earth, Venus never cooled down enough to allow the dense water vapor of its primordial atmosphere to condense. Excessive solar radiation and dense clouds on the night side of the planet prevented this, as the researchers report in the specialist journal “Nature”.
According to common assumption, Venus was as life-friendly as the earth in its early days. They could have possessed a mild climate, liquid water, and even oceans. One of the possible indicators for the latter is the deviating rock composition of the Venusian plateaus. Only when the young sun gained strength did the stronger radiation trigger a galloping greenhouse effect, which evaporated the oceans and made Venus hostile to life. So much for the theory.
The cool down phase was crucial
But now a comparative climate model of Venus and Earth raises doubts about this current scenario. In their study, Martin Turbet from the University of Geneva and his first colleagues reconstructed in the climate model what it looked like on Venus right at the beginning of its history. “Before one asks the question whether conditions are necessary for the maintenance of a liquid ocean, the water that is present in the young warm planetary atmosphere must first condense on the surface,” explain the researchers.
Therefore, the modeling starts immediately after the planet formation: “We simulate the climate on Earth and Venus from the beginning of their development, when their surface was still glowing more than four billion years ago,” explains Turbet. “At these high temperatures, all of the water was available as steam – like in a gigantic pressure cooker.” In the simulation, he and his team reconstructed the conditions under which the water vapor condenses and rains down after this magma ocean phase.
Critical Sun Exposure Security Service
The result: In order for the atmosphere of a young planet to cool down enough to allow water to condense, solar radiation must not exceed a certain threshold. With the young Venus 325 watts per square meter die, with the earth 312.5 watts per square meter – this corresponds to about 92 percent of the current situation.
If the radiation is below this threshold, clouds can form on the day side of the planet. “These clouds cause the albedo to die abruptly and quickly, causing the atmosphere to cool, causing water vapor to condense on the surface and ultimately forming oceans,” the researchers explain. This was the case with the young earth: IT was far enough outside, the solar radiation below the limit to lower around – and therefore created liquid water and oceans.
Night clouds as a thermal blanket
Not so with the young Venus: Even in the early days of the solar system with a still weak sun, it never received less than around 500 watts per square meter of irradiation, as Turbet and his colleagues report. As a result, the sun-facing side of the atmosphere heated up and strong winds arose, which blew on the night side of the planet. There the gas envelope cooled down so much that thick clouds formed there.
The problem with this: Dense clouds on the night side act like an insulating thermal blanket. They prevent heat from being radiated from the planet’s surface and the lower atmosphere into space. Unlike clouds on the day side, they do not cool the atmosphere, but rather heat it up. “These clouds caused a strong greenhouse effect on Venus, which prevented the planet from cooling down as much as previously assumed,” says Turbet.
Weak sun as a godsend
Taken together, this could mean that dying young Venus never has enough to allow liquid water and oceans on its surface. In the early days of the solar system, it received that flowing ounce of too much solar radiation. If the orbit of the earth were just a little closer to the sun or if the young sun had had a little more luminosity, our home planet could have flourished the same fate.
“The luminous young sun, die for a long time as rather counterproductive for life-friendly conditions on the earliest earth, so it could even die for the emergence of life act, prerequisite for the emergence of life act, Turbet and his colleagues. Because it shone almost four billion years ago with only around 75 percent of its current luminosity, it made it possible for water to condense on our planet.
Three space probes could provide clarity
Three space probes on site could soon clarify whether there actually never was liquid water and seas on Venus. NASA is planning two Venus missions with DAVINCI + and VERITAS in the next few years, which will investigate the composition of the Venus atmosphere, but also the geology of its crust. In the 2030s, ESA will send the EnVision space probe to Venus, which will also analyze the gas envelope and crust.
“The observations of these three future Venus missions are definitely explained in order to confirm or refute our modeling”, co-author David Ehrenreich of the University of Geneva. One indication of this could be the evidence of oceanic crust in the lowlands of the planet.
Also relevant for exoplanets
Should this scenario and model be confirmed, then it would not only shed a whole new light on the history of Venus. It would also have an impact on the assessment of exoplanets, as Turbet and his colleagues explain. Because the primordial cloud effect pushes the boundaries of the classic habitable zone – the zone in which liquid water is possible on a planet.
Some exoplanets that were previously thought to be dying could have been too hot for the local water condensation. (Nature, 2021; doi: 10.1038 / s41586-021-03873-w)
Source: University of Geneva