Astrophysicists led by UNIGE and PRN PlanetS investigated the past of Venus to find out if Earth’s sister planet once had oceans.
The planet Venus can be seen as the evil twin of Earth. At first glance, its mass and size are comparable to that of our home planet, it is also composed mainly of rocky material, contains water and has an atmosphere. Still, a closer look reveals some striking differences between them: the thick CO2 atmosphere of Venus, extreme surface temperature and pressure, and acid clouds are indeed a stark contrast to the conditions necessary for life on Earth. This may not always have been the case.
Previous studies have suggested that Venus might have been a much more hospitable place in the past, with its own oceans of liquid water. A team of astrophysicists led by the University of Geneva (UNIGE) and the National Center for Research Competence (NCCR) PlanetS, Switzerland, studied whether our planet’s twin indeed had milder periods. The results, published in the journal Nature, suggest that this is not the case.
Venus has recently become an important research subject for astrophysicists. ESA and Nasa decided this year to send no less than three space exploration missions over the next decade to the second closest planet to the Sun. One of the key questions these missions aim to answer is whether Venus ever hosted the first oceans. Astrophysicists led by Martin Turbet, researcher at the Department of Astronomy of the Faculty of Sciences of UNIGE and member of PRN PlanetS, tried to answer this question with the tools available on Earth.
“We simulated the climate of the Earth and Venus at the very beginning of their evolution, more than four billion years ago, when the surface of the planets was still molten,” explains Martin Turbet. “The associated high temperatures meant that any water would have been present in vapor form, like in a gigantic pressure cooker. “
Using sophisticated three-dimensional models of the atmosphere, similar to those scientists use to simulate Earth’s current climate and future evolution, the team studied how the atmospheres of the two planets would evolve over time. and whether oceans could form in the process.
“Thanks to our simulations, we were able to show that climatic conditions did not allow water vapor to condense in the atmosphere of Venus”, specifies Martin Turbet. This means that the temperatures have never been 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 preferentially form on the night side of the planet. These clouds cause a very powerful greenhouse effect which prevented Venus from cooling down as quickly as previously thought ”, continues the Geneva researcher.
Small differences with serious consequences
Surprisingly, astrophysicists’ simulations also reveal that Earth could easily have suffered the same fate as Venus. If the Earth had been just a little closer to the Sun, or if the Sun had shone as brightly in its “youth” as it does today, our home planet would be very different today. It was probably the relatively weak radiation from the young Sun that allowed the Earth to cool enough to condense the water that forms our oceans. For Emeline Bolmont, professor at UNIGE, member of PlaneS and co-author of the study, “this is a complete reversal of the outlook on what has long been called the ‘faint Young Sun paradox. ‘. It has always been considered a major obstacle to the appearance of life on Earth! The argument was that if the Sun’s radiation was much weaker than it is today, it would have turned the Earth into a life-hostile ice ball. “But it turns out that for the young and very hot Earth, this weak Sun may have in fact been an unexpected opportunity”, continues the researcher.
“Our results are based on theoretical models and constitute an important element in answering the question of the history of Venus”, explains the co-author of the study David Ehrenreich, professor at the Department of Astronomy at UNIGE and member of PRN PlanetS. “But we won’t be able to definitively resolve the issue on our computers. Observations from the three future Venusian space missions will be essential to confirm – or deny – our work. “These prospects delight Emeline Bolmont, for whom” these fascinating questions can be tackled by the new Center for Life in the Universe, which has just been created within the Faculty of Sciences of UNIGE “.
Reference: “Day-night cloud asymetry prevents early oceans on Venus but not on Earth” by Martin Turbet, Emeline Bolmont, Guillaume Chaverot, David Ehrenreich, Jérémy Leconte and Emmanuel Marcq, October 13, 2021, Nature.
DOI : 10.1038/s41586-021-03873-w