Venus could have had oceans long after life began on Earth


Venus taken by the Magellan spacecraft. Credit: NASA/JPL

Today, Venus has a dry, oxygen-poor atmosphere. However, recent studies have suggested that the early planet may have had liquid water and reflective clouds that could have sustained habitable conditions. Researchers at the University of Chicago, Department of Geophysical Sciences have created a new time-dependent model of Venus’ atmospheric composition to investigate these claims. Their results were published in the Proceedings of the National Academy of Sciences.

Water is everywhere in our solar system, usually in the form of ice or atmospheric gas, but occasionally in liquid form. There is water on all planets, many moons, from the outer ring of the inner asteroid belt to the icy Kuiper Belt and far out to the distant Oort Cloud two light-years away.

Venus is a hot, dry, rocky planet, slightly smaller than our own, with only traces of water vapor in its thick CO2 Atmosphere, and previous studies have attempted to model its atmospheric past. Depending on how the previous models were built, there are drastically different climate images.

Venus may have always been an uninhabitable hot mess that lost its oxygen through absorption during the crystallization of its magma ocean and never formed liquid water on its surface. Without a way to sequester carbon, atmospheric CO continues to rise2 wrapped the planet in a thick, heavy blanket, causing the current surface atmospheric pressure to be 92 times that of Earth, making Venus hotter than Mercury despite being twice as far from the Sun . Even a possible barrage from icy comets would not be enough to keep the water on the surface.

On the other hand, other models suggest that Venus in the early Solar System, when solar irradiance was 30% less, may have had a moderate surface temperature as early as 700 million years ago, with a much thinner atmosphere and liquid water masses on its surface – perhaps oceans. before a runaway greenhouse effect boiled it away.

The researchers at the University of Chicago decided to pursue the question with their own model. They took the unique approach of first assuming that there once was an ocean with a habitable climate, populating the computer model with a variety of different sea levels, and running those oceans through three distinct processes of evaporation and oxygen depletion. They ran the model a total of 94,080 times using three different time-dependent starting points, using a scoring system that allowed them to identify the runs with results that most closely matched Venus’ actual atmosphere today.

According to the study results published in PNAS, out of 94,080 runs, only a few hundred were within range of the actual Venusian atmosphere we see today. The hypothetical habitable epochs on Venus had to end 3 billion years ago with a maximum sea depth of 300 meters over its entire surface (total hydrosphere). The results suggest that Venus has been uninhabitable for more than 70% of its history, four times longer than some previous estimates.

Scientists are fairly confident that liquid water is needed for life to exist on a rocky planet, since we have an example of life on a wet rocky planet and nothing else to compare it to. According to the fossil record, life on Earth is thought to have started around 3.5 to 4 billion years ago and goes back even further, to around 4.5 billion years, using the evolutionary molecular clock estimate. If Venus had had liquid water on its surface 3 billion years ago, it could have hosted life as well.

More information:
Alexandra O. Warren et al., Narrow range of early habitable Venus scenarios allowed by modeling oxygen loss and radiogenic argon outgassing, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2209751120

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