Researchers also determined that a similar story could have played out on Earth as well had things been just a bit different.
Venus, our closest planetary neighbor, is called Earth’s twin because of the similarity in size and density of both planets. Otherwise, the planets differ radically.
While Earth is a natural hub for life, Venus is a lifeless planet with a toxic carbon dioxide atmosphere 90 times thicker than ours, clouds of sulphuric acid and surface temperatures that reach 864 degrees Fahrenheit (462 degrees Celsius) — hot enough to melt lead.
To understand how these two rocky planets turned out so differently, a team of astrophysicists decided to try to simulate the beginning, when our solar system’s planets formed 4.5 billion years ago.
They used climate models — similar to what researchers use when simulating climate change on Earth — to peer back in time at young Venus and Earth.
When Earth and Venus were furnaces
More than 4 billion years ago, Earth and Venus were piping hot and covered in magma.
Oceans can only form when temperatures are cool enough for water to condense and fall as rain over thousands of years. That’s how Earth’s global ocean formed over tens of millions of years. Venus, on the other hand, remained hot.
At the time, our sun was about 25% fainter than it is now. But that wouldn’t have been enough to help Venus cool off, since it’s the second-closest planet to the sun. The researchers questioned whether clouds could have played a role in helping Venus cool down.
Their climate model determined that clouds did contribute, but in an unexpected way. They clustered on the night side of Venus where they wouldn’t have been able to shield the planet’s day side from the sun. While Venus is not tidally locked to the sun, where one side of the planet always faces the star, it has an extremely slow rotation rate.
Rather than shielding Venus from heat, the night side clouds contributed to a greenhouse effect that trapped heat within the planet’s dense atmosphere and kept temperatures high. With such consistent, trapped heat, Venus would have been too hot for rain to fall. Instead, water could only exist as its gaseous form, water vapor, in the atmosphere.
“The associated high temperatures meant that any water would have been present in the form of steam, as in a gigantic pressure cooker,” said Martin Turbet, lead study author, researcher at the University of Geneva’s Department of Astronomy of the Faculty of Science and member of the National Centre of Competence in Research PlanetS, Switzerland, in a statement.
Why Earth might have gone the same way
Things could have turned out the same way for Earth if our planet had been slightly closer to the sun or if the sun was as bright back then as it is now.
Because the sun was dimmer billions of years ago, Earth was able to cool down enough from its molten state for water to form and create our global ocean. The faint young sun “was a key ingredient to actually form the first oceans on Earth,” Turbet wrote in an email.
“This is a complete reversal in the way we look at what has long been called the ‘Faint Young Sun paradox,'” said Emeline Bolmont, study coauthor and professor at the University of Geneva, in a statement. “It has always been considered as a major obstacle to the appearance of life on Earth. But it turns out that for the young, very hot Earth, this weak Sun may have in fact been an unhoped-for opportunity.”
Previously, scientists believed if the sun’s radiation was weaker billions of years ago, Earth would have just turned into a snowball. Instead, the opposite was true.
The findings show the variety of ways rocky planets have evolved in our solar system.
Earth’s ocean has existed for nearly 4 billion years. There is evidence that Mars was covered in rivers and lakes between 3.5 billion and 3.8 billion years ago. And now it seems less likely that Venus could have ever supported liquid water on its surface.
Beyond our solar system
The new research could also be applied to exoplanets (planets outside of our solar system).
“Our results have strong implications for exoplanets, as they suggest that a large fraction of the exoplanets that were thought to be capable of having surface oceans of liquid water are probably now desiccated because they never succeeded in condensing and thus forming their first oceans,” Turbet said.
Future missions to Venus can help test the theory put forth by Turbet and his team.
“Our results are based on theoretical models and are an important building-block in answering this question,” he said. “But observations are needed to rule on the matter definitively! Let’s hope that the future space missions EnVision, VERITAS and DAVINCI+ will bring us a definitive answer.”
These NASA and European Space Agency missions, set for launch at the end of the decade, will help scientists understand the oldest surface features on Venus called tesserae, which “may hold pieces of evidence of past traces of the presence (or absence) of liquid water on the surface of Venus,” Turbet said.