The story of the inner early Solar System goes something like this: Billions of years ago, there were three rocky worlds with oceans of liquid water. Perhaps all three could have been primed for life. But as Mars lost its atmosphere and Venus’ atmosphere experienced a runaway greenhouse effect, only Earth could support life by the end.
But a trio of researchers at the University of Cambridge, U.K., have a different view — that all those billions of years ago, Venus was already too hot to support oceans. There was water vapor (we still see evidence of this today), but it never had the chance to condense into oceans. Instead, Venus was a steam world, one that could reach surface temperatures as high as 1,340 degrees Fahrenheit (727 degrees Celsius). By most metrics, this means the surface of Venus was already a hellish, inhospitable world — and it never got much better.
“Over billions of years, this atmospheric steam would undergo photodissociation, breaking water molecules into hydrogen and oxygen, with the lighter hydrogen gradually escaping into space,” says Tereza Constantinou, a Cambridge PhD student and first author of a paper detailing the work published today in Nature Astronomy. “This process ultimately left Venus with the arid atmosphere we observe today.”
The team’s work offers one scenario for the early Solar System that could drastically revise our understanding of Venus’ climate history and significantly alter the chance that it was ever habitable. If Venus was, in fact, a steam world, it wouldn’t be alone. Recent exoplanet discoveries like GJ 9827 d show evidence of worlds shrouded in hot steam.
Redefining the habitable zone
Astronomers often work off of two concepts for the habitable zone of a star system. There’s the conservative habitable zone, defined as the region around a star where temperatures would be a lot like Earth’s. There’s also the optimistic habitable zone, which allows for the fact that Venus and Mars are both theorized to have once been habitable worlds; this suggests that Venus is on what’s called the inner edge of the habitable zone, and Mars on the outer.
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If Venus is beyond the inner edge, that could revise our understanding of what kinds of planets could be habitable — and drastically narrow the search for life on other worlds.
“However, knowing that Venus was likely never habitable, the Venus-like exoplanets that JWST is characterizing are unlikely candidates for habitable conditions,” Constantinou says. “This narrows the search for habitable and inhabited conditions on exoplanets, directing attention to Earth analogues.”
Missions to Venus
There are a few missions, either current or in the preparatory stages, that could answer a lot of questions around Venus’ climate history and confirm the findings of this team. EnVision, a European mission expected to launch in 2031, will investigate Venus from orbit. It will be equipped with various instruments that will examine how the atmosphere, the surface, and the innards of Venus interact. Among them are mass spectrometers, which will look for gases in the atmosphere that indicate the past (or, in trace amounts, present) existence of water.
There’s also NASA’s DAVINCI mission, which has a similar timeline to EnVision. It’s teamed up with another NASA mission, VERITAS. DAVINCI will be equipped with an atmospheric probe, which will be able to learn more about Venus’ atmosphere as it descends, and “speak to whether Venus was ever wet or habitable,” Constantinou says.
Of course, there’s still a slight chance for life at Venus. You just need to look higher. Conditions in the upper atmosphere are more temperate and Earth-like, and under certain scenarios could have still fostered life, even if Venus’ surface was too far gone.
In 2020, a team of researchers reported what they claimed was evidence of a gas called phosphine in the atmosphere of Venus. On Earth, phosphine is made by life and not much else, so that would be a strong indication of either some microbial life in Venus’ atmosphere or some non-biological process. But this claim remains controversial and disputed. The team’s initial analysis was affected by data reduction errors at the observatory that collected the data, and many researchers think the signal is more likely a different compound that happens to be absorbing light at the same wavelength as phosphine.
“Any potential life in the Venusian atmosphere would have originated and evolved under entirely different conditions, perhaps adapted to survive in sulfuric acid clouds — so very much life as we do not yet know it,” Constantinou says.
