A team of astronomers has found that Venus has never been habitable, despite decades of speculation that our closest planetary neighbour was once much more like Earth than it is today.
The researchers, from the University of Cambridge, studied the chemical composition of the Venusian atmosphere and inferred that its interior is too dry today for there ever to have been enough water for oceans to exist at its surface. Instead, the planet has likely been a scorching, inhospitable world for its entire history.
The results , reported in the journal Nature Astronomy, have implications for understanding Earth's uniqueness, and for the search for life on planets outside our Solar System. While many exoplanets are Venus-like, the study suggests that astronomers should narrow their focus to exoplanets which are more like Earth.
From a distance, Venus and Earth look like siblings: it is almost identical in size and is a rocky planet like Earth. But up close, Venus is more like an evil twin: it is covered with thick clouds of sulfuric acid, and its surface has a mean temperature close to 500°C.
Despite these extreme conditions, for decades, astronomers have been investigating whether Venus once had liquid oceans capable of supporting life, or whether some mysterious form of 'aerial' life exists in its thick clouds now.
"We won't know for sure whether Venus can or did support life until we send probes at the end of this decade," said first author Tereza Constantinou, a PhD student at Cambridge's Institute of Astronomy. "But given it likely never had oceans, it is hard to imagine Venus ever having supported Earth-like life, which requires liquid water."
When searching for life elsewhere in our galaxy, astronomers focus on planets orbiting their host stars in the habitable zone, where temperatures are such that liquid water can exist on the planet's surface. Venus provides a powerful limit on where this habitable zone lies around a star.
"Even though it's the closest planet to us, Venus is important for exoplanet science, because it gives us a unique opportunity to explore a planet that evolved very differently to ours, right at the edge of the habitable zone," said Constantinou.
There are two primary theories on how conditions on Venus may have evolved since its formation 4.6 billion years ago. The first is that conditions on the surface of Venus were once temperate enough to support liquid water, but a runaway greenhouse effect caused by widespread volcanic activity caused the planet to get hotter and hotter. The second theory is that Venus was born hot, and liquid water has never been able to condense at the surface.
"Both of those theories are based on climate models, but we wanted to take a different approach based on observations of Venus' current atmospheric chemistry," said Constantinou. "To keep the Venusian atmosphere stable, then any chemicals being removed from the atmosphere should also be getting restored to it, since the planet's interior and exterior are in constant chemical communication with one another."
The researchers calculated the present destruction rate of water, carbon dioxide and carbonyl sulphide molecules in Venus' atmosphere, which must be restored by volcanic gases to keep the atmosphere stable.
Volcanism, through its supply of gases to the atmosphere, provides a window into the interior of rocky planets like Venus. As magma rises from the mantle to the surface, it releases gases from the deeper portions of the planet.
On Earth, volcanic eruptions are mostly steam, due to our planet's water-rich interior. But, based on the composition of the volcanic gases necessary to sustain the Venusian atmosphere, the researchers found that volcanic gases on Venus are at most six percent water. These dry eruptions suggest that Venus's interior, the source of the magma that releases volcanic gases, is also dehydrated.
At the end of this decade, NASA's DAVINCI mission will be able to test and confirm whether Venus has always been a dry, inhospitable planet, with a series of flybys and a probe sent to the surface. The results could help astronomers narrow their focus when searching for planets that can support life in orbit around other stars in the galaxy.
"If Venus was habitable in the past, it would mean other planets we have already found might also be habitable," said Constantinou. "Instruments like the James Webb Space Telescope are best at studying the atmospheres of planets close to their host star, like Venus. But if Venus was never habitable, then it makes Venus-like planets elsewhere less likely candidates for habitable conditions or life.
"We would have loved to find that Venus was once a planet much closer to our own, so it's kind of sad in a way to find out that it wasn't, but ultimately it's more useful to focus the search on planets that are mostly likely to be able to support life – at least life as we know it."
The research was supported in part by the Science and Technology Facilities Council (STFC), part of UK Research and Innovation (UKRI).
