Venus was never capable of supporting life as we know it

A team of astronomers has determined that Venus has never been capable of supporting life, countering decades of theories suggesting that our closest planetary neighbor may have once resembled Earth more closely than it does today.

Researchers from the University of Cambridge examined the chemical makeup of Venus’s atmosphere and concluded that the planet’s interior is currently too arid to have ever contained sufficient water for surface oceans. 

Consequently, Venus has likely remained an extremely hot and uninhabitable world throughout its existence.

Published in the journal Nature Astronomy, these findings have significant implications for understanding Earth’s uniqueness and in the ongoing search for extraterrestrial life. 

Although many exoplanets exhibit Venus-like characteristics, the study advises astronomers to prioritize exoplanets that are more Earth-like in nature.

The hostile environment of Venus

From afar, Venus and Earth appear similar: Venus is nearly the same size and, like Earth, is a rocky planet. However, upon closer inspection, Venus resembles an “evil twin” with thick sulfuric acid clouds and surface temperatures averaging around 500°C.

Despite these harsh conditions, astronomers have long explored whether Venus once had liquid oceans capable of sustaining life or if some form of “aerial” life might exist within its dense clouds today.

“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.”

A benchmark for the habitable zone

In the quest to find life beyond our galaxy, astronomers target planets within the habitable zone of their host stars, where conditions allow for liquid water on the surface. Venus serves as a critical benchmark for defining the boundaries of this habitable zone.

“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.

Two competing theories on Venus’s past

There are two main hypotheses regarding the evolution of Venus since its formation 4.6 billion years ago. 

The first suggests that Venus once had temperate surface conditions with liquid water, but a runaway greenhouse effect triggered by extensive volcanic activity caused the planet to become increasingly hotter. 

The second hypothesis proposes that Venus was born hot and has never been able to sustain liquid water on its 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.”

Volcanic evidence and dehydration

The researchers calculated the current rates at which water, carbon dioxide, and carbonyl sulfide molecules are destroyed in Venus’s atmosphere. These molecules must be replenished by volcanic emissions to maintain atmospheric stability.

Volcanic activity, by releasing gases into the atmosphere, provides insights into the interiors of rocky planets like Venus. As magma ascends from the mantle to the surface, it emits gases from deeper layers of the planet.

On Earth, volcanic eruptions primarily emit steam due to our planet’s water-rich interior. However, based on the composition of the volcanic gases needed to sustain Venus’s atmosphere, the researchers determined that volcanic emissions on Venus contain at most six percent water. 

These dry eruptions indicate that Venus’s interior, which supplies the magma releasing volcanic gases, is also devoid of significant water.

Refining the search for life-supporting planets

By the end of this decade, NASA’s DAVINCI mission will have the capability to test and verify whether Venus has always been a dry, inhospitable planet through a series of flybys and a surface probe. 

The mission’s outcomes could assist astronomers in refining their search for life-supporting planets 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.”

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