Could Venus Have Once Been Habitable?

An image showing what a habitable Venus may have looked like.

Venus is the closest planet to the Earth, so you would probably expect it to have some similarities to Earth. However, apart from size, Venus is about as far away from Earth, characteristically speaking, as it gets. First off, the average surface temperature on Venus is about 860°F (460°C), which is hot enough to melt lead. This high temperature is mainly a result of the amplified effects of the runaway greenhouse effect on Venus, where the atmosphere is thick and rich with carbon dioxide. Second, the atmospheric pressure is about 92 times that of what we experience on Earth. To put that into perspective, this is the pressure you would face if you were over 3,000 feet underwater! It’s also pretty much impossible to see sunshine on Venus, as clouds of sulfuric acid dominate the sky, leaving the planet hot, dry, and without water. These harsh conditions lead scientists to question if Venus could have ever even supported basic life forms. However, evidence from recent studies and models of planetary evolution suggest that Venus might have had a very different past—one filled with liquid water and, potentially, even life. 

The Formation of Venus

Venus roughly formed around 4.6 billion years ago, the same time as Earth. Venus is only 76.783 million miles away from Earth, and given that it’s so close to us, chances are that it had access to many of the same ingredients during formation, including water. This water was likely delivered in the form of ice by asteroids and comets. Models of planetary formation suggest that there might’ve been enough water delivered to form small oceans on Venus. A solid piece of evidence that supports this theory is the trace amounts of water vapor detected in the Venusian atmosphere. However, the biggest evidence is the high deuterium-to-hydrogen ratio in the atmosphere.

Evidence of Past Water

First, we need to understand what deuterium is, and what its ratio to hydrogen means. Deuterium is a heavier isotope of hydrogen. The most abundant form of hydrogen has only one proton and no neutrons. Deuterium, on the other hand, has an extra neutron, thus making it heavier. The higher amount of Deuterium, relative to hydrogen, suggests that a lot of water had undergone evaporation in the past. This means that Venus may have once had a lot of water, which is a major indication of life, as it provides a stable environment for life-forming chemical reactions.

Computer simulations show that Venus may have been in a habitable state until up to 2.6 billion years ago. Back then, the Sun was a lot younger and only shined 70% as brightly, reducing solar heating. The atmosphere was also thinner, and combined with a less powerful Sun, surface temperatures were likely suitable for liquid water to exist. However, it’s important to note that these computer simulations don’t account for uncertainties in how much water was actually delivered to Venus on asteroids and comets, since there is no direct evidence regarding the initial amount of water. So, while Earth got a lot of water during its formation period, the same cannot be told for Venus.

Venus’s rotation may have also played a role in the potential existence of water. Currently, Venus spins very slowly, with every day on Venus lasting 243 Earth days. But earlier in its history, Venus may have spun faster, leading to more balanced day-and-night cycles and temperature fluctuations that somewhat resemble Earth’s.

Geological Clues and Surface Features

A river-like channel on Venus, as captured during NASA’s Magellan mission.

The surface of Venus was heavily reshaped by volcanic activity and tectonic processes. As part of the Magellan mission, the surface of Venus was mapped. During this mission, it was revealed that there are countless volcanic plains, lava flows, and large shield volcanoes, indicating long-term geological activity. Most importantly, the presence of Tesserae—highly deformed regions on Venus’s surface—was noticed. Tesserae have similar features often linked to water-rich environments, and it may indicate past tectonic activity involving water. There is also rock and mineral evidence collected by Soviet landers in the 1970s and 1980s that shows basalt, a type of volcanic world that usually only forms in water-rich environments (at least on Earth). Lastly, river-like channels were detected in radar images, which means that some sort of liquid flow probably carved them out. However, the liquid doesn’t necessarily have to have been water, as it could have also been something else like lava. 

Could Venus Have Really Been Home to Life?

In the end, the question of whether Venus could have once been home to life or, at the very least, habitable remains open, but evidence from atmospheric studies, geological features, and planetary models suggests that it could have had liquid water and a stable climate billions of years ago. Its early environment, possibly with oceans and milder temperatures, might have been suitable for life, though no direct proof of life exists. The planet’s transformation into a hot, dry world was likely driven by the loss of water and a runaway greenhouse effect. Ongoing research and upcoming missions will help clarify Venus’s history and whether it ever supported life. By studying Venus, scientists hope to not only unravel the story of our planetary neighbor but also gain insights into the conditions that make planets habitable across the universe.