Venus likely started with the same basic ingredients as Earth and may have supported liquid water on its surface for up to 2 billion years. That’s the picture emerging from climate models, atmospheric chemistry, and spacecraft data collected over the past few decades. But the story is more nuanced than a simple yes, and some scientists argue Venus may never have had oceans at all.
Why Scientists Think Venus Started Like Earth
Venus and Earth formed from the same cloud of dust and gas roughly 4.5 billion years ago. They’re nearly the same size, made of similar rock, and orbit in the same neighborhood of the solar system. For a long time, planetary scientists assumed Venus simply took a different path at some point, diverging from its twin into the hellscape we see today, where surface temperatures hit 465°C (870°F) and the atmosphere is 96% carbon dioxide.
The first real hint that Venus might have once had water came from NASA’s Pioneer mission in the 1980s. Those measurements detected an unusually high ratio of deuterium (heavy hydrogen) to regular hydrogen in the Venusian atmosphere. One interpretation: Venus once had far more water, and as lighter hydrogen atoms escaped to space over billions of years, the heavier deuterium was left behind. That chemical fingerprint suggested an enormous amount of water had been lost.
However, more recent analysis complicates this reading. A 2024 study in Nature Astronomy pointed out that the deuterium-to-hydrogen ratio alone doesn’t actually prove liquid water ever existed on the surface. The ratio could instead reflect a steady-state process where comets and volcanic outgassing continuously deliver small amounts of water that are then stripped away. The past surface conditions of Venus, the authors concluded, “are still unknown.”
The Case for a Habitable Venus
Climate modelers at NASA’s Goddard Institute for Space Studies built simulations of an early Venus with an atmosphere similar to Earth’s, a shallow ocean filling the planet’s low-lying basins, and the highlands left exposed as continents. They used real topographic data from NASA’s Magellan radar mission in the 1990s to map where water would have pooled. The result: Venus could have maintained habitable surface temperatures for up to 2 billion years.
A key factor in these models is Venus’s slow rotation. A day on Venus lasts 243 Earth days, and the GISS team found this actually helps habitability. The slow spin creates a pattern of thick cloud cover on the sun-facing side that reflects sunlight back into space, keeping the surface cooler than you’d expect for a planet 30% closer to the sun than Earth. The models also noted that ancient Venus likely had more dry land than Earth, especially in the tropics, which would have limited the amount of water evaporating into the atmosphere and reduced the warming effect of water vapor.
Under these conditions, the climate could have remained stable with liquid water on the surface until at least 715 million years ago. That’s a remarkably long window of potential habitability, stretching from the planet’s formation well into a period when complex life was already developing on Earth.
How Venus Lost Its Water
If Venus did have oceans, several forces conspired to destroy them. The basic sequence goes like this: as the sun gradually brightened over billions of years, more energy hit Venus’s surface. Ocean water evaporated into the atmosphere. Water vapor is a powerful greenhouse gas, so more evaporation meant more warming, which meant more evaporation. At some tipping point, this feedback loop became unstoppable, a process called a runaway greenhouse effect.
Once the water was in the upper atmosphere, ultraviolet radiation from the sun broke water molecules apart into hydrogen and oxygen. The lightweight hydrogen drifted into space. But that’s not the whole story. NASA researchers discovered that Venus has an unexpectedly powerful “electric wind,” an electric field in the upper atmosphere at least five times stronger than Earth’s. This field is strong enough to accelerate even heavy oxygen ions to escape velocity, effectively vacuuming the remaining components of water off the planet. As one NASA researcher described it, oxygen ions in Venus’s upper atmosphere have “won a terrible, terrible lottery” because they get dragged into space by an invisible force.
The solar wind, a stream of charged particles blowing off the sun at a million miles per hour, also eroded the atmosphere over time. Earth is largely protected from this by its global magnetic field, which deflects the solar wind. Venus has no such shield. The combination of intense ultraviolet light (twice as strong as at Earth, due to Venus’s closer orbit), the powerful electric wind, and the unrelenting solar wind stripped away an ocean’s worth of water over geological time. With no water left, carbon dioxide from volcanic activity accumulated in the atmosphere with nothing to absorb it, locking Venus into its current state.
The Counterargument: Venus May Have Always Been Dry
Not all planetary scientists are convinced Venus ever had a temperate period. An alternative hypothesis holds that Venus was born hot and stayed hot. During planetary formation, both Venus and Earth would have been covered in magma oceans. On Earth, conditions cooled enough for water vapor to condense into liquid oceans. On Venus, being closer to the sun, the surface may never have cooled below the point where steam could condense. In this scenario, Venus’s water remained as vapor in the atmosphere from the very beginning, and was gradually stripped away without ever forming oceans.
The geological record on Venus doesn’t settle the debate. The planet’s surface is relatively young, between 200 million and 1.6 billion years old, because volcanic activity periodically resurfaces it. The oldest visible terrain, called tesserae, represents only the last 10 to 20 percent of Venus’s total geological history. Everything that came before has been buried or destroyed. Critically, the tesserae show no clear signature of plate tectonics. Venus’s geological history instead shows cycles of compression and tension with declining intensity, a pattern fundamentally different from Earth’s tectonic plate system. Whether this rules out an earlier Earth-like period or simply reflects the limited window we can observe remains an open question.
What Could Settle the Debate
NASA’s DAVINCI mission is designed to directly test whether Venus once had water. A probe will descend through the atmosphere carrying instruments specifically built for this question. A tunable laser spectrometer will measure the deuterium-to-hydrogen ratio with far greater precision than any previous mission, and a mass spectrometer will cross-check those readings. Together, these instruments should be able to distinguish between the “lost ocean” scenario and the “steady-state supply” scenario that has muddied interpretation of the Pioneer-era data.
Perhaps most exciting, a descent camera will photograph the surface during the probe’s fall, analyzing the composition of rocks in the tessera highlands. The camera can distinguish between rock types that form in the presence of water (lighter-colored, silica-rich rocks similar to Earth’s continental crust) and those that form from dry volcanic processes (darker, denser basaltic rocks). If the tesserae turn out to contain minerals that only form through water-rock interaction, or layered sediments that suggest water deposition, it would be strong evidence that Venus once had a wet surface. If they’re purely volcanic basalt, the always-dry hypothesis gains ground.
The answer matters beyond Venus. Understanding whether a planet so similar to Earth in size and composition could maintain habitable conditions for billions of years, then lose them entirely, has direct implications for how we evaluate the habitability of rocky planets orbiting other stars. Venus may be the closest example we have of a world that crossed the line from livable to lethal.