By David Warner 
When we look at our solar system, Earth stands out as a beautiful blue marble, covered in water. Its neighbor, Venus, is a very different place. Often called Earth’s “twin” because it is a similar size, Venus today is a scorching, toxic world. It is the hottest planet in our solar system, with a thick, crushing atmosphere. But scientists have a big question: was Venus always this way? For decades, researchers have been gathering clues to figure out if Venus, long ago, could have been a gentle, watery world just like Earth.
The idea of a “habitable” Venus with oceans is an exciting one. It would mean that our solar system might have once hosted two planets with liquid water on their surfaces at the same time. Understanding what happened to Venus is more than just a history lesson about another planet. It helps us understand how planets become habitable and, more importantly, how they can lose that habitability. The story of Venus could be a warning about how a planet’s climate can change dramatically over time.
Solving this mystery is like piecing together a puzzle with very few pieces. We cannot simply look at the surface and see old, dry ocean beds. The planet’s extreme conditions have hidden much of its past. Instead, scientists must use computer models, chemical clues in the atmosphere, and data from space probes to look back in time. What happened to turn Earth’s twin from a potential paradise into the inferno we see today?
To understand why the idea of oceans is so surprising, we first need to look at the Venus we know today. In simple terms, Venus is a hellscape. The surface temperature is a staggering 465 degrees Celsius (about 869 degrees Fahrenheit). This is hot enough to melt lead. This extreme heat is not because Venus is just a little closer to the Sun; it is because of its atmosphere. The air on Venus is incredibly thick and heavy, pressing down on the surface with a force over 90 times greater than Earth’s. Standing on Venus would feel like being more than half a mile deep in the ocean.
This thick atmosphere is made almost entirely of carbon dioxide, which is a powerful greenhouse gas. On Earth, carbon dioxide traps some of the Sun’s heat, which keeps our planet warm enough for life. On Venus, this process went into overdrive. The massive blanket of carbon dioxide traps so much heat that the surface can never cool down, not even at night. This is known as a “runaway greenhouse effect,” and it is the key to understanding Venus’s current state.
To make matters worse, the planet is covered in permanent clouds. But these are not clouds of water like we have on Earth. The clouds on Venus are made of droplets of corrosive sulfuric acid. These clouds are so thick that they completely hide the surface from view, and they rain sulfuric acid that evaporates long before it ever reaches the hot ground. Between the crushing pressure, the toxic air, and the lead-melting heat, Venus today is one of the most inhospitable places imaginable in our solar system.
If Venus is such a terrible place now, why would anyone think it once had oceans? The biggest clue is not on the surface, but high up in its atmosphere. This clue is a special kind of hydrogen. On Earth, a normal hydrogen atom is just a single proton. But a tiny fraction of hydrogen is “heavy hydrogen,” also called deuterium. A deuterium atom has one proton and one neutron, making it twice as heavy as regular hydrogen. Water (H2O) can be made with either kind.
When scientists measured the atmosphere of Venus, they found something shocking. The ratio of deuterium to regular hydrogen is extremely high. Venus has about 100 to 150 times more deuterium, relative to its hydrogen, than Earth does. This finding is what scientists call a “smoking gun.” It is powerful evidence that Venus must have lost a massive amount of water.
The theory goes like this: In the early solar system, Venus and Earth formed with similar materials, which means both planets likely started with a lot of water. If Venus had oceans, this water would have been H2O. Over time, as the planet heated up, this water evaporated into the atmosphere. Once in the upper atmosphere, intense ultraviolet (UV) light from the Sun would have zapped the water molecules, breaking them apart into hydrogen and oxygen. The regular, lightweight hydrogen atoms were so light that they easily escaped Venus’s gravity and drifted off into space. But the heavy deuterium atoms were not as lucky. They were too heavy to escape as easily and were left behind. Over billions of years, as more and more water was broken apart and the light hydrogen escaped, the concentration of the heavy deuterium left behind grew larger and larger. The high D/H ratio, as it is called, is like the “ghost” of a lost ocean, pointing to a past where Venus had a vast amount of water that is now gone.
This is a great question that applies to Earth as well. When the inner planets formed, the area near the Sun was very hot. It was likely too hot for water ice to exist as a solid. So, the water that formed the oceans on both Earth and potentially Venus must have been delivered after the planets cooled down a bit. Scientists have two main theories for how this happened, and it is likely that both played a role.
The first and most popular theory is delivery by comets and asteroids. The outer solar system, beyond Mars and Jupiter, is very cold and filled with icy bodies. Comets are essentially giant, dirty snowballs, and many asteroids also contain a lot of water ice. In the first few hundred million years of the solar system, this was a chaotic time. These icy bodies were constantly crashing into the inner planets. Every comet or water-rich asteroid that struck the young Venus would have delivered its water, contributing to a growing supply on the surface. This process, known as the “Late Heavy Bombardment,” is thought to be how Earth got the water for its oceans. It is very likely the same thing happened to Venus.
The second theory is that the water came from inside the planet itself. When planets are young, they are very hot and volcanically active. The rock deep within the planet’s mantle contains minerals that have water (H2O) and its components (hydrogen and oxygen) locked inside them. As this hot rock, or magma, rises to the surface during a volcanic eruption, it releases massive amounts of gas. This process is called “outgassing.” This gas is mostly carbon dioxide, but it also includes a lot of water vapor (steam). On Earth, this volcanic steam cooled, condensed, and fell as rain for millions of years, filling the first oceans. If early Venus was similar, its own volcanoes could have “outgassed” enough steam to create vast, shallow oceans. Most scientists believe it was a combination of both comets bringing water from the outside and volcanoes releasing it from the inside.
Using powerful computer models, scientists have tried to simulate what an early Venus with oceans might have been like. The results are fascinating. These models, many of which were run by NASA scientists, suggest that for a very long time, Venus could have been a habitable world. Some models show that Venus may have had stable, liquid-water oceans for as long as two or three billion years. This is longer than life has existed on Earth’s land.
These simulations show a planet with a much thinner atmosphere, perhaps more like Earth’s. It would have had a global ocean, possibly shallower than Earth’s, but still vast. The models also suggest that the planet’s slow rotation—Venus spins backward very slowly—might have actually helped keep it cool. This slow spin would have created thick clouds on the side of the planet facing the Sun, and these bright white clouds would have reflected a lot of sunlight back into space, preventing the planet from heating up too quickly.
In this scenario, ancient Venus could have had temperatures similar to a warm day on Earth. With liquid water, a stable climate, and the same basic chemical building blocks that led to life on our planet, it is possible that Venus was the first habitable world in our solar system, long before life on Earth became complex. This “habitable Venus” idea suggests a long, gentle period where life could have potentially emerged, before a major catastrophe turned the planet into the inferno it is today. This model of a “blue” Venus is the driving force behind the new missions we are sending to uncover its past.
If Venus did have beautiful oceans for billions of years, what went so terribly wrong? The answer is a climate catastrophe called the runaway greenhouse effect. This is the same process that heats Venus today, but it would have been the event that destroyed its oceans. This likely started because the Sun, like all stars, has been slowly getting brighter and hotter over its lifetime. Billions of years ago, the Sun was fainter, which helped Venus stay cool. But as the Sun’s intensity increased, it began to heat Venus more and more.
This extra sunlight started to evaporate the oceans. The water turned into water vapor, which is itself a very powerful greenhouse gas. This is the start of a terrible feedback loop. More sunlight led to more evaporation. More evaporation meant more water vapor in the atmosphere. More water vapor trapped more heat, which made the planet even hotter. This hotter planet then caused the oceans to evaporate even faster. This cycle would have repeated, gaining speed, until a “tipping point” was reached.
Once this runaway process began, it could not be stopped. The oceans would have literally boiled away into the atmosphere, turning the entire planet into a thick, steamy sauna. The atmosphere would have become incredibly dense with water vapor and carbon dioxide (which was likely released from rocks by the heat). This is the event that transformed Venus from a potentially habitable twin of Earth into the scorching hot, high-pressure world we see. As this steam-filled atmosphere built up, the UV light from the Sun would have had plenty of water molecules to break apart, allowing the hydrogen to escape and erasing the water from Venus forever.
This is a very important question, as it helps us understand our own planet. Both Earth and Venus have greenhouse effects; in fact, Earth’s is essential for life. Without its natural greenhouse effect (from gases like water vapor and carbon dioxide), Earth’s average temperature would be below freezing, and the oceans would be solid ice. This natural, balanced effect is good. It acts like a blanket, trapping just enough heat to keep the planet comfortable.
The difference is a matter of balance and scale. On Earth, we have a stable water cycle. If the planet gets a bit warmer, more water evaporates, which can form clouds. These clouds can reflect sunlight, helping to cool the planet back down. More importantly, Earth has a process that removes carbon dioxide from the atmosphere. This is called the carbonate-silicate cycle. Carbon dioxide in the air dissolves in rainwater, forming a weak acid. This acid rain falls on rocks, and the resulting chemical reaction eventually washes minerals into the ocean. In the ocean, tiny creatures use these minerals to build shells, and they eventually become rock (like limestone). This process locks away carbon dioxide for millions of years.
Venus has no such cycle. It lost its water, so it has no rain and no oceans to dissolve carbon dioxide. It also does not appear to have plate tectonics, the process that helps recycle rocks on Earth. As a result, every bit of carbon dioxide that was ever released from its volcanoes—and Venus has had a lot of volcanic activity—just stayed in the atmosphere. Over billions of years, this CO2 built up, creating the massive, thick atmosphere that traps heat so efficiently. Venus shows what happens when the greenhouse effect is out of control and there is no natural system to regulate it. It is an extreme example, but it serves as a powerful reminder of how important these delicate climate balances are.
For a long time, the story of a “warm, wet Venus” that was destroyed by a runaway greenhouse effect was the leading theory. It is a compelling story and is supported by the deuterium evidence. However, recent science has offered a different, and perhaps sadder, possibility. Some new computer models, published in the last few years, suggest that Venus may never have had oceans at all. This is the “born hot” theory.
This theory argues that Venus, being closer to the Sun, was in a very different situation from the very beginning. Even with a fainter young Sun, it may have received too much heat to ever allow water to become liquid on its surface. In this scenario, as Venus was forming and its volcanoes were “outgassing” steam and carbon dioxide, the surface was already too hot. The water that came from volcanoes and comets never had a chance to rain down and form oceans. Instead, it just immediately became part of a thick, steamy atmosphere.
This massive steam-filled atmosphere would have started a greenhouse effect from day one. The planet would have been hot and steamy for millions of years, allowing the Sun’s UV light to begin breaking that water vapor apart almost immediately. In this model, Venus was never a habitable blue world. It was always a hot, toxic planet, and its water was lost to space while it was still just steam. This theory is supported by some analyses of Venus’s geology, which suggest the types of rock on its surface did not form in the presence of large amounts of liquid water. This debate between “born wet” and “born hot” is now one of the biggest questions in planetary science.
Since the surface of Venus is so hard to study, scientists are desperate for any clues hidden in its geology. The planet’s surface is relatively young in geologic terms. It appears to have been completely repaved by massive volcanic eruptions around 300 to 700 million years ago. This means any evidence of ancient oceans from billions of years ago—like beaches, riverbeds, or sedimentary rocks—was likely buried under miles of lava. This makes the job incredibly difficult.
However, there are still things to look for. One of the biggest clues would be the type of rock found in certain areas. On Earth, we have two main types of crust: oceanic crust (dense, dark basalt) and continental crust (lighter, less dense granite). Granite is a special rock that forms through complex processes that require plate tectonics and, crucially, large amounts of water. Scientists are very interested in the “highlands” of Venus, which are raised areas that look a bit like continents. If we could find out what these highlands are made of, it would tell us a lot. If they are made of granite-like rock, it would be very strong evidence that Venus once had both water and plate tectonics, just like Earth.
This is a primary goal for future missions. We need to get back to Venus with better technology. We need to map the surface in much higher detail to look for any landforms that might have survived the volcanic paving, perhaps an ancient shoreline at the edge of a highland “continent.” But most importantly, we need to find out the chemical makeup of the surface rocks. Finding granite on Venus would be one of the biggest discoveries in the history of astronomy, as it would almost certainly confirm the “wet, habitable” past.
After decades of focusing on Mars and the outer solar system, NASA and other space agencies are finally turning their attention back to Venus. Two upcoming NASA missions, set to launch around the end of the 2020s or in the early 2030s, are designed specifically to answer the question of whether Venus had oceans. These missions are called VERITAS and DAVINCI.
VERITAS, which stands for “Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy,” will be an orbiter. It will circle the planet and use advanced radar to create a stunning, high-resolution 3D map of the entire surface. This map will be far more detailed than any we have had before. It will search for the tell-tale signs of past water, like old coastlines, and will look for evidence of plate tectonics. Critically, VERITAS will also carry an instrument that can “see” the chemical makeup of the surface rocks from orbit. It will be able to tell us if those highland “continents” are made of water-formed granite or just volcanic basalt. This mission will essentially be a global geologist, mapping the past of Venus.
The other mission is DAVINCI, which stands for “Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging.” This mission is even more daring. It will drop a special probe, about the size of a beach ball, directly into Venus’s thick atmosphere. This probe will fall for about an hour, taking measurements all the way down. It will “sniff” the air, precisely measuring all the gases, including the different types of hydrogen. This will give us a definitive answer on the deuterium-to-hydrogen ratio, confirming the old evidence. It will also look for other gases that could tell us about the planet’s history. As it gets close to the surface, it will take the first high-resolution pictures of a unique, crumpled landform called a “tessera.” Scientists believe these tesserae might be the oldest parts of Venus’s crust, our last, best chance to see a piece of the original planet that may have formed in the presence of water.
The quest to understand Venus is not just about curiosity. Venus is the closest thing we have to a laboratory for studying how a planet-wide climate can go terribly wrong. Venus and Earth are “sister planets.” They started out roughly the same size, in the same part of the solar system, and were made of the same stuff. Yet, one became a temperate paradise filled with life, and the other became a toxic, lead-melting inferno. We need to know why.
By studying the runaway greenhouse effect on Venus, we learn more about the greenhouse effect on Earth. While Earth is not in any danger of becoming as hot as Venus, the same physical laws apply. Venus shows us what happens when greenhouse gases in an atmosphere reach a “tipping point.” It provides a stark warning about the long-term consequences of loading an atmosphere with carbon dioxide. The more we learn about how Venus lost its habitability, the better we can understand and protect our own.
Furthermore, in the last few decades, we have discovered thousands of planets orbiting other stars, called exoplanets. Many of these planets are in the “Venus Zone” of their stars—a region where they get a similar amount of light to Venus. Right now, we do not know if these planets are hellish worlds or if they could be habitable. Understanding Venus’s history—whether it was “born hot” or “born wet”—will directly help us figure out which of these thousands of other worlds might also be home to liquid water, and maybe even life.
The question of whether Venus ever had oceans is one of the most exciting mysteries in our solar system. For now, the answer is a very strong “maybe.” We have compelling clues, like the “ghost” of lost water in the high D/H ratio, that point to a watery past. We have computer models that show a habitable Venus could have existed for billions of years. But we also have new theories and models suggesting Venus may have been born hot and was never a blue planet at all.
To find the truth, we must go back. The upcoming VERITAS and DAVINCI missions are our best chance to finally solve this puzzle. They will read the history written in Venus’s atmosphere and on its ancient, rocky surface. By learning what really happened to Earth’s twin, we will not only discover the secrets of a lost world but also learn a profound lesson about our own. It makes you wonder: if Venus was habitable, could life have started there, and could any sign of it be left?
The average surface temperature on Venus is about 465 degrees Celsius (869 degrees Fahrenheit). It is the hottest planet in our solar system, even hotter than Mercury, because its thick carbon dioxide atmosphere traps heat in a runaway greenhouse effect.
Venus is often called Earth’s twin because the two planets are very similar in size, mass, and density. They are also made of similar rocky materials and are neighbors in the solar system. The name refers only to its physical stats, not its environment, which is very different.
The main evidence is the deuterium-to-hydrogen (D/H) ratio in its atmosphere. Venus has about 100 to 150 times more “heavy hydrogen” (deuterium) than Earth. This suggests Venus once had a large amount of water, and as the water molecules were broken by sunlight, the lighter hydrogen escaped to space while the heavier deuterium was left behind.
A runaway greenhouse effect is an unstoppable feedback loop. It likely happened on Venus when the Sun became hotter, causing its oceans to evaporate. This water vapor (a greenhouse gas) trapped more heat, which caused more evaporation, which trapped even more heat, until the entire ocean boiled away into the atmosphere.
If Venus had liquid water oceans for two or three billion years, as some models suggest, it is possible that simple, microbial life could have emerged. It is a key question scientists hope to answer, though finding any evidence would be extremely difficult.
On Earth, carbon dioxide is locked away in rocks and dissolved in the oceans. Venus lost its oceans and does not appear to have plate tectonics, so it has no way to remove CO2 from its atmosphere. Over billions of years of volcanic eruptions, the carbon dioxide just built up to the extreme levels seen today.
They are two upcoming NASA missions to Venus. VERITAS will be an orbiter that will map the entire surface in high-resolution 3D and figure out what the rocks are made of, looking for signs of past water. DAVINCI will be a probe that drops through the atmosphere to “sniff” the gases and take pictures of the surface.
Some recent (2024) computer models suggest Venus was “born hot.” This theory proposes that Venus was too close to the Sun for water to ever be liquid. The water from volcanoes and comets just stayed as steam in the atmosphere, which was then broken apart by sunlight and lost to space.
Once the water evaporated into steam (H2O), it rose to the upper atmosphere. There, ultraviolet (UV) light from the Sun split the water molecules into hydrogen and oxygen. The hydrogen was so light that it escaped Venus’s gravity and drifted away into space, while the oxygen likely reacted with rocks or was also lost.
The surface of Venus is completely hidden by a thick, permanent blanket of clouds. These clouds are not made of water but of corrosive sulfuric acid. To see the surface, we must use radar, which can pass through the clouds, as the VERITAS mission will do.