Today, the atmosphere of our neighboring planet Venus is as hot as a pizza oven and drier than the driest desert on Earth – but it wasn’t always that way.
Billions of years ago, Venus had as much water as Earth does today. If that water was once liquid, Venus might once have been habitable.
Over time, almost all of that water has been lost. By figuring out how, when, and why Venus lost its water, planetary scientists like me can understand what makes a planet habitable—or what can transform a habitable planet into an uninhabitable world.
Scientists have theories to explain why most of that water disappeared, but more water has disappeared than they predicted.
In a May 2024 study, my colleagues and I revealed a new water removal process that has gone unnoticed for decades but could explain this mystery of water loss.
Energy balance and premature water loss
The solar system has a habitable zone: a narrow ring around the sun in which planets can have liquid water on their surfaces. Earth is in the middle, Mars is out on the too cold side, and Venus is out on the too warm side. Where a planet falls on this habitability spectrum depends on how much energy the planet gets from the sun, and how much energy the planet radiates away.
The theory of how most of Venus’s water loss occurred is tied to this energy balance. On early Venus, sunlight broke the water in the atmosphere into hydrogen and oxygen. Atmospheric hydrogen warms a planet – like having too many blankets on the bed in the summer.
When the planet gets too hot, it throws off the blanket: the hydrogen escapes into space in a stream, a process called hydrodynamic escape. This process removed one of the key ingredients for water from Venus. It is not known exactly when this process took place, but it was probably within the first billion years or so.
The hydrodynamic escape stopped after most of the hydrogen was removed, but a small amount of hydrogen remained. It’s like emptying a water bottle: a few drops will remain at the bottom. These leftover droplets cannot escape in the same way. There must be another process at work on Venus that continues to remove hydrogen.
Small reactions can make a big difference
Our new study shows that an overlooked chemical reaction in Venus’ atmosphere could produce enough escaping hydrogen to close the gap between expected and observed water loss.
This is how it works. In the atmosphere, gaseous HCO⁺ molecules, which each consist of one atom of hydrogen, carbon and oxygen and have a positive charge, combine with negatively charged electrons, because opposites attract.
But when the HCO⁺ and the electrons react, the HCO⁺ breaks down into a neutral carbon monoxide molecule, CO, and a hydrogen atom, H. This process gives energy to the hydrogen atom, which can then exceed the planet’s escape velocity and travel to space escape. . The whole reaction is called HCO⁺-dissociative recombination, but we like to call it DR for short.
Water is the original source of hydrogen on Venus, so DR effectively dries out the planet. DR has likely occurred throughout Venus’ history, and our work shows that it likely continues to this day. It doubles the amount of hydrogen escape previously calculated by planetary scientists, upending our understanding of current hydrogen escape on Venus.
Understanding Venus with data, models and Mars
To study DR on Venus, we used both computer modeling and data analysis.
The modeling actually started as a Mars project. My PhD research involved investigating what kinds of conditions made planets habitable for life. Mars also had water, albeit less than Venus, and also lost most of it to space.
To understand hydrogen escape on Mars, I developed a computational model of the Martian atmosphere that simulates Martian atmospheric chemistry. Despite being very different planets, Mars and Venus actually have similar upper atmospheres, so my colleagues and I were able to extend the model to Venus.
We found that HCO⁺ dissociative recombination produces a lot of escaping hydrogen in the atmospheres of both planets, which matched measurements made by the Mars Atmosphere and Volatile EvolutioN, or MAVEN, mission, a satellite orbiting Mars.
It would be valuable to have collected data in Venus’ atmosphere to support the model, but previous missions to Venus have not measured HCO⁺ – not because it isn’t there, but because they were not designed to detect it. However, they did measure the reactants that produce HCO⁺ in the atmosphere of Venus.
By analyzing measurements from Pioneer Venus, a combined orbiter and probe emission that studied Venus from 1978-1992, and using our knowledge of chemistry, we showed that HCO⁺ should be present in the atmosphere in similar amounts to our model .
Follow the water
Our work has filled in a piece of the puzzle about how water is lost from planets, which affects how habitable a planet is for life. We learned that water loss does not happen all at once, but over time through a combination of methods.
The faster hydrogen loss via DR means that less time is needed overall to remove the remaining water from Venus. This means that if oceans were ever present on early Venus, they could have been present longer than scientists thought before water loss through hydrodynamic escape and DR began. This would allow more time for possible life. However, our results do not mean that oceans or life were definitively present; Answering that question will require much more science in the coming years.
There is also a need for new Venus missions and observations. Future Venus missions will provide some atmospheric measurements, but they will not focus on the upper atmosphere where most of the dissociative recombination of HCO⁺ occurs. A future mission into Venus’ upper atmosphere, similar to the MAVEN mission on Mars, could greatly increase everyone’s knowledge of how the atmospheres of terrestrial planets form and evolve over time.
With technological advances in recent decades and a booming new interest in Venus, now is an excellent time to turn our eyes to Earth’s sister planet.
This article is republished from The Conversation, an independent nonprofit organization providing facts and trusted analysis to help you understand our complex world. It was written by: Eryn Cangi, University of Colorado Boulder
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This material is based on work supported by the National Science Foundation Graduate Research Fellowship Program under Grant DGE 1650115. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.