by 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 had ever been liquid, Venus might once have been habitable.
Over time, that water is almost lost. Figuring out how, when and why Venus lost its water helps planetary scientists like me understand what makes a planet habitable – or what can turn a habitable planet into an uninhabitable world.
Scientists have theories that explain why most of that water has disappeared, but more water has disappeared than they predicted.
In a study in May 2024, my colleagues and I uncovered a new process of water removal that has not been noticed for many years, but which may explain this water loss mystery.
Energy balance and early water loss
The solar system has a habitable zone – a narrow ring around the Sun where planets can have liquid water on their surface. Earth is in the middle, Mars is out on the too cold side, and Venus is out on the too hot side. Where a planet sits on this habitable spectrum depends on how much energy the planet receives from the Sun, as well as how much energy the planet radiates.
The theory of how most of Venus’ water loss occurred is tied to this energy balance. Early on Venus, sunlight broke water in the atmosphere into hydrogen and oxygen. Atmospheric hydrogen warms a planet – like too many blankets on the bed in summer.
When the planet gets too hot, it throws off the blanket: the hydrogen escapes in a stream out into space, a process known as hydrodynamic escape. This process removed one of the main ingredients for water from Venus. It is not known exactly when this process occurred, but it was probably within the first billion years or so.
Hydrodynamic escape was stopped after most of the hydrogen was removed, but some hydrogen was left behind. It’s like dumping a bottle of water – there will still be a few drops left at the bottom. The remaining drops cannot escape in the same way. There must still be some other process at work on Venus that continues to remove hydrogen.
Small reactions can make a big difference
Our new study reveals that a chemical reaction in the atmosphere of Venus can 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 consist of one atom each of hydrogen, carbon and oxygen and have a positive charge, combine with negatively charged electrons, since opposites attract.
But when the HCO⁺ and the electrons react, the HCO⁺ breaks into a neutral carbon monoxide molecule, CO, and a hydrogen atom, H. This process energizes the hydrogen atom, which can then exceed the planet’s escape velocity and escape to space. . 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 the planet’s DR effectively dries up. DR has likely occurred throughout the history of Venus, and our work shows that it likely continues to the present day. It doubles the amount of hydrogen escape previously calculated by planetary scientists, furthering our understanding of present-day hydrogen escape on Venus.
Understanding Venus with data, models and Mars
To study DR on Venus, we used computer modeling and data analysis.
The actual modeling began as the Mars project. My Ph.D. Research involved exploring the conditions that made planets habitable for life. Mars also had water, although less than Venus, and lost most of it to space as well.
To understand the escape of Martian hydrogen, I developed a computational model of the Martian atmosphere that simulates the atmospheric chemistry of Mars. Despite being very different planets, Mars and Venus have similar upper atmospheres, so my colleagues and I were able to extend the model to Venus.
We found that the atmospheres of both planets contain a lot of hydrogen escaping through dissociative recombination of HCO⁺, which agreed with measurements taken by the Mars Atmosphere and Volatile Evolution mission, or MAVEN, a satellite orbiting Mars.
Collecting data in the atmosphere of Venus would be valuable to back up the model, but previous missions to Venus have not measured HCO⁺ – not because it doesn’t exist, but because they weren’t designed to detect it. They did, however, measure the reactants that produce HCO⁺ in the atmosphere of Venus.
By analyzing measurements made by Pioneer Venus, a combination mission of orbiters and probes that studied Venus from 1978-1992, and by 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 completed a piece of the puzzle of how water is lost from planets, affecting how habitable a planet is for life. We have learned that water loss occurs not only in one fell swoop, but over time through a combination of methods.
Today’s faster loss of hydrogen through DR means that it takes less time overall to remove the remaining water from Venus. This means that if oceans were ever present on early Venus, they could have been present for longer than scientists thought before water loss through hydrodynamic escape and DR began. This would provide more time for life to develop. Our results do not mean that the oceans or life were definitely present, however – the answer to that question will require much more science over many years.
New missions and observations of Venus are also needed. Future Venus missions will provide some atmospheric measurements, but will not focus on the upper atmosphere where most dissociative HCO⁺ recombination occurs. A future Venus upper atmosphere mission, like the MAVEN mission at Mars, could greatly enhance everyone’s knowledge of how the atmospheres of the terrestrial planets form and evolve over time.
With the technological advances of recent years and a blossoming new interest in Venus, now is a great time to turn our eyes to our fellow planet Earth.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of 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 the material are those of the author or authors. and do not necessarily reflect the views of the National Science Foundation.
