Where Did Venus's Water Go?

It should not be surprising that Venus is dry. It is famous for its hellish conditions, with dense sulphurous clouds, rains of acid, atmospheric pressures comparable to a 900 meter deep lake, and a surface temperature high enough to melt lead. But it’s lack of water is not just a lack of rain and oceans: there’s no ice or water vapour either. Like Earth, Venus is found within our Solar System’s goldilocks zone, so it would have had plenty of water when it was first formed. So where did all of Venus’s water go?

Venus is an extremely dry planet, although it wasn’t always like this. At some point in its history, a run-away greenhouse effect began, ending with its current extreme state. Most models agree that this process would have driven off most of its original water, but that there should still be some remaining. And yet, observations show us that there is practically no water at all. Planetary scientists at the University of Colorado Boulder believe that they have found an explanation: a molecule called HCO+ high in Venus’s atmosphere may be responsible. Unfortunately, they may have to wait for future missions to Venus before they can confirm it.

Until the middle of the 20th century, Venus was thought of as Earth’s twin. Both planets are approximately the same size and mass, and they’re both within the sun’s habitable zone – the region where temperatures can exist that are warm enough to melt ice, but not so hot that water boils into steam. It was long assumed that, beneath its shining white cloud cover, Venus must have a similar climate to Earth. Science fiction authors even wrote stories about visitors to Venus exploring verdant jungles and meeting exotic civilizations. But the truth is much harsher: Venus is an extreme place, with sulphuric acid rains, crushing atmospheric pressure, and a surface temperature hot enough to melt lead. But it wasn’t always like that.

The general assumption among astronomers and planetary scientists is that both Earth and Venus started life with similar amounts of water. But something happened to release enormous quantities of carbon dioxide into its atmosphere, leading to an extreme runaway greenhouse effect. The high temperatures melted off any ice, and boiled away any liquid water, filling the atmosphere with water vapour. Much of this hot vapour would eventually blow off into space, drying out the planet, but some should remain. The puzzle is that the usual models predict a great deal more remaining water vapour than what is actually there. So, what happened?

According to a study, led by Dr Eryn Cangi and Dr Mike Chafin, both of the Laboratory for Atmospheric and Space Physics (LASP), the answer may be a molecule named HCO+. In their earlier work studying the atmosphere of Mars, they discovered a process by which this molecule can remove water from planetary atmospheres. In their new paper, they suggest that the same process could be at work on Venus. The only catch is that this molecule has never been detected in the Venusian atmosphere.

Unfortunately, there is little evidence to confirm this theory. HCO+ has never been detected in the atmosphere of Venus. However, Cangi and Chafin point out that this is because nobody has ever looked for it, and none of the missions sent to Venus so far were equipped with instruments that could detect it. They are optimistic for future missions, however.

NASA's DAVINCI probe falling to the surface of Venus.
Illustration of NASA’s DAVINCI probe falling to the surface of Venus. (Credit: NASA GSFC visualization by CI Labs Michael Lentz and others)

“One of the surprising conclusions of this work is that HCO+ should actually be among the most abundant ions in the Venus atmosphere,” says Chaffin.
“There haven’t been many missions to Venus,” adds Cangi. “But newly planned missions will leverage decades of collective experience and a flourishing interest in Venus to explore the extremes of planetary atmospheres, evolution and habitability.”

The planetary science community has gotten increasingly interested in Venus, and a number of future missions are planned to study it in more detail. NASA’s planned Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging (DAVINCI) mission is one example. DAVINCI will drop a probe down to the surface, which will study the atmosphere at different altitudes as it falls. Unfortunately for Cangi and Chafin, it is not designed specifically to look for HCO+, but it may reveal other clues to either confirm or disprove their theory. But they remain optimistic that additional missions will be sent in future that will carry the necessary instruments that they can use to test their work.

For more information, visit CU Boulder’s announcement at https://www.colorado.edu/today/2024/05/06/venus-has-almost-no-water-new-study-may-reveal-why