Planetary scientists at the University of Colorado Boulder have discovered how Venus, Earth’s boiling, uninhabitable neighbor, became so dry.
The results could help explain what happens to water on a large number of planets throughout the galaxy. The work is published in ‘Nature’.
The new study fills a big gap in what researchers call “the story of water on Venus.” Using computer simulations, the team found that hydrogen atoms in the planet’s atmosphere shoot out into space through a process known as “dissociative recombination,” causing Venus to lose about twice as much water each day compared to Venus. with previous estimates.
“Water is really important for life,” says Eryn Cangi, a research scientist at the Laboratory for Atmospheric and Space Physics (LASP) and co-lead author of the new paper. “We need to understand the conditions that support liquid water in the universe, and that may have produced the very dry state of Venus today.”
Venus, he explains, is absolutely parched. If you took all the water on Earth and spread it over the planet like jam on toast, you would get a liquid layer about 3 kilometers deep. If you did the same on Venus, where all the water is trapped in the air, you would end up with only 3 centimeters, barely enough to wet your toes. “Venus has 100,000 times less water than Earth, although it is basically the same size and mass,” specifies Michael Chaffin, co-lead author of the study and scientific researcher at LASP.
In the current study, researchers used computer models to understand Venus as a giant chemistry laboratory, zooming in on the various reactions that occur in the planet’s swirling atmosphere. The group reports that a molecule called HCO+ (an ion made up of one hydrogen, one carbon, and one oxygen atom) high in Venus’ atmosphere may be to blame for the planet’s water escape.
For Cangi, co-lead author of the research, the findings reveal new clues about why Venus, which probably once seemed almost identical to Earth, is almost unrecognizable today.
“We are trying to discover what small changes occurred on each planet to bring them to these very different states,” says Cangi, who earned his doctorate in astrophysical and planetary sciences in Boulder in 2023.
Scientists suspect that billions of years ago, during the formation of Venus, the planet received about as much water as Earth. At some point, catastrophe struck. Clouds of carbon dioxide in Venus’ atmosphere triggered the most powerful greenhouse effect in the solar system, eventually raising surface temperatures to a scorching 480 degrees Celsius. In the process, all the water on Venus evaporated and most of it went into space.
But that ancient evaporation can’t explain why Venus is as dry as it is today, or how it continues to lose water into space. «As an analogy, let’s say I threw the water out of my water bottle. There would still be a few drops left,” explains Chaffin. On Venus, however, almost all the remaining drops also disappeared. The culprit, according to the new work, is the elusive HCO+.
Chaffin and Cangi explained that in the upper atmospheres of the planets, water mixes with carbon dioxide to form this molecule. In previous research, researchers reported that HCO+ may be responsible for Mars losing a large portion of its water.
Here’s how it works on Venus: HCO+ is constantly produced in the atmosphere, but individual ions don’t survive for long. Electrons in the atmosphere find these ions and recombine to split them in two. In the process, hydrogen atoms drift away and may even escape into space entirely, robbing Venus of one of the two components of water.
In the new study, the group calculated that the only way to explain Venus’s dry state was if the planet harbored larger-than-expected volumes of HCO+ in its atmosphere. There is a twist in the team’s findings. Scientists have never observed HCO+ around Venus. Chaffin and Cangi suggest that’s because they never had the instruments to look properly.
Researchers are hopeful that a future mission can do so, revealing another key piece of the story of water on Venus. “There have not been many missions to Venus,” Cangi concludes. “But the newly planned missions will leverage decades of collective experience and a burgeoning interest in Venus to explore the extremes of planetary atmospheres, evolution and habitability.” (Europa Press)
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