Planetary scientists at the University of Colorado Boulder have discovered how Venus, Earth's scalding and uninhabitable neighbor, became so dry.

The new study fills in a big gap in what the researchers call "the water story on Venus." Using computer simulations, the team found that hydrogen atoms in the planet's atmosphere go whizzing into space through a process known as "dissociative recombination" -- causing Venus to lose roughly twice as much water every day compared to previous estimates.

The team will publish their findings May 6 in the journal Nature.

The results could help to explain what happens to water in a host of planets across the galaxy.

"Water is really important for life," said 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, she added, is positively parched. If you took all the water on Earth and spread it over the planet like jam on toast, you'd get a liquid layer roughly 3 kilometers (1.9 miles) deep. If you did the same thing on Venus, where all the water is trapped in the air, you'd wind up with only 3 centimeters (1.2 inches), barely enough to get your toes wet.

"Venus has 100,000 times less water than the Earth, even though it's basically the same size and mass," said Michael Chaffin, co-lead author of the study and a research scientist at LASP.

In the current study, the researchers used computer models to understand Venus as a gigantic chemistry laboratory, zooming in on the diverse reactions that occur in the planet's swirling atmosphere. The group reports that a molecule called HCO+ (an ion made up of one atom each of hydrogen, carbon and oxygen) high in Venus' atmosphere may be the culprit behind the planet's escaping water.

For Cangi, co-lead author of the research, the findings reveal new hints about why Venus, which probably once looked almost identical to Earth, is all but unrecognizable today.

"We're trying to figure out what little changes occurred on each planet to drive them into these vastly different states," said Cangi, who earned her doctorate in astrophysical and planetary sciences at CU Boulder in 2023.