When Venusformed billions of years ago, the planet probably had about as much water as Earth.

Thick carbon dioxide clouds in Venus' atmosphere trap heat, making it the hottest planet in the solar system, despite it being farther from the sun in spacethan Mercury. The runaway greenhouse effect has raised the planet's temperature to a sizzling 900 degrees Fahrenheit, according to NASA. 

But scientists say evaporation from heat can't completely explain why Venus, sometimes known as Earth's evil twin, is the arid wasteland it is today. 

"As an analogy, say I dumped out the water in my water bottle. There would still be a few droplets left," said Michael Chaffin, a researcher at the University of Colorado Boulder, in a statement. 

On Venus, almost all of those remaining "drops" are gone, too, and a new study co-led by Chaffin puts forward a theory for how it has happened. The answer, according to the research, may be a molecule that could be in the planet's upper atmosphere, known as HCO+, which forms when water mixes with carbon dioxide. 

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What makes the finding somewhat odd is that scientists have never detected this molecule around Venus. The team published its researchin the journal Naturethis week. 

With computer simulations of the planet's chemistry, the scientists discovered that hydrogen atoms in the Venusian atmosphere could escape into space through so-called "dissociative recombination," causing the planet to lose twice as much water daily than previous estimates. 

To get a better sense of just how much less water Venus has than Earth, co-author Eryn Cangi explains it this way: If all of Earth's water could be spread evenly around the world, the planet would be covered in a nearly 2-mile-deep ocean. But on Venus, where almost all of the available water is steam in the air, that liquid would blanket the planet in a paltry single inch of water. 

"Venus has 100,000 times less water than the Earth, even though it's basically the same size and mass," Chaffin said. 

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The HCO+ molecule would form in the atmosphere, but individual ions wouldn't last long, according to the new theory. Instead, electrons in the atmosphere would seek these ions and recombine to split the ions. Through this process, hydrogen atoms would drift away, depleting the planet of one of the two necessary ingredients to make water. This could explain why Venus continues to lose water to space. 

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Despite scientists never having observed HCO+ around Venus, the research team thinks that might be because no spacecraft that has visited the world has had the right instruments to check. And even though there are upcoming missions planned to study Venus from the sky in the 2030s, such as NASA's DAVINCIand the European Space Agency's Envision, these robotic probes won't have the right tools to detect the molecule, either. 

But getting to the bottom of how Venus dried out could help scientists better understand what can happen to water elsewhere. 

"Water is really important for life," Cangi said. "We need to understand the conditions that support liquid water in the universe."