The reason Venus is so dry has been discovered by scientists.
Earth’s scalding and uninhabitable planetary neighbour has almost no water, although it once had as much water as our own planet.
Now researchers at the University of Colorado Boulder in the United States believe they have worked out how Venus became so dry.
The study fills in a big gap in what they call “the water story on Venus.”
Using computer simulations, the research 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 around twice as much water every day compared to previous estimates.
The findings, published in the journal Nature, could help to explain what happens to water in many planets across the galaxy.
Study co-lead author Dr Eryn Cangi, a research scientist at the Laboratory for Atmospheric and Space Physics (LASP), said: “Water is really important for life.
“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.”
She explained that Venus is “positively parched”.
The researchers explained that 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 kilometres (1.9 miles) deep.
If you did the same thing on Venus, where all the water is trapped in the air, you’d end up with only three centimetres (1.2 inches).
Co-lead author Dr Michael Chaffin, a research scientist at LASP, said: “Venus has 100,000 times less water than the Earth, even though it’s basically the same size and mass.”
The research team used computer models to understand Venus as a gigantic chemistry laby, zooming in on the diverse reactions that occur in the planet’s swirling atmosphere.
They found that a molecule called HCO+ – an ion made up of one atom each of hydrogen, carbon and oxygen – high in Venus’ atmosphere may be behind the planet’s escaping water.
Dr Cangi says the findings reveal new hints about why Venus, which probably once looked almost identical to Earth, is all but unrecognisable today.
She said: “We’re trying to figure out what little changes occurred on each planet to drive them into these vastly different states.”
Dr Cangi says Venus wasn’t always such a desert.
Scientists believe that during the planet’s formation billions of year ago itreceived about as much water as Earth.
But, at some point, clouds of carbon dioxide in Venus’ atmosphere kicked off the most powerful greenhouse effect in the solar system, eventually raising temperatures at the surface to a roasting 900 degrees Fahrenheit.
In the process, all of Venus’ water evaporated into steam, and most drifted away 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 to space.
Dr Chaffin said: “As an analogy, say I dumped out the water in my water bottle. There would still be a few droplets left.”
However, on Venus almost all of those remaining drops also disappeared.
The researchers explained that in planetary upper atmospheres, water mixes with carbon dioxide to form HCO+.
Previous research has suggested that HCO+ may be responsible for Mars losing a big chunk of its water.
In the new study, the researchers calculated that the only way to explain Venus’ dry state was if the planet hosted larger than expected volumes of HCO+ in its atmosphere.
But scientists have never observed HCO+ around Venus.
The research team suggest that’s because they’ve never had the instruments to properly look.
Fewer spacecraft have travelled to the second planet from the sun, and none had instruments capable of detecting the HCO+ that powers the team’s newly discovered escape route.
Dr Chaffin said: “One of the surprising conclusions of this work is that HCO+ should actually be among the most abundant ions in the Venus atmosphere.”
NASA’s planned Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging (DAVINCI) mission is due to drop a probe through the planet’s atmosphere all the way to the surface. It’s scheduled to launch by the end of the decade.
DAVINCI won’t be able to detect HCO+, but the researchers are hopeful that a future mission might – revealing another key piece of the story of water on Venus.
Dr Cangi added: “There haven’t been many missions to Venus.
“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.”
