Researchers found that Venus lost much more water than previously thought through a process called “dissociative recombination,” in which hydrogen atoms are ejected into space. Planetary scientists at the University of Colorado at Boulder have discovered how Venus, Earth’s fiery and uninhabitable neighbor, became so dry.

The new study fills a major gap in what researchers call the “history of water on Venus.” Using computer simulations, the team found that hydrogen atoms in the planet’s atmosphere were escaping into space through a process known as “dissociative recombination,” causing Venus to lose nearly twice as much water each day as previously predicted. The results could help explain what happens to water on many planets in the galaxy.

“Water is really important for life,” said Erin Kangy, a research scientist at the Laboratory for Atmospheric and Space Physics (LASP) and co-author of the new paper. “We need to understand the conditions in the universe that support liquid water, and this may have led to the very dry state of Venus today.”

He added that Venus had dried up completely. If you took all the water on Earth and spread it across the planet like jam on toast, you’d get a layer of liquid about 3 kilometers (1.9 miles) deep. If you did the same on Venus, where all the water was trapped in the air, only 3 centimeters (1.2 inches) would remain, enough to wet your toes.

“Despite being essentially the same size and mass, Venus has 100,000 times less water than Earth,” said study co-author and LASP research scientist Michael Chaffin.

In the current study, researchers used computer models to understand Venus as a giant chemistry laboratory, taking a close look at the various reactions occurring in the planet’s rotating atmosphere. The team reports that a molecule called HCO+ (an ion composed of one atom of hydrogen, carbon and oxygen), found in high amounts in Venus’ atmosphere, may be responsible for water leakage on the planet.

According to Kangy, one of the study’s lead authors, the results reveal new clues as to why Venus, which once likely looked nearly identical to Earth, is almost unrecognizable today.

“We’re trying to understand what little changes are happening on each planet that put them in these very different states,” said Kangy, who received her doctorate in astrophysics and planetary sciences from CU Boulder in 2023.

water spill

He noted that Venus was not always this desert. Scientists suspect that during the formation of Venus billions of years ago, the planet received as much water as Earth. At some point disaster struck. Clouds of carbon dioxide in Venus’ atmosphere caused the strongest greenhouse effect in the Solar System, eventually raising surface temperatures to 900 degrees Fahrenheit. In this process, all of the water on Venus evaporated into steam and most of it flew into space.

But this ancient evaporation can’t explain why Venus is as dry as it is today, or how it continues to lose water to space.

“To use an analogy, let’s say I spilled the water from my bottle. There’s still a few drops left,” Chaffin said.

However, almost all of the remaining droplets on Venus have disappeared. The culprit, according to the new study, is the elusive HCO+.

Missions to Venus

Chaffin and Kangy explained that in the upper atmosphere of the planet, water mixes with carbon dioxide to form this molecule. In previous studies, researchers reported that HCO+ may be responsible for Mars losing most of its water.

Here’s how it works on Venus: HCO+ is constantly produced in the atmosphere, but individual ions don’t survive long. Electrons in the atmosphere find these ions and recombine, splitting the ions into two. In the process, hydrogen atoms shift and may even escape into space entirely, leaving Venus deprived of one of its two water components.

In the new study, the team calculated that the only way to explain Venus’s dry state is that there are higher than expected amounts of HCO+ in the planet’s atmosphere. There is a change in the team’s findings. Scientists have never observed HCO+ around Venus. Chaffin and Kangy suggest it’s because they never had the tools to look decent.

Although dozens of missions have visited Mars in recent years, far fewer spacecraft have traveled to the second planet from the Sun. None of them were carrying devices that could detect the HCO+ that powered the team’s newly discovered escape route.

“One of the surprising findings of this study is that HCO+ should actually be one of the most abundant ions in the Venusian atmosphere,” Chaffin said.

But in recent years, more and more scientists have turned their attention to Venus. For example, NASA’s planned Noble Gases, Chemistry, and Imaging Deep Atmosphere Venus Probe (DAVINCI) mission will drop a probe through the planet’s atmosphere all the way to the surface. Its launch is planned for before the end of this decade.

DAVINCI will also fail to detect HCO+, but researchers hope a future mission will reveal another important part of the water story on Venus.

“There haven’t been many missions to Venus,” Kangy said. “But newly planned missions will use decades of collective experience and growing interest in Venus to probe the extreme limits of planetary atmospheres, evolution, and habitability.”