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Venus has lost almost all of its water, but why did it disappear?

A team of researchers led by the University of Colorado Boulder (CU Boulder) has found why Venus, Earth’s nearest planetary neighbor, is so devoid of moisture. The experts delved deep into what they term “the water story on Venus,” revealing that Venus loses about twice as much water into space each day than previously thought.

The phenomenon occurs through a process known as “dissociative recombination,” in which hydrogen atoms from Venus’s atmosphere are lost to space. 

Water retention and loss across planets

By using computer simulations, the experts hope to shed light on water retention and loss across many planets in the galaxy, enhancing our understanding of planetary climates and their capability to support life.

Water is really important for life,” said co-lead author Eryn Cangi, a research scientist at the Laboratory for Atmospheric and Space Physics (LASP) at CU Boulder. “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.” 

Earth versus Venus

Contrasting Venus with Earth highlights the stark differences in water abundance. “Venus has 100,000 times less water than the Earth, even though it’s basically the same size and mass,” explained co-lead author Michael Chaffin, a research scientist at LASP. 

If Earth’s water were evenly distributed across its surface, it would form a layer about three kilometers deep. In contrast, Venus would have a shallow pool of just three centimeters deep.

Rapid water loss from Venus 

The team utilized advanced computer models to simulate the complex chemical processes occurring in Venus’s dense and turbulent atmosphere

The research pointed to a molecule known as HCO+, an ion consisting of hydrogen, carbon, and oxygen, as a key player in the rapid loss of water from Venus. This molecule forms high in the atmosphere and facilitates the escape of hydrogen atoms into space.

Planetary changes and their outcomes 

For Cangi, who completed her doctorate in astrophysical and planetary sciences at CU Boulder in 2023, the study not only fills a significant gap in our understanding of Venus’s climate history but also illustrates how minor planetary changes can lead to dramatic environmental outcomes. 

“We’re trying to figure out what little changes occurred on each planet to drive them into these vastly different states,” she said.

Ancient water loss on Venus

Reflecting on Venus’s past, the researchers suggest that it once harbored as much water as Earth. However, a catastrophic escalation in greenhouse gasses led to runaway temperatures, vaporizing all its water, which then was lost to space. 

This ancient water loss does not fully account for Venus’s current dryness, which led the scientists to explore ongoing atmospheric processes.

“As an analogy, say I dumped out the water in my water bottle. There would still be a few droplets left,” Chaffin said. On Venus, even those residual droplets have disappeared, a mystery that the study attributes to the elusive HCO+ molecule.

Chaffin and Cangi pointed out that while many missions have explored Mars, Venus has seen fewer scientific visitors, which has limited our understanding of its atmospheric composition. 

“One of the surprising conclusions of this work is that HCO+ should actually be among the most abundant ions in the Venus atmosphere,” Chaffin remarked.

Future missions to Venus

Looking ahead, the researchers are enthusiastic about upcoming missions to Venus, such as NASA’s Deep Atmosphere Venus Investigation of Noble gasses, Chemistry, and Imaging (DAVINCI) mission, which will probe the planet’s atmosphere down to the surface. Although DAVINCI won’t detect HCO+ directly, it represents a step forward in collecting data that could confirm their findings and enhance our understanding of Venus.

“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,” Cangi concluded.

The study is published in the journal Nature.


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