Once covered in oceans, now Venus has almost no water. Here's why

Venus, often dubbed Earth’s twin due to its similar size and density, starkly contrasts with our vibrant, life-sustaining planet. While Earth flourishes with life, Venus, residing as the second planet from the sun, presents a hostile environment with temperatures soaring to 880 degrees Fahrenheit (471 degrees Celsius) and oppressive surface pressures.

However, the most striking divergence lies in the absence of water on Venus, despite being within the sun’s “Goldilocks Zone” conducive to liquid water.

Recently, a team from the University of Colorado in Boulder, US delved into this enigma, aiming to decipher “the water story on Venus.”

The team published 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.

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The mystery of Venus’s water loss

Billions of years ago, Venus may have boasted oceans just like Earth’s. However, a catastrophic greenhouse effect triggered by carbon dioxide clouds led to its evaporation, leaving Venus barren. Despite this understanding, the mechanism behind Venus’s continued water loss perplexes scientists.

“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.

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“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.

Why is Venus leaking water into space?

As Venus wasn’t always a barren expanse, scientists speculate that billions of years ago, during Venus’s formation, the planet likely received a comparable amount of water to Earth. However, a catastrophic event ensued.

Clouds laden with carbon dioxide in Venus’s atmosphere triggered an unparalleled greenhouse effect, elevating surface temperatures to a scorching 900 degrees Fahrenheit. Consequently, all of Venus’s water evaporated into steam, with much of it escaping into space.

Yet, this ancient evaporation fails to fully account for Venus’s current aridity or its ongoing water loss into space.

“As an analogy, say I dumped out the water in my water bottle,” Cangi said. “There would still be a few droplets left.”

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On Venus, however, almost all of those remaining drops also disappeared. The culprit, according to the new work, is elusive HCO+.

HCO+: The culprit behind Venus’s water loss?

In their latest investigation, the researchers employed computer models to analyse Venus akin to a vast chemistry laboratory, focusing on the intricate reactions within its tumultuous atmosphere. Their findings suggest that a molecule known as HCO+—comprising hydrogen, carbon, and oxygen atoms—abundant in Venus’ upper atmosphere, might be accountable for the planet’s water loss.

For Cangi, these revelations offer fresh insights into the transformation of Venus, which likely bore resemblance to Earth in the past but has evolved into an alien landscape today.

“We’re trying to figure out what little changes occurred on each planet to drive them into these vastly different states,” added Cangi in the study.

What role does HCO+ play on Venus?

Here’s the mechanism at play on Venus: HCO+ is continually generated in the atmosphere, but these individual ions have a short lifespan. Electrons in the atmosphere promptly encounter these ions, prompting their recombination and subsequent splitting into two. This process results in the rapid departure of hydrogen atoms, potentially escaping into space entirely, thus depriving Venus of one of the essential components of water.

In their recent study, the team calculated that Venus’s desiccated condition could only be elucidated if the planet harbored larger-than-anticipated quantities of HCO+ in its atmosphere. However, there’s a twist to the team’s revelations: HCO+ has never been observed around Venus. Chaffin and Cangi posit that this oversight is due to the lack of suitable instrumentation.

While numerous missions have been dispatched to Mars in recent years, Venus, as the second planet from the sun, has received far less attention. None of the previous missions have been equipped with the necessary tools to detect HCO+, the crucial element powering the team’s newly unveiled water escape mechanism.

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

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Past missions lacked the necessary instruments, and upcoming ones like NASA’s DAVINCI mission, set for 2029, aren’t equipped for this task. Nonetheless, the team remains optimistic, banking on future missions to unveil Venus’s hidden secrets.

“There haven’t been many missions to Venus,” Cangi said. “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.”

As humanity’s curiosity turns towards Venus, propelled by upcoming missions like DAVINCI and EnVision, the stage is set for unraveling the mysteries of our planetary neighbor. By piecing together these findings, scientists inch closer to answering the burning question: Why is Venus leaking water into space?

With inputs from agencies