Context

Four billion years ago, Venus had enough water to cover its surface with an ocean 3 km deep.  Presently, planet retains only enough water for this ocean to be a mere 3 centimeters deep.

Venus Water Mystery

Explanation of Loss of Water on Venus: A team of researchers in the U.S. has achieved a critical breakthrough in understanding this phenomenon.

• The team’s findings, reported in a paper in Nature could bridge the gap between the amount of water that satellite observations indicate Venus has likely lost over the past 4.5 billion years and what scientists had previously predicted.

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Reasons for Loss of Water on Venus

• Harsh Atmosphere: One primary factor contributing to Venus’s harsh conditions is its infernal atmosphere, a consequence of its high concentration of carbon dioxide, which induces a potent greenhouse effect. 
  • With surface temperatures exceeding the boiling point of water, reaching 450 degrees Celsius, water can only exist in Venus’s atmosphere as vapor.
• Proximity to Sun: The intense heat and ultraviolet radiation from the Sun interacted to break down water molecules into their component hydrogen and oxygen atoms within Venus’s ionosphere.
  • Ionosphere is the uppermost layer of the atmosphere characterized by high-speed movement of charged atoms, molecules, and electrons.
  • However, the rates at which these processes occurred remain unknown. 

Theories Related to Loss of Water on Venus

• Thermal Process:  Known as hydrodynamic escape, involves the expansion of Venus’s outer atmosphere due to heating from the Sun, enabling hydrogen gas to escape into space. 
  • This phenomenon persisted until the outer atmosphere sufficiently cooled, around 2.5 billion years ago.
• Non-thermal process: The research has been focused on the present-day mechanism of water loss, specifically through a non-thermal process. 
• Their study centered on the escape of hydrogen atoms from Venus into space, leading to a reduction in water levels as the oxygen atoms remaining have fewer hydrogen atoms available to form water molecules.

Key Findings from the Study

• HCO+ Reaction in Venus’ Upper Atmosphere: On Venus, the team found that a particular reaction, called the HCO+ dissociative recombination reaction (DR) occurs in bulk at an altitude of about 125 km, above the clouds made of sulphuric acid.

• • Noting the similarities between the upper atmospheres of Venus and Mars, the same fundamental reactions were modeled in Venus’ ionosphere.
• Impact of HCO+ DR: The team built models to simulate the influence of this reaction on the upper atmosphere, and found that it accelerated water decline once the hydrodynamic escape of hydrogen gas ended.
  • Specifically, the researchers found HCO+ DR could have doubled the rate at which Venus lost water by hydrogen escape.
  • This means if Venus had oceans in the past, they could have lasted longer than expected — because the faster rate of hydrogen escape means the planet could have lost more water in the same amount of time.
• Long-Term Water Depletion on Venus: The model predicted that the amount of water on Venus would have stayed roughly the same from nearly 2 billion years ago. 

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• • This is because, as a non-thermal process, the HCO+ DR reaction would’ve gone on indefinitely and drained all the water. (The thermal process was time-bound because the upper atmosphere returned to thermal equilibrium). Yet Venus still has some water today.
• The Missing Molecule: There is no evidence confirming the presence of HCO+ ions in Venus’s atmosphere.
• Neglecting Search for HCO+ ions: Previous space missions had overlooked the search for HCO+ ions, and the orbiters sent to Venus were unable to detect the chemical signatures of HCO+ DR from a distance. 
  • There would have needed to be a clear link between HCO+ DR and water loss on Venus for scientists to prioritize this investigation.