TAG: GS 3: SCIENCE AND TECHNOLOGY
THE CONTEXT: A recent study published in Nature has provided a significant advance in understanding the water loss on Venus, addressing a long-standing discrepancy between the expected and observed amounts of water lost by the planet over the past 4.5 billion years.
EXPLANATION:
- This research, led by a team of U.S. scientists, focuses on a previously overlooked chemical reaction that may have played a crucial role in Venus’s water depletion.
Historical Context of Water on Venus
- Over four billion years ago, Venus possessed enough water to cover its surface with an ocean approximately 3 kilometers deep.
- In stark contrast, present-day Venus only has enough water to create a shallow layer of 3 centimeters.
- Scientists have been able to account for a significant portion of this lost water, but a considerable gap remained unexplained until now.
Mechanisms of Water Loss on Venus
- Thermal and Non-Thermal Processes
- Venus’s water loss can be attributed to two primary processes:
- Thermal Process (Hydrodynamic Escape): This process involved the heating of Venus’s outer atmosphere by the Sun, causing it to expand and allowing hydrogen gas to escape into space. This escape continued until the atmosphere cooled, approximately 2.5 billion years ago.
- Non-Thermal Processes: These involve the disintegration of water molecules due to the Sun’s ultraviolet radiation, resulting in the release of hydrogen and oxygen atoms.
- Venus’s water loss can be attributed to two primary processes:
New Insights from the Study
- The research led by Dr. Eryn Cangi and her colleagues concentrated on the present-day non-thermal process of hydrogen escape.
- They discovered that accounting for the formyl cation (HCO+), a positively charged molecule, could explain the discrepancy between previous estimates and satellite observations of water loss.
- The HCO+ ion has been encountered during the research on Mars.
- Scientists have known that HCO+ molecules facilitate hydrogen escape on Mars, prompting the team to investigate if similar reactions occur in Venus’s ionosphere.
- On Venus, the team identified that the HCO+ dissociative recombination reaction (DR) is a significant process at an altitude of about 125 kilometers.
- This reaction involves HCO+ molecules absorbing electrons and breaking down into carbon monoxide (CO) and hydrogen atoms.
- These hydrogen atoms, now energetic, escape into space, accelerating water loss.
Modeling and Predictions
- The researchers created models to simulate the effect of the HCO+ DR reaction on Venus’s upper atmosphere.
- Their findings indicated that this reaction could have doubled the rate of hydrogen escape, suggesting that Venus might have lost its water more quickly than previously thought.
Implications for Venus’s Water History
- The study’s model suggests that if Venus had oceans, they would have persisted longer than expected because of the accelerated hydrogen escape rate.
- The researchers also predicted that Venus’s water content would have remained relatively stable for nearly the past 2 billion years, as the non-thermal HCO+ DR process would have continually depleted water.
Evidence and Future Research
- The team found indirect evidence of HCO+ DR in the data from the NASA Pioneer Venus orbiter, noting the presence of other molecules necessary for forming HCO+.
- Recent findings published in Nature Astronomy reported signatures of carbon ions escaping Venus, which qualitatively support the HCO+ DR model.
- There is a need for future missions to focus on Venus’s upper atmosphere to directly detect HCO+ ions.
- The MAVEN mission to Mars as a successful example of such an approach.
Broader Implications
- This research has broader implications for understanding planetary habitability, particularly in comparing Venus’s history of water with Earth’s.
- The need to determine whether Venus is abnormally dry or if Earth is abnormally wet, as this distinction has significant implications for our understanding of planetary habitability in the universe.