International Space Agencies – Missions and Discoveries

Understanding Water Loss on Venus


From UPSC perspective, the following things are important :

Prelims level: Solar System; Venus and its physiography; Non-Thermal Dissociative Recombination;

Mains level: NA

Why in the News?

Over four billion years ago, Venus had enough water to potentially cover its surface with an ocean approximately 3 km deep, but today, it would remain with only 3 cm.

  • A research by US scientists explain the Non-Thermal Dissociative Recombination (DR) responsible for faster loss of water from Venus.

About Venus

  • Venus is the second planet from the Sun. It is a terrestrial planet and is the closest in mass and size to its orbital neighbour Earth.
  • Venus is notable for having the densest atmosphere of the terrestrial planets, composed mostly of carbon dioxide with a thick, global sulphuric acid cloud cover.
  • At the surface it has a mean temperature of 464 °C (737 K) and a pressure of 92 times that of Earth’s at sea level.
  • These extreme conditions compress carbon dioxide into a supercritical state close to Venus’s surface.
  • Internally, Venus has a core, mantle, and crust. Venus lacks an internal dynamo, and its weak induced magnetosphere is caused by atmospheric interactions with the solar wind.
  • Venus is one of two planets in the Solar System (the other being Mercury), that have no moons.
  • The rotation of Venus has been slowed and turned against its orbital direction (retrograde) by the currents and drag of its atmosphere.
  • It takes 224.7 Earth days for Venus to complete an orbit around the Sun, and a Venusian solar year is just under two Venusian days long.

Water Loss on Venus:

  • Venus lost its water primarily due to two factors:
      • Evaporation due to Greenhouse Effect: Its dense atmosphere rich in carbon dioxide, creating a strong greenhouse effect and surface temperatures around 450 degrees Celsius, which prevents water from existing in liquid form.
      • Proximity to the Sun: This leads to the disintegration of water molecules into hydrogen and oxygen in the ionosphere under solar heat and ultraviolet radiation.
  • Mechanism of Water Loss:
  1. Thermal Process: Initially, hydrodynamic escape was significant, where solar heating caused the outer atmosphere to expand, allowing hydrogen to escape into space. This process cooled and slowed about 2.5 billion years ago.
  2. Non-Thermal Process: Focus of recent study; involves hydrogen escaping into space, reducing water formation as oxygen atoms lack hydrogen to bond with.

Key Research Findings: Non-thermal Dissociative Recombination (DR)

The discrepancy in water loss rates was addressed by identifying a previously overlooked chemical reaction involving the formyl cation (HCO+).

  • HCO+ dissociative recombination (DR) reaction occurs when HCO+ gains an electron and splits into CO and a hydrogen atom, which then escapes into space.
  • This reaction is responsible for losing out water without evaporation.
  • This reaction was modelled to significantly increase the rate of hydrogen escape, potentially doubling the rate at which Venus lost water.
  • The model suggests that water levels on Venus would have been stable from nearly 2 billion years ago due to the ongoing non-thermal HCO+ DR reaction, yet some water remains today.

Future Research on Venus

  • Existence of HCO+ Ions: Direct evidence of HCO+ ions in Venus’s atmosphere is still missing. Past missions did not focus on this molecule, and its involvement in water loss was not previously considered crucial.
  • Future Missions: The findings underscore the importance of future Venus missions to investigate the presence of HCO+ in the upper atmosphere, similar to the MAVEN mission to Mars.


[2011] What is the difference between asteroids and comets?

  1. Asteroids are small rocky planetoids, while comets are made of ice, dust and rocky material.
  2. Asteroids are found mostly between the orbits of Jupiter and Mars, while comets are found mostly between Venus and Mercury.
  3. Comets show a perceptible glowing tail, while asteroids do not.

Which of the statements given above is/ are correct?

(a) 1 and 2 only

(b) 1 and 3 only

(c) 3 only

(d) 1, 2 and 3

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