Origin of Water on Earth (HL) (HL IB Biology)

Extraplanetary origin of water

• Water is crucial for the existence of life but when Earth formed around 4.5 billion years ago, conditions were too hot for water vapour to condense into liquid water
• This has led scientists to believe that Earth's water must have originated from somewhere else
• One such hypothesis is that asteroids, and the meteorites that break off from them, may be the origin of Earth's water, since many of them contain ice and other organic materials that would have made it possible for life to evolve
• One of the oldest group of meteorites in the solar system are called carbonaceous chondrites
  • These meteorites contain hydrogen isotopes similar to those found in seawater
• Another group of ancient meteorites called eucrite achondrites, contain ratios of hydrogen isotopes that are similar to that found on Earth, providing more support for this hypothesis
• It is possible that during an impact with Earth, these meteorites would have released water vapour which would have been trapped by Earth's gravity
• Temperatures on Earth would have been low enough to allow this water vapour to condense to form liquid water which would have been retained on the surface by gravity

The search for extraterrestrial life and the presence of water

• Living organisms depend on water for their existence, so this would be a requirement for any planet to support life
• For water to exist in liquid form, the temperature on a planet should not be too hot or too cold, which in turn would depend on the distance of that planet from its nearest star
• The area around a star where temperatures are favourable for water to exist in liquid form is known as the Goldilocks zone
  • Named after the story of Goldilocks and the three bears

The Goldilocks zone diagram

A planet must be the correct distance from its nearest star to be located in the Goldilocks zone where temperatures are "just right" for the existence of liquid water

• In the search for life outside our solar system, scientists are looking at planets located in the Goldilocks zone of other solar systems
• These planets are called exoplanets
• They are able to use a technique called transit spectroscopy, which analyses light passing through the planet's atmosphere as it passes in front of its nearest star
• Based on the wavelengths of light being absorbed or deflected, an analysis can be made about the elements and molecules present in the atmosphere
  • If it indicates that water may be present, the planet is said to have a water signature
• For an exoplanet to support life it must have the following characteristics:
  • A water signature
  • Located in the Goldilocks zone of its solar system
  • Be large enough to support an atmosphere