IB Biology Topic A1.1.7: Extraplanetary Origin of Water on Earth and Reasons for Its Retention Notes

Early Earth Was Too Hot for Water

1. During Earth's formation, temperatures were extremely high.
2. Any water present would have evaporated and been lost to space.
3. Therefore, water had to come from an extraplanetary source, beyond Earth itself.

Asteroids Delivered Earth's Water

1. The most widely accepted explanation is that water was delivered by asteroid collisions.
2. During Earth's early history, there was a period of intense asteroid bombardment.
3. Many of these asteroids contained water (as ice or hydrated minerals).
4. When asteroids collided with Earth, they delivered water to the planet's surface.
5. Evidence:
   1. Traces of water have been found in material from asteroids.
   2. The isotopic composition of water in some asteroids matches Earth's water.
6. Modern vs. ancient asteroids:
   1. Ancient asteroids (during Earth's formation) likely contained much more water.
   2. Modern asteroids have lost most of their water due to:
      1. Heat from the Sun evaporating water.
      2. Weak gravity allowing water vapor to escape into space.

Why Earth Retained Its Water

1. Once water arrived on Earth, two key factors allowed the planet to retain it over billions of years:
2. Gravity:
   1. Earth's size gives it strong gravitational pull.
   2. Earth's gravity holds onto its atmosphere and prevents water molecules from escaping into space.
   3. Without sufficient gravity, water vapor would drift away, as has happened on smaller planets like Mars.

1. Low enough temperatures to condense water:
   1. Earth's distance from the Sun creates temperatures that allow water to exist in liquid form.
   2. This places Earth in the habitable zone, where liquid water is stable, explored further in A1.1.8.
   3. Temperatures are low enough for water vapor to condense into liquid water.
   4. As a result, most of Earth's water exists as liquid rather than vapour or ice.

Abundant Water Made Life Possible

1. Because Earth both captured and kept water, it has held vast amounts of liquid water for billions of years.
2. This long-term abundance of water provided the stable medium in which life could originate and evolve.