A1.1.7 Extraplanetary origin of water on Earth and reasons for its retention (HL only) Notes

Origins of Water on Earth

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.

The Asteroid Hypothesis

The most widely accepted explanation is that water was delivered by asteroid collisions.

1. During Earth's early history, there was a period of intense asteroid bombardment.
2. Many of these asteroids contained water (as ice or hydrated minerals).
3. When asteroids collided with Earth, they delivered water to the planet's surface.
4. 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.
5. 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

Once water arrived on Earth, two key factors allowed the planet to retain it over billions of years:

1. 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.

2. Low Enough Temperatures to Condense Water

1. Earth's distance from the Sun creates temperatures that allow water to exist in liquid form.
2. Earth is in the "habitable zone" (sometimes called the Goldilocks zone), the region around the Sun where temperatures are neither too hot nor too cold.
3. Temperatures are low enough for water vapor to condense into liquid water.
4. Most of Earth's water (98.3% of approximately 1.4 billion cubic kilometers) exists in liquid form.