IB Biology Sub-topic A1.1 Notes

Origin of the Solar System

Now that you have learned about the properties of water, you can appreciate how vital it is to life on Earth. In fact, 70% of our bodies and 70% of the Earth's surface are composed of water, highlighting its importance to life. However, the Earth did not always have this much water.

You are expected to understand how these water levels formed on Earth. To understand this, let's start from the beginning:

1. Approximately 4.6 billion years ago, part of a molecular gas and dust cloud collapsed on itself.
2. It begun to rotate faster during its collapse and form a hot protostar center and colder protoplanetary disc.
3. Gravity condensed the center into the Sun and dust and gas in the disc into hundreds of protoplanets.
4. Many of these were ejected, destroyed or collided and merged to form the early planets of the Solar System. 
5. Solar winds and the high temperatures of the inner Solar System removed hydrogen and helium, forming the rocky planets of Mercury to Mars.
6. Further planets were far enough away that solar winds and low temperatures allowed gases to remain and expand, forming the gas giants of Jupiter to Uranus.
7. Leftover debris congregated in asteroid belts.

Extraplanetary origin of water

Knowing the initial formation of the Solar System, we can move to the development of the Earth:

1. Early Earth was mostly molten due to intense volcanic activity.
2. Eventually, the Earth to cool enough to form a solid crust and core, leading to the formation of the Earth's magnetic field.
3. The magnetic field stabilized Earth's atmosphere and preventing it from being stripped away by solar winds.
4. At some point, a Mars-sized object collided with the Earth, releasing material from the mantle which became trapped in orbit, forming the Moon.
5. Further plate tectonic activity caused mantle flow, leading to mountain formation and volcanic activity, releasing gases.
6. Cooling of the planet and gravity caused the gases to be retained, creating a denser atmosphere.
7. The arrival of water is explained by the Asteroid Theory. This states that water abundant asteroids from the asteroid belt entered the inner Solar System and collided with Earth, depositing much water on Earth's surface.
8. Coupled with Earth's gravitational pull, this helped water retention to form oceans and seas. Due to the atmosphere and ideal distance from the Sun, vaporization did not occur, maintaining its liquid form.

Evidence of these theories is largely derived from studies exploring the Hydrogen isotope Deuterium. Its presence in equal ratios in water and asteroids and unequal ratios in many parts of the Earth's crust support the Asteroid Theory.