Why is there liquid water on Earth?

The liquid water covering three-quarters of the surface of our planet, which we believe is essential to life, can only remain there because of four things: 

  1. Earth’s geology
  2. Earth’s global magnetic field—generated by convection and rotation within its liquid outer core (the geodynamo)
  3. Earth’s atmosphere
  4. Earth’s watery surface

That’s right: liquid water can exist on Earth’s surface because liquid water exists on Earth’s surface. That statement seems tautological, but it actually isn’t intended to be. The reason water has to be included in this list is that if it weren’t there each of the other three items in the list would either vanish or change drastically and, as a result, there would no longer be conditions suitable even to sustain liquid water on the surface of our planet.

In fact, this is true of all four items in the above list: remove or significantly alter any one of them, and the other three would either vanish or be significantly altered as well, and as a result there could be no liquid water on the surface of our planet. Earth’s internal heat drives both geological activity and the fluid motion within its liquid outer core that sustains the magnetic field. Together these maintain a stable atmosphere and surface conditions in which liquid water can persist, while the presence of that water in turn regulates surface temperature and influences geological processes such as plate tectonics and erosion.

The sequence just stated tells you roughly what would happen if any of the four items in this list were removed; e.g. without a magnetic field, Earth would lose its atmosphere, without which liquid water could not remain on its surface, which would then affect its geology. However, knowing “what” is different from how “how” or “why” this sequence of events would occur. The answers to those deeper questions are required if you are to understand “what” is going on.

Learning Activity

The following Crash Course Astronomy video covers most of the relevant details while using graphics that enhance the description. It is therefore an excellent source for learning what you need to know about

  • the Earth’s different layers—its core, mantle and crust
  • plate tectonics,
  • different types of volcanoes and their role in replenishing Earth’s atmosphere
  • the sources of heat in Earth’s core, and the relationship between Earth’s core and its global magnetic field,
  • the manner in which Earth’s atmosphere is protected by its magnetic field, and the cause of the auroras.
  • the greenhouse effect, and the potential dangers of excess greenhouse gases such as carbon dioxide (CO2), methane (CH4), and water vapour (H2O).

As you watch the video, you’ll see that it is packed with information. Take careful notes on each of the above points, pausing or going back as necessary to ensure you understand them.

 

In terms of explaining how or why each of the above four aspects of the Earth link together to create the necessary conditions for water to exist on our planet, all this video lacks is an explanation of the geological impact of removing all the liquid water from its surface. However, the omission actually sets us up very well as we move on to explore Venus, where scientists think liquid water actually was the first link in the chain to have broken, when it boiled off the planet’s surface early in its history. Therefore, rather than having to investigate what would happen hypothetically, Venus effectively enables us to investigate the scientific consequence, a few billion years down the road, of boiling off all the Earth’s liquid water. Mars, conversely, illustrates the long-term atmospheric loss that can occur without sufficient magnetic shielding or volcanic replenishment.