Earth's water most likely had an extraplanetary origin: water was delivered to the early Earth by asteroids. Earth then retained this water because its gravity prevented most of it from escaping and temperatures became low enough for water vapour to condense.
This is Additional Higher Level (AHL) content in IB Biology subtopic A1.1 Water. The explanation distinguishes between the origin of Earth's water and the conditions that allowed it to remain on the planet.
| Stage | Mechanism |
|---|---|
| Delivery | Water-containing asteroids collided with the early Earth, bringing water from beyond the planet. |
| Retention by gravity | Earth's gravitational field was strong enough to prevent most water vapour from escaping into space. |
| Condensation | As Earth cooled, temperatures became sufficiently low for water vapour to condense into liquid water. |
| Long-term presence | The retention of abundant water over billions of years provided the medium in which life could originate and evolve. |
Water is essential because it acts as the medium for life. Most metabolic reactions occur in aqueous solution, and water enables the transport and interaction of many hydrophilic substances. Its long-term presence therefore made the evolution and persistence of life possible.
A common misconception is that IB students must discuss several competing sources, such as comets, volcanic outgassing and asteroids. For this syllabus statement, the required hypothesis for the extraplanetary origin of water is limited to asteroids. Another misconception is that gravity produced the water; gravity only helped Earth retain water after its arrival.
Exam technique: For an “outline” or “explain” question, include both parts: asteroids delivered water, while gravity and sufficiently low temperatures allowed its retention and condensation. Do not give the source without explaining why the water remained on Earth.