How Did Water Form on Earth?

Water is essential for life, but its origin on Earth is still debated among scientists. Two main theories exist.

Theory 1 — Born With Water

Some scientists argue that Earth formed with water already embedded in its rocky materials. Evidence includes enstatite chondrites — meteorites with a similar isotopic composition to Earth — which contain traces of water-bearing minerals, suggesting Earth’s water may be primordial.

Theory 2 — Asteroid and Comet Delivery

The more widely supported theory is that water was delivered to early Earth through impacts during the Late Heavy Bombardment (~4.1–3.8 billion years ago), a period when asteroids and comets bombarded the inner solar system. Comets and certain asteroids carry ice and water-bearing minerals. The isotopic ratios of hydrogen in Earth’s water closely match those of specific asteroid classes (carbonaceous chondrites).

Sources: planetary.org, cneos.jpl.nasa.gov

Conclusion

Earth’s water likely came from a combination of primordial minerals and delivery by asteroids and comets during the Late Heavy Bombardment. The isotopic evidence currently favours asteroid delivery as the dominant source. Either way, water made Earth unique among inner planets and set the stage for life.