Asteroids and comets and the early solar nebula all delivered water to the Earth.
- Comets are 80% water, and would deliver vast amounts of water to Earth, but they’re also volatile, and would have a difficult time surviving the harsh radiation of the young Sun.
- Asteroids have a lower ratio of water, but they could protect that water a little better, delivering less with each catastrophic impact.
- Large amounts of water just came directly from the solar nebula. As we orbited around the young Sun, it passed through the water-rich material in the nebula and scooped it up. Gravitational interactions between the planets would have transferred material around the Solar System, and it would have added to the Earth’s volume of water over hundreds of millions of years.