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How may comet impacts have added water to early Earth?

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Plumb the depths of geologic time. — geologic history of Earth Discover how old Earth is and discover the timing of important events in Earth's history in our interactive.

The origin of Earth's hydrosphere remains a subject of significant scientific debate, as there is no direct evidence for water on the planet between its formation 4.6 billion years ago and the appearance of the oldest known rocks roughly 3.9 to 4.0 billion years ago.1

One prominent theory suggests that much of Earth's water was delivered by comet and meteor impacts during the planet's early history. Cometary nuclei are composed of roughly equal parts of silicate dust, organic materials, and volatile ices, with water ice accounting for approximately 80 percent of those ices. Because these celestial objects contain significant amounts of water, their frequent collisions with the early Earth could have provided a substantial source of the water that eventually formed the oceans.12

Conversely, other scientists argue that Earth's water originated from chemical reactions within the planet's interior, with water released through degassing as the planet cooled and differentiated.13