The most likely primary source of Earth's water is asteroids.
While the exact origin of Earth's water remains a topic of scientific debate, compelling evidence points to asteroids as the dominant contributor. Early theories suggested comets, icy bodies from the outer solar system, delivered Earth's water. However, the deuterium-to-hydrogen (D/H) ratio in cometary water doesn't match that of Earth's oceans as well as some asteroids do.
Evidence Supporting Asteroid Origin:
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D/H Ratio Matching: The D/H ratio, a measure of the heavy isotope of hydrogen (deuterium) relative to normal hydrogen, is a key indicator. Certain types of asteroids, particularly carbonaceous chondrites, possess a D/H ratio that closely resembles Earth's ocean water.
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Asteroid Composition: Carbonaceous chondrites are rich in water-bearing minerals. These minerals, hydrated silicates, release water when heated, suggesting a mechanism for water delivery during the early Earth's formation.
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Late Heavy Bombardment: The Late Heavy Bombardment, a period of intense asteroid impacts in the early solar system, provides a plausible timeframe for delivering significant amounts of water to Earth.
Alternative Theories and Considerations:
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Comets: While not the primary source, comets may have contributed a portion of Earth's water. Different types of comets have varying D/H ratios, and some could be a better match than others.
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Volcanic Outgassing: Water trapped within the Earth's mantle could have been released through volcanic activity over geological time. However, this process is unlikely to account for the vast quantity of water present on Earth today.
Summary:
Based on current scientific understanding, asteroids, particularly carbonaceous chondrites, are considered the most probable primary source of Earth's water. The matching D/H ratios and the presence of water-bearing minerals in these asteroids provide strong support for this hypothesis. While other sources like comets and volcanic outgassing may have played a role, their contribution is believed to be secondary compared to that of asteroids.
