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How did Earth get its water?
Human cannot live without water. This compound, made of two hydrogen and one oxygen elements, plays such a critical role in our life by making up to 70% of our body. Yet, its origin is still a subject of debate among scientists. Two hypotheses are currently available: The exogenous and indigenous hypotheses. The exogenous hypothesis means the Earth formed in dry condition and later received water after its formation, whereas the indigenous hypothesis favors the concept of Earth was born in wet condition. These controversies suggest that the origin of H2O on Earth is still a mystery.
Scientists who work on this problem have used the ratio of hydrogen isotope to discriminate the source region of Earth's water. Deuterium (D) is an isotope (read: close sibling) of Hydrogen (H), which is an important element in H2O. Determining deuterium/hydrogen (D/H) ratio helps researchers to characterize not only Earth's ocean, but also water in comets, asteroids, and meteorites.
The Exogenous Hypothesis
Exogenous hypothesis suggests that Earth formed in dry condition and later received water from extraterrestrial materials after Earth's formation. This hypothesis is favorable among scientists because the inner solar, the region where Earth formed, was too hot for water condensation. Temperature during Earth's formation reached 800-1000 K (500-700oC), which was too hot for water to be thermodynamically stable. Therefore, scientists who propose this hypothesis suggest that the source of water on Earth must come from the outer region of the solar system.
Scientists believe that extraterrestrial materials, such as comets or meteorites, bombarded the Earth and brought water into this planet during an event called the 'late veneer'. Based on the D/H analysis, the likely candidate for the exogenous water source on Earth is carbonaceous chondrites -- a type of meteorites with high carbon and water content. Although comets also have high water content, they cannot be the source of Earth's water because their D/H ratios are way higher than the Earth's ocean. Carbonaceous chondrites, however, share similar D/H values with Earth's ocean, making these rocky meteorites the most likely source of Earth's water.
The Indigenous Hypothesis
Scientists who propose the indigenous hypothesis believe that Earth gained water in the early stage of its formation and was born in wet condition. This view also suggests that some of the minerals that formed the Earth were hydrous. Thus, exogenous source of water on Earth is not necessary.
A group of scientists has proposed that water adsorption in the inner solar during Earth's formation could be the reason we can enjoy our water everyday. Numerous models and simulations show that silicate grains were able to absorb water and later formed the Earth. Scientists who support this hypothesis also believe that these grains can absorb 1-4 Earth oceans within 800 - 1000 K of the inner solar temperature, indicating that indigenous source of Earth's water is plausible.
Interestingly, geologists who work on samples from Baffin Island, Greenland, and Iceland discovered tiny parts of basaltic rocks called mineral inclusions, which show Earth's primitive D/H ratio. The helium and oxygen isotope ratios of these rocks also suggest an isolated ancient reservoir in Earth's mantle. These evidences suggest that primordial water is present deep inside the Earth.
Water is Old!
Despite the fight among scientists in explaining the origin of Earth's water, they seem to agree on two things: The water that we drink everyday is old and has undergone a long journey for more than 4 billion years. Now, go ahead and take a glass of ancient water! Cheers!