Earth is the only planet we know of that harbors life. Crucial for life on Earth is water. Was water on Earth from the beginning? Or did it come from space? 

Approximately 4.5 billion years ago, when the sun was still in its final stages of formation, the solar system was a harsh place. During this time many planets formed, and many planets collided. Because of the turbulence and extreme heat of the inner ring of planets in this period, it’s impossible for water to have existed on Earth at the time.

However, scientists theorize that 400 million years later, during the inner solar system’s 100-million-year period of giant impacts, countless asteroids containing ice crashed into Earth. This ice melted on the volcanic surface of Earth and then evaporated into the planet’s atmosphere. This evaporation turned into rain as Earth’s temperature started to cool down, forming enough rain to create our oceans. In support of this theory, scientists have found that earlier asteroids carried OH⁻ (hydroxide), which turns into H₂O (water) after melting.

Others hypothesize that water on Earth may have come from Theia, a protoplanet during the early years of our solar system that collided with Earth and formed our Moon in the process. Scientists hypothesize that Theia was created in the outer ring of planets, an area less turbulent during those early days of our solar system. Water (or, at this far distance from the sun, ice) could have existed in large quantities on Theia. Theia could have deposited some of this water on Earth through the collision.

In our solar system, water can only exist on asteroids that are carbon rich, which are only found in the outer regions of the solar system. Using mass spectroscopy, a group of scientists at the University of Münster recently found that a large amount of molybdenum, an element mostly found in Earth’s mantle and core, is also present in asteroids from these same outer regions. Perhaps Theia, from the same area as these asteroids, brought these molybdenum isotopes and water to Earth during the collision. Scientists theorize that this small protoplanet could have transferred enough carbon rich material to account for all water on Earth. So, scientists haven’t yet found the single answer to why there’s water on Earth, but some exciting theories are in the works!

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References

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Budde, G., Burkhardt, C., & Kleine, T. (2019). Molybdenum isotopic evidence for the late accretion of outer Solar System material to Earth. Nature Astronomy, 3(8), 736–741. https://doi.org/10.1038/s41550-019

University of Münster. (2019, May 21). Formation of the Moon Brought Water to Earth: New Research Explains How Earth Became a Habitable Planet. ScienceDaily. https://www.sciencedaily.com/releases/2019/05/190521101505.htm

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