On the Origin of Earth's Water
The Earth today is covered in water—huge oceans spanning across far more area than Earth’s land. Yet early in the solar system’s formation, violent gusts of solar wind stripped the inner planets of volatiles, including water. So how is it possible that Earth harbors so much of it now? Where did Earth’s water come from? Understanding the answers to these questions is key for understanding planetary formation.
Our solar system began as a massive cloud of gas (mainly hydrogen) and dust, called a molecular cloud. This cloud underwent gravitational collapse, which precipitated a swirling movement—the cloud began to spin. Most of the material was concentrated in the center of the cloud (due to gravity) and began to form our proto-Sun. Meanwhile the rest of the material continued to swirl around it, in a disk referred to as the solar nebula.
Within the solar nebula, the slow process of accretion began. Particles collided with one another to build up larger and larger pieces of material, similar to using a piece of Play Doh to pick up other pieces (creating a larger and larger mass of the substance). The material continued to accrete to form planetesimals, or pre-planetary bodies. Planetesimals obtained sufficient mass to gravitationally alter the movement of other bodies, which made collisions more common and accelerated the process of accretion. The planetesimals grew into “planetary embryos” which gained enough mass to eventually clear out their orbits of most of the remaining debris.
Within our solar system there is a dividing boundary called the frost line. The frost line is the imaginary line which splits the solar system between where it is warm enough to harbor liquid volatiles (such as water) and where it is cold enough for them to freeze. It is the point away from the Sun beyond which volatiles cannot remain in their liquid state. It could be thought of as the dividing line between the inner and outer planets within our solar system (Ingersoll 2015).
The Sun ultimately amassed enough material and reached a sufficient temperature to begin the process of nuclear fusion, fusing atoms of hydrogen into helium. The onset of this process spurred a massive ejection of violent gusts of solar wind, which stripped the inner planets of much of their atmospheres and volatiles. This means that the Earth either had some way to retain some of its water, its water was delivered later in its formation, or some combination of the two.
One of the leading theories is delivery via comets and asteroids. We know from research and studies of comets and asteroids that many contain vast amounts of water, and it’s possible that the Earth was bombarded by a lot of them. This would obviously have increased the amount of water on the planet. It would take a very high number of impacts to amass all the water we have on Earth today, but perhaps comets and asteroids did not do it alone.
From studies of the composition of our water it seems the Earth’s water can’t have come exclusively from comets and asteroids, so there must be another factor in play. According to an article in Nature science journal, “Measurements of the chemical composition of Moon rocks suggest that Earth was born with its water already present, rather than having the precious liquid delivered several hundred million years later” (Cowen 2013).
One thing that is helping to source the Earth’s water is chemical isotope analysis. Some water is made up of oxygen and "normal" hydrogen (the common H2O we know and love), but some is made of a heavier isotope of hydrogen called deuterium. It can be thought of as something like a ‘chemical fingerprint.’ In studying the isotopic ratio of each in rock samples from the Earth and the moon, it appears there must be a common source for each body (Cowen 2013).
However, it seems not all of Earth’s water was delivered by comets and/or asteroids. A team of researchers studying the isotopic content of rocks specifically located in Baffin Island, Canada have discovered evidence supporting the idea of Earth having “native water”—water not delivered by comets or asteroids, but here since its formation. The rocks the team studied were sourced “directly from the mantle, and were not been affected by material from the crust. In them, the researchers found glass crystals that have trapped small droplets of water” (Carpineti 2015). By studying the water contained within the glass crystals, the researchers discovered that it was of the same composition as Earth’s water today. So how did it survive during the solar system’s chaotic formation? Why was it not scorched off with the rest?
Deep within the Earth, it is possible that volatiles would have been safer. There, water could have been preserved and expelled or otherwise brought to the surface at a later date--at a time when the temperature and other conditions were right to support its preservation at the surface of the planet. Water vapor in the Earth's interior acts as a propellant for volcanoes, producing the blasting effect we all associate volcanoes with.
The fact that there is this water vapor harbored within the Earth now could be a key factor in understanding how the Earth’s native water likely survived the violent gusts of solar wind present earlier in the solar system’s formation. If water was contained deep within the Earth, it is very possible that it would have been protected from the forces that would have blasted away the surface water. Then it could be later expelled via volcanic eruptions, geysers, etc to bring it up to the Earth’s surface. It is most likely that this occurred along with water delivery via comets and/or asteroids to produce the oceans that we have now.
Research is being ever continued to discover more about Earth’s history, including the origin of its water. Additional missions and studies will be performed on comets and asteroids as well as samples found on Earth to learn more about potential sources and links. Understanding this topic will lead to further overall understanding of planetary formation, and perhaps solar system formation altogether.