Chapter 5: The Earth-Moon System

Earth has a hydrosphere, lithosphere, atmosphere, and biosphere.

Our Solar System alone has eight planets and dozens of moons. Yet the only one of these planetary bodies that is absolutely suitable for life is Earth. You might pause for a moment to wonder what Earth has that the other bodies don't. There is increasing evidence that some of the other planets once hosted oceans, but Earth is the only planet to maintain significant amounts of water. Scientists surmise that liquid subsurface oceans are present on Jupiter's large moon Europa. As a result, this moon is now receiving attention from scientists of many disciplines as a potential habitat for life. 

If Earth is unique because of its ready supply of water, how did the water get there? It turns out there is no simple answer to this question. The Earth is approximately 4.5 billion years old. The oldest physical remnants we have obtained from the Earth are crystals of zirconium silicate — or zircons — which were thought to have formed 4.4 billion years ago. These crystals seem to have formed in an aqueous environment, indicating that the Earth had significant water soon after formation. There is a paucity of evidence to help scientists concretely identify the source of Earth's water. However, there are several plausible explanations for our oceans.

A diagram of the Earth.

Earth and all of the planets in the Solar System formed at roughly the same time. As one of four terrestrial planets, Earth is composed primarily of rocky material. During accretion, the dusty debris that came together to form Earth would have had relatively little water due to the high temperatures associated with the close proximity to our host star. Some scientists believe that a little water could have been incorporated into the rocks and dust that eventually became our planet. Just how much water is unclear. Volcanic activity on Earth could have released this water into the atmosphere. It is probable, however, that water in the early atmosphere would have been broken apart (photodissociated) by ultraviolet (UV) light, turning water into oxygen and hydrogen. Hydrogen high up in the atmosphere could have been lost to outer space because it is so light and escapes from Earth's gravitational attraction easily. Even if the incorporation of water into Earth during accretion was more favorable than predicted, processes such as photodissociation of water by UV light and subsequent loss of hydrogen cause us to look for other sources of water.

When we look at our Solar System, we notice that there are large amounts of water in the outer regions. This water is mostly in the form of ice, but it is still prevalent. Comets are bodies in the outermost regions of the Solar System that contain large amounts of water. They also have highly elliptical orbits that frequently bring them into the inner solar system — and sometimes on a path that will cause them to collide with Earth.  Although we have a general sense of the composition of comets — they are like dirty snowballs that contain organic matter — we do not yet know for sure their exact makeup. Astronomers have studied at least four well-known comets: Halley's Comet and Comets Hyakutake, Hale-Bopp, and 67P/Churyumov-Gerasimenko. They have noticed one commonality between them. In these comets, the percentage of deuterium (a form of hydrogen with double the mass of normal hydrogen) is twice that of what we find in average seawater. It is not yet clear whether or not these comets are representative of the types of comets that would have been colliding with early Earth. Regardless, if Earth had received the majority of its water from comets with similar composition, we would expect the percentage of deuterium to be similar. We could even accept it if the percentage of deuterium in our oceans were greater than that of the comets, due to photodissociation and subsequent loss of deuterium to outer space. Neither of these is the case, so we cannot currently make any conclusions about the contribution of water by comets. Based on the deuterium composition of the oceans, scientists have estimated that no more than 10% of the water on Earth originally came from comet collisions.

Although we have just focused on comets as a possible water bearer for Earth, there are other planetary bodies to consider. Meteoritic material originating in the asteroid belt could also be responsible for bringing water to Earth. Objects in the outer part of the asteroid belt are predicted to contain up to twenty percent water. Jupiter's large mass and gravitational influence could easily have perturbed the orbits of asteroids in the belt, sending some of them on a one-way trip to Earth. The composition of asteroids within the belt varies depending on their distance from the Sun, so it is difficult to estimate exactly how much water could have been contributed to Earth. However, scientists such as Alessandro Morbidelli have predicted that if asteroids had collided with Earth to bring it the majority of its water, delivery by comets could have contributed some water, resulting in more appropriate deuterium to hydrogen ratios in Earth's oceans.

Like so many things in science, the exact origin of water on Earth is still unknown. Yet scientists continue to make observations, collect data, and analyze information to try and develop a complete understanding of the natural world. The current best bet is that asteroids delivered most of the Earth's water within the first 100 million years after it formed.