The origin of Earth’s oceans

Little is known about the origin of Earth's oceans, but there are currently two main hypotheses: One is that the water was always here; the other is that it came from somewhere else. The first of these hypotheses stems from the simple observation that, among all the gases released by the Hawaiian volcanoes, steam (water vapor) is the most prevalent. Thus it could be that simple volcanic outgassing of water trapped within Earth's rocky mantle produced the planet's oceans.

The premise of the second hypothesis, that the source of Earth's water is extraterrestrial, seems unlikely at first glance because, at least in our solar system, liquid water is quite rare. In the solid form of ice, however, water in actually quite abundant. Comets, for instance, are made of ice mixed with other debris. Specialists believe that large comets may contain fifty or more cubic kilometers of water in the form of ice. Although 10 of these comets, if they collided with Earth and melted, would provide enough water to fill Lake Erie, another 240 would be needed to fill Lake Superior, and about 7 million would be necessary to fill the Atlantic Ocean. That so many millions of large comets would have impacted Earth seems not very likely, but comets may well have been the source of a great deal of the water on Earth.

Another potential source of extraterrestrial water is the asteroid belt between Mars and Jupiter. Occasionally, the gravitational field of Jupiter shifts one of these asteroids into a new orbit that now crosses the orbit of Earth. Given enough time, this asteroid and Earth will occupy the same intersection point at the same time, resulting in a collision. It has been estimated that forty thousand tons of matter rains down on Earth every year as a result of this process. Examination of these meteorites has shown that the rock in their cores is often infused with a high concentration of water molecules. (A meteorite is the portion of an asteroid that survives its passage through the atmosphere and impact with the ground without being vaporized.)

Although neither the from-here nor the from-elsewhere hypothesis is particularly compelling, both are plausible, and perhaps both are true. It seems most likely that several processes were at work over hundreds of millions of years to make Earth the only planet known to be covered with liquid water. (The Bedside Baccalaureate, edited by David Rubel)