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)