Question

How and when did water come into existence on our earth?

You have asked a good question. Planetary scientists have been working on this question for a number of years, and the answers have kept changing, at least until recently. We are pretty certain the the water did not originate at the same distance from the Sun that the Earth is now, that is, 1 AU (Astronomical Unit). The solar nebula, from which all the planets formed, is thought to have been too hot at this distance for water to condense, and water vapor does not react fast enough with rocks to form hydrated minerals. Thus, the water is thought to have originated farther away from the Sun, either in the asteroid belt (2-3.5 AU) or out in the giant planets region (5-30 AU), where comets are thought to have formed.

For many years, comets were a leading candidate for supplying Earth's water. After all, comets are a mixture of 50% water and 50% other materials, mostly silicate dust. Furthermore, the cratering on the Moon is thought to have been produced by a rain of objects, which could have been comets, that hit the Moon (and Earth) between 4.5 and 3.8 billion years ago. This period is called the heavy bombardment period. Comets were thought to be good candidates for producing this bombardment because accretion times in the outer Solar System are longer than in the region where the terrestrial planets formed.

The cometary hypothesis for the origin of Earth's water has a serious problem, however, that was recognized when scientists were able to measure their D/H ratios. Deuterium (D) is an isotope of hydrogen (H) that has a neutron, as well as a proton, in its nucleus. The D/ H ratio in Earth's oceans is 1.56¥10-4. The D/H ratio in the three comets that have been studied (Halley, Hyakatake, and Hale-Bopp) is about twice that value. One might be able to explain the difference between comets and Earth's water if comets had less D than Earth because the normal, lighter isotope of hydrogen escapes more rapidly to space. But if the starting material had more deuterium than Earth's water, it is difficult to see how the D/H ratio could have been reduced to its current value.

This leaves the asteroid belt as the most likely source of Earth's water. The water in meteorites (which come from the asteroid belt) has a wide range of D/H values, but the average is close to that for Earth's water. There are some problems with this theory, too-specifically in trying to explain the concentrations of noble gases, which are different in Earth's atmosphere than in meteorites. However, the asteroid theory has gained a lot of supporters in the past few years. A question that remains controversial is whether most of this water came in during the heavy bombardment period or during the main accretion period. Models of accretion predict that large planetesimals can interact with each other gravitationally and end up on orbits very different from where they formed. Earth's water could be supplied, in theory, by one Moon-sized planetesimal from the outer asteroid belt. But some must have come in during the heavy bombardment period as well.

This was a long-winded answer, but yours was a very tough question..

Prof. James Kasting
Penn State University
January 28, 2002