What is the origin of the life on this planet? That is certainly an important question, and one that’s very difficult to answer. An equally important, yet slightly less difficult, question to answer concerns the origin of the single most important substance to the development of life: water. There are three well-established theories on the origin of water on the earth, that it was released by volcanoes, transferred by asteroids and meteorites, or delivered by comets.
The first theory of the origin of water concerns volcanic activity. It says that as the earth cooled, certain elements broke apart and recombined to form new substances. Two of these elements were hydrogen, which broke away from hydrocarbons like methane, and oxygen, which broke away from iron oxide. These elements were carried up to the surface by molten magma and spewed into the atmosphere by volcanoes. Then they combined to form water and fell to the earth as rain. While this theory explains the accumulation of water at the surface of the earth, it does not seem sufficient to account for a substance that covers seventy percent of the surface of the earth.
Another popular theory says that water was delivered to earth by earth-grazing comets. While water molecules have been found within nebulae at remote corners of the universe; analysis of water contained within comet ice has shown that it isn’t the same as water in the oceans of the earth. The water from comets contains higher levels of deuterium, an isotope of hydrogen, than ocean water. Deuterium is found in one of about every thirty-two hundred molecules of ocean water. Any explanation of the origin of water on earth would have to account for the disperse ratio of deuterium in the earth’s oceans. Recently, an international team of astronomers discovered a comet that does contain water with the correct deuterium ratio. This means it is possible that comets are responsible for at least some of the water on the earth.
The third mainstream theory for the origin of water is that the necessary elements were carried here on the backs of meteorites and asteroids as they collided with the earth. Astronomers have refuted this theory, saying that the concentration of xenon gas in the earth atmosphere is too low to account for mass collisions. Meteorites and asteroids are known to contain this inert gas, and a high number of past collisions would definitely have left large traces. As with comets, meteorites and asteroids may have played only a minor role in the origin of water on this planet.
Although comets and meteorites may have had a part in the delivery of water to the earth, evidence showed that neither one of them could be the primary method. A new theory has been proposed that says water adhered to the interstellar dust that accreted to form this planet. There was a question as to whether the adhesive bonds would have been strong enough to withstand the conditions of accretion. Computer simulations carried out at UCL in London have shown that the fractal surfaces of the interstellar dust grains are suitable for the retention of water under such extreme conditions.
Water covers about seventy percent of the earth’s surface. If comets and meteors had been the primary delivery methods of water, that number would be higher considering the high rates of cosmic activity over the last 4.5 billion years that the earth has existed. It seems most likely that the primary source of the water on this planet was the accretion of interstellar dust.