The Making of a Water Wonderland
Defant, Marc J., The World and I
Conceived in space, born from molten rock or flying in as snowballs, and shielded from solar winds by a great magnetic umbrella, the ubiquitous liquid filling our planet's basins and coursing through our veins has a story to tell us.
Water is probably the most important compound on Earth. Eighty percent of all life on Earth lives in the oceans. Water is the ice of glaciers, the potable liquid filling underground aquifers across the globe, and the stuff of rivers, lakes, and oceans. The precious fluid has carved Earth's caves and weathered its land features. Water gives us a cloudy day, a foggy night, and a winter landscape blanketed in white. Three- quarters of Earth's surface is covered by water, but only 2.5 percent of it is fresh and only a small fraction of that is available for human use. (Much of the fresh water exists as ice in Antarctica and Greenland.Even more of it is underground in aquifers.)
Water, making up between 50 and 70 percent of our bodily weight, is an essential medium of all body fluids. Even our teeth have a 5 percent water content. The approximately 6 billion people on Earth carry a total of about 300 billion pounds of water within them as they walk upon the land. That's enough to fill 27 Lake Michigans. And these figures don't even consider the other life-forms on land which, like us, are predominantly water. Water is a requisite for all life and, fortunately, Earth is a water wonderland. How did it come to be so?
Water and the solar system
A startling fact about our universe is its virtual absence of water. It is made up of 75 percent hydrogen, 24 percent helium, and about 1 percent of the rest of the elements, one of which is the oxygen that combines with hydrogen to make water. Earth is unique in its large collection of liquid water. To gain an understanding of how Earth became a repository for so much liquid water requires a brief review of our solar system's history.
Our solar system is thought to have originated from a cloud, called a solar nebula, consisting primarily of hydrogen and helium produced during the Big Bang. Elsewhere, dying stars would have produced oxygen and other heavier elements, and all must have been injected into the solar nebula through a supernova, the explosive death of a large star. Once in the solar nebula, oxygen, craving electrons, would have combined with hydrogen, its preferred electron source. Thus began our water.
Computer models show that a slightly rotating solar nebula would collapse inward from gravitational forces into a disklike structure, with most of the mass concentrated in the center. In fact, the Sun makes up 99.9 percent of the mass of our solar system. As the solar nebula coalesced to form the Sun, hydrogen atoms were stripped of their electrons, exposing naked protons. When these fused into helium nuclei, the released nuclear energies lit up the heavens.
At the same time, the Sun would have thrown off an immense flux of charged particles, an electrified solar wind. It would have swept lighter elements and molecules, including water, away from the newly forming planets nearest the Sun (that is, the terrestrial planets: Mercury, Venus, Earth, and Mars).
The blue planet
Conditions for today's seasons and lunar tides are thought to have been established during the early formation of the solar system roughly 4.5 billion years ago, when a Mars-sized object probably struck a glancing blow to a barren, partially molten Earth. That impact likely produced not only the tilt of Earth's axis and the capture of the Moon but the melting of both the impactor and Earth. In the process, computer simulations show, Earth would have captured water, other vaporlike compounds, and the impacting object's iron-rich core.
Earth is like an onion, with progressively less dense layers from the interior outward. These layers range from the dense core's iron and nickel to the mantle's lighter minerals, the lightest minerals forming the crust, and finally the liquid and gaseous oceans of the hydrosphere and atmosphere. …