![]() ![]() Instead it is bound into the molecular structure of “hydrated” rocks and minerals that have been dragged below the crust by tectonic processes. More water may lurk deep underneath our feet, in the planet's rocky mantle, which extends more than 3,000 kilometers from the crust down to the cusp of the liquid-iron core. Freshwater locked up in polar ice caps, clouds, rivers, lakes, soil and Earth's biota contributes only a tiny fraction to this total. Put another way, if our planet was a 300,000-kilogram Boeing 777, then all the water in the oceans would have the mass of a single passenger. Yet many people are surprised to learn that all this ocean water constitutes only about 0.02 percent of the mass of Earth. The oceans, which have an average depth of four kilometers, hold enough water to fill a sphere more than 1,300 kilometers across. An ocean planet drier than a boneĪs viewed from space, planet Earth might instead be “planet Ocean.” Water covers more than two thirds of the surface and makes up more than two thirds of the typical earthling. As timeless and ineffable as Earth's oceans may seem, new evidence is bringing us closer to answering exactly how and when they formed and whether it was mostly comets, asteroids or some entirely different delivery mechanism that brought all that water to our once dry planet. Even so, many aspects of the grand tale of our ocean's formation remain mysterious and are subjects of intensive ongoing research. ![]() Looking into the depths of interstellar space, we can see the same primordial processes that took place here in our own solar system unfolding far away, before our telescopic eyes. Recently further evidence of snow lines and late-stage planetesimal collisions has emerged from observations of other stars in the midst of forming planets. Astronomers call this transition point the “snow line,” and conventional wisdom holds that most of Earth's water came from beyond it, in showers of icy asteroids and comets that were perhaps flung down into the inner solar system by the outer giant planets during the last gasps of planet formation. As dry, rocky bodies were growing rapidly near the sun, farther out, somewhere in the vicinity of what is now the asteroid belt and Jupiter, temperatures in the disk were low enough to allow water and other volatiles to form ices. In the disk's inner regions near the sun, intense frictional heating of the gas and more sunlight probably cooked off hydrogen and other light elements, leaving only relatively dry material from which to form planets. Many of the planetesimals left over from planet formation then became the objects that we know today as asteroids and comets. The formation of rocky worlds is thought to be a progressive, step-by-step process where smaller objects in the disk collide and stick together to form larger ones: microscopic grains become pebbles, which become boulders, which become kilometer-scale planetary building blocks called planetesimals. ![]() As the cloud that would become our solar system collapsed, its angular momentum flattened the material into a whirling disk, in which all the planets formed. Their relative lack of water is a product of where and how they were born. Earth and its neighbors Mercury, Venus and Mars are rocky, not water worlds. Much of the remaining oxygen bonded with other atoms, such as carbon and magnesium, but the hydrogen and oxygen left over were sufficient to produce several times more water than rock in our solar system.Īnd yet this is not what we see. Most of the gas was sopped up by the sun and the gas-giant planets, which formed earlier than the rocky planets. That enrichment is no surprise because hydrogen and oxygen are also the first and third most abundant elements in the universe (chemically inert helium is the second). The cloud was rich with hydrogen and oxygen, the two atomic ingredients for water, H2O. Its water-as well as every drop of rain, every gust of humid air and every sip from your cup-is a memory from eons ago, when the seas literally fell from the sky.Īll the water in our solar system can be traced to the giant primordial cloud of gas and dust that collapsed to form the sun and planets more than four and a half billion years ago. Today we realize that Earth's global ocean has not been around forever. In numerous creation myths, a watery abyss was present before the emergence of land and even light. Standing on the seashore, watching waves roll in from over the horizon-it is easy to see the ocean as something timeless. ![]()
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