planet. Over time this gas began to escape into the atmosphere and in so doing rapidly changed the composition of the planet’s gaseous envelope. We have a clue about the nature of the gas still trapped within Earth by studying the gas composition of volcanoes. Present-day composition of volcano effluents are 50-60 percent water vapor, 24 percent carbon dioxide, 13 percent sulfur, and about 6 percent nitrogen, with traces of other gases, a composition that differs markedly from the current atmospheric composition.

Our world ocean (since it is interconnected, even though we give parts of it separate names) has also changed its chemistry, mainly by changes in salinity through time. Most scientists believe the oceans have gradually become saltier through time, although a smaller but vocal group advocates that the oceans have become less salty through time. (The amount of salt in the oceans has no effect on the atmosphere and thus plays no part in our story.) The most characteristic aspect of our planet is its envelope of liquid water, and it would seem reasonable to assume that the voluminous oceans of planet Earth were created as part of the natural evolution of the cooling planet. This may not be the case, however. While the outer planets and moons of our solar system, from Jupiter outward, are rich in water, astronomers modeling how solar systems form have discovered that water should be in short supply among the inner parts of the solar system. Because of this, it is now believed that an appreciable volume of Earth’s surface water was brought here from the outer reaches of the solar system by comets impacting the planet early in its history. If this is the case, it indicates that much of our oceans and perhaps an appreciable portion of our atmosphere are exotic to Earth. Most of this delivery happened in the first 500 million years of Earth’s history, and the rain of comets onto the planet during the period from 4.2 billion to 3.8 billion years ago, known as the Heavy Bombardment period, may have caused Earth’s early oceans to be repeatedly vaporized into steam.

The composition of Earth’s atmosphere early in its history is a controversial and heavily researched topic. While the amount of nitrogen may have been similar to that of today, there are abundant and diverse lines of evidence indicating that there was little or no oxygen available. Carbon dioxide, however, would have been present in much higher



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