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Until a few decades ago, planetary scientists thought the early Earth might have attracted small amounts of gases such as hydrogen, helium, and hydrogen compounds from the solar nebula to form a primeval atmosphere . According to that old hypothesis, slow decay of radioactive elements eventually heated Earth’s interior; melted it; caused it to differentiate into core, mantle, and crust; and triggered widespread volcanism. When a volcano erupts, 50 to 80 percent of the gas released is water vapor. The rest is mostly carbon dioxide, nitrogen, and smaller amounts of sulfur gases such as hydrogen sulfide—the rotten-egg gas that you smell if you visit geothermal pools and geysers such as those at Yellowstone National Park. These gases could have diluted the primeval atmosphere and eventually produced a secondary atmosphere rich in carbon dioxide, nitrogen, and water vapor. In contrast, a modern understanding of planet building implies that Earth formed so rapidly that it was substantially heated by the impacts of infalling material, as well as by radioactive decay. If Earth’s surface was molten as it formed, then outgassing would have been continuous, and the early atmosphere would have been rich in carbon dioxide, nitrogen, and water vapor from the beginning. In other words, planetary scientists now think Earth went straight to the volcanic “secondary atmosphere” and never had a hydrogen-rich primeval atmosphere. You will find in the final chapter of the book that the lack of hydrogen in Earth’s original atmosphere has important implications for how life began on our planet. Astronomers also have suspected that some of the abundant water on Earth arrived late in the formation process as a bombardment of volatile-rich planetesimals. These icy bodies, the theory goes, were scattered by the growing mass of the outer planets and by the outward migrations of Saturn, Uranus, and Neptune. The inner Solar System, including Earth, would then have been bombarded by a storm of comets, some of which could have supplied some or all of Earth’s water. That hypothesis once faced a serious objection. Spectroscopic studies of a few comets revealed that the ratio of deuterium to hydrogen in comets does not match the ratio in the water on Earth. Some astronomers thought this meant that the water now on Earth could not have arrived in cometlike planetesimals. However, studies of Comet LINEAR, which broke up in 1999 as it passed near the Sun, show that the water in that comet had a ratio of isotopes similar to water on Earth. Those data suggest that there may be major differences in composition among comets. Icy planetesimals that formed far from the Sun may be richer in deuterium, whereas those that formed closer to the orbit of Jupiter may contain water with isotope ratios more like those in Earth’s water. This is a subject of continuing research, and it shows that the origins of Earth’s atmosphere and oceans are yet to be fully understood. In whatever way Earth’s atmosphere originated, the mix of gases must have changed over time. The young atmosphere would have been rich in water vapor, carbon dioxide, and other gases. As it cooled, the water condensed to form the first oceans. Carbon dioxide is easily soluble in water—which is why carbonated beverages are so easy to manufacture—and the first oceans began to absorb atmospheric carbon dioxide. Once in solution, the carbon dioxide reacted with dissolved substances in the seawater to form silicon dioxide, calcium carbonate (limestone), and other mineral sediments in the ocean floor, freeing the seawater to absorb more carbon dioxide. Thanks to those hemical reactions in the oceans, the carbon dioxide was transferred from the atmosphere to seafloor sediments. When Earth was young, its atmosphere had no free oxygen, that is, oxygen not combined with other elements. Oxygen is very reactive and quickly forms oxides in the soil or combines with iron and other substances dissolved in water. Only the action of plant life keeps a steady supply of oxygen in Earth’s atmosphere via photosynthesis, which makes energy for plants by absorbing carbon dioxide and releasing oxygen. Beginning about 2 billion to 2.5 billion years ago, photosynthetic plants in the oceans had multiplied to the point where they made oxygen at a rate faster than chemical reactions could remove it from the atmosphere. After that time, atmospheric oxygen increased rapidly. (This topic will be discussed again in the final chapter regarding life in the universe.) It is a Common Misconception that there is life on Earth because of oxygen. The truth is exactly the opposite: There is oxygen in Earth’s atmosphere because of life. Except for the minority of creatures, including us, that are animals, most life forms on Earth do not need oxygen, and some are even poisoned by it. An ozone molecule consists of three oxygen atoms linked together (O 3 ). Ozone molecules are very good at absorbing ultraviolet photons. Earth’s lower atmosphere is now protected from ultraviolet radiation by an ozone layer about 15 to 30 km above the surface that exists because the atmosphere contains abundant ordinary oxygen (O 2 ) from which ozone is made. Because the atmosphere of the young Earth did not contain oxygen, an ozone layer could not form, and the Sun’s ultraviolet radiation was able to penetrate deep into the atmosphere. There the energetic ultraviolet photons would have broken up weaker molecules such as water (H 2 O). The hydrogen from the water then escaped to space, and the oxygen formed oxides in the crust. Earth’s atmosphere could not reach its present composition ( Table 20-1) until it was protected by an ozone layer, and that required oxygen.