Planet Earth is surrounded today by a gassy envelope called the atmosphere. How did it acquire this atmosphere?

Question

What will be an ideal response?

Answer 1

Half of the atmosphere lies below an altitude of 3.5 miles (5.6 kilometers), and 90 percent is found below an altitude of 10 miles (16 kilometers). This thin blanket of gas provides the air that humans breathe and protects them from the sun’s heat and dangerous radiation. Scientists are able to identify four stages in the history of Earth’s atmosphere:

Stage 1: No Atmosphere. During the earliest stages of the formation of Earth, the planet was too small to have a strong gravitational field. Any free (that is, not chemically bound) gases in the region would not be held around the planet and would have drifted off into space.

Stage 2: Early Atmosphere—Outgassing or Comets? The gases that formed the planet’s first atmosphere may have bubbled up from volcanoes or been brought to Earth by comets. Proponents of the former process, known as outgassing, offer a precise chemical description of Earth’s first atmosphere by analyzing the sort of gases still emitted by volcanoes today (mainly carbon dioxide and nitrogen). Proponents of the cometary gassing and water vapor theory suggest that comets may have brought to Earth 10 times more water than is now present in the oceans, and 1,000 times more gas than is found in today’s atmosphere. (The collision that formed the moon must have produced an extraordinary amount of heat, whichmay have resulted in the complete loss of gas and water vapor amassed on the planet up to that time.)

Stage 3: The Oxygen Revolution. For a period of over 3 billion years, single-cell organisms floating in the oceans evolved to acquire the ability to photosynthesize, a word that denotesthe conversion of sunlight, water, and carbon dioxide into oxygen and energy-rich carbohydrates. As photosynthesis took place, these living organisms absorbed much of thecarbon dioxide (CO2) in the atmosphere and converted it to oxygen instead, which slowly changed the chemical composition of the atmosphere. At first, oxygen combined with iron to form bands of red, rusted rock; only when most of the exposed iron had been turned to rust did free oxygen begin to accumulate in the atmosphere.

Stage 4: The Atmosphere Today. Photosynthesis was thus responsible for creating the atmosphere of today, which consists of 78 percent nitrogen; 21 percent oxygen; and 1 percent argon, carbon dioxide, and other gases. This provides a wonderful example of the power of living organisms to shape Earth’s surface. The atmospheres of planets without life are very different because they lack processes such as photosynthesis that can continually alter the chemical composition of the surface, so they are shaped purely by physical and chemical processes. The atmosphere of Mars has only 1 percent of the density of Earth and consists mainly of carbon dioxide and a tiny amount of water vapor. Jupiter’s atmosphere is dominated by alternating bands of light cloud regions (where gases are rising and cooling) and dark cloud regions (where gases are sinking). This generates high-speed winds and massive storms, like the Great Red Spot observed from only 26,000 miles (42,000 kilometers) above the surface by Pioneer 11 in 1974. Earth’s atmosphere has come under sustained chemical assault since the Industrial Revolution, and particularly since the second half of the twentieth century, creating the potential for disastrous global warming that will be explored toward the end of the book.

The impact of humans on today’s atmosphere is one more reminder of how life has shaped the planet. It is clear then that living organisms have played a critical role in the processes that have shaped the physical Earth, which means that geological and organic processesare closely intertwined.