LC2. Where Did Earth’s Atmosphere Come From?
Chapter 2
Early Ideas About the Origin of the Solar System
In 1755, Immanuel Kant, a German philosopher, published his theory of the origin of the solar system. Kant acknowledged in his essay that he first got his ideas from reading a detailed account of Thomas Wright’s theory of the universe’s formation. Wright was a British professor of mathematics and natural philosophy. Kant’s ideas went beyond Wright’s and also took into account Isaac Newton’s better-known theory of gravity and the formation of the solar system.
Kant proposed that first there was a huge cloud of dust and gas in space. Under the pull of gravity the gas molecules and dust particles were attracted to each other, and so the cloud grew denser and became smaller. As it did so, the cloud began to revolve, at first slowly, and then more and more rapidly. We can envision this phenomenon as what occurs when an ice skater pulls her arms in toward her body and spins faster as a result.
Over millions of years, the cloud became smaller and matter became more concentrated. Eventually, the gas and dust formed a disk, and then the disk broke into rings around the central core. From these rings the planets and their moons formed while the remainder of the material in the center contracted and became the Sun. A cloud of dust and gas in space is called a nebula, so this explanation of the formation of the solar system is called the “nebula theory.”
Kant’s theory is similar in many respects to modern theories of the origin of the solar system. On the next few pages we will describe the currently favored theory of how the solar system formed. We’ll end the chapter with a description of Earth as it was about a billion years after it formed, with an atmosphere in which human beings could not have survived for more than a few seconds.
I. Modern Ideas About the Origin of the Solar System
Our description of the current theory takes the form of a storyboard for an imaginary movie that represents scientists’ current “best guess” about how the solar system formed:
Scene 1 (time: 5 billion years ago): The scene opens with a magnificent, full-screen view of our galaxy—three or our hundred billion stars arranged in a pinwheel, with nebulae (glowing clouds of gas and dust) in some of the spiral arms. The camera zooms in on one of these clouds, and we see that it is slowly spinning. A star explodes nearby. Shock waves from the exploding star compress the cloud, so part of it becomes denser. The denser cloud starts to spin faster as it gets smaller. This is the solar nebula—the cloud of gas and dust that will evolve into the solar system.
Scene 2 (time: 4.6 billion years ago): While Kant thought that the solar nebula would divide into rings, modern theory suggests that the gas cloud condensed into small chunks of rock and metal. When two chunks collided, they would stick together because of gravity. Bigger and bigger chunks would be created in this way, until huge balls of metal and rock formed, eventually creating the rocky planets we call Mercury, Venus, Earth, and Mars. (Gravity is the force that acts between all objects in the universe, pulling them together.)
Scene 3 (time: 4.6 billion years ago): Billions of meteoroids—small chunks of rock and metal in space—continue to orbit the Sun, occasionally falling toward the large body that will become our home planet. Occasionally, larger bodies, called asteroids, slam into Earth, sometimes adding mass, and sometimes blasting material back into space.
Scene 4 (time: 4.5 billion years ago): As Earth forms, billions of meteors rain down on it, heating it continuously. More heat is added by radioactive elements. The heat is so intense when Earth first forms that its entire surface is a sea of molten lava. Above the lava is Earth’s first atmosphere, which probably consists of gases attracted by gravity from interplanetary space—mostly hydrogen.
Scene 5 (time: 4.5 billion years ago): As Earth forms, billions of meteors rain down on it, heating it continuously. More heat is added by radioactive elements. The heat is so intense when Earth first forms that its entire surface is a sea of molten lava. Above the lava is Earth’s first atmosphere, which probably consists of gases attracted by gravity from interplanetary space—mostly hydrogen.
Scene 6 (time: 4.3 billion years ago): The daily bombardment of Earth’s surface by meteorites slows, and Earth begins to cool. While the crust solidifies, there are hundreds of active volcanoes. When molten rock from Earth’s interior moves closer to the surface, the pressure on the molten rock decreases, and the gases within it start to form bubbles—in a way similar to uncapping a bottle of soda and releasing the pressure. We see the volcanoes ejecting ash and lava as well as huge quantities of gas, resulting in an atmosphere that consists of about 60% water vapor, 25% carbon dioxide, and the rest nitrogen and sulfur.
Scene 7 (time: 4.0 billion years ago): When the atmosphere cools further, water vapor condenses and forms clouds. When the clouds cool even further, it rains. Eventually there is enough rain to begin filling areas where the crust is depressed, creating the early ocean.
Scene 8 (time: 3.9 billion years ago): In this scene, the ocean plays an important role in stabilizing Earth’s temperature. Water absorbs carbon dioxide, one of the major heat-holding gases of the atmosphere. As carbon dioxide is removed from the atmosphere, the entire Earth cools. (The ocean is probably the reason why Earth did not become as hot and inhospitable as Venus. Today, Venus’s atmosphere is mostly carbon dioxide and so it remains too hot for life as we know it.) At the end of the movie—not quite a billion years after our planet first formed—we see that our oceans continue to absorb carbon dioxide, keeping our planet cool enough for life to begin.
LC2.1. Investigation: Timeline
Create a timeline representing the entire history of our planet. Use a long roll of paper about 2 inches wide (like the paper rolls used in cash registers). In your timeline a paper strip 1 meter long will represent 100 million (100,000,000) years. Cut a paper strip to represent the entire history of our planet. How long should it be?
Using a felt-tipped marker, indicate major events in Earth’s history as described in this chapter. Using scotch tape or masking tape, put up the strip of paper in your classroom so you can view Earth’s history as one continuous timeline.
As you complete each chapter in this book, use markers of different colors to add information about the evolution of life and major changes in climate.
II. Conclusion
Our planet evolved from a huge cloud of gas and dust in space. The Solar System that we know today—including Earth, our home planet—was formed when gravity pulled together the tiny particles of dust and gas molecules into a number of large spherical bodies.
By about 3.9 billion years ago, Earth had acquired an atmosphere consisting of water vapor, carbon dioxide, nitrogen, and a few other gases. There was some dry land and an ocean, but as yet there was no oxygen in the atmosphere and as far as we can tell, no life.
How Earth evolved an atmosphere containing oxygen, and land and seas teeming with life, is more amazing than fiction. It has taken the creativity of thousands of scientists to figure it out. In the next chapter we’ll start by looking at the evidence used by scientists to learn about Earth’s past history and the evolution of life—evidence embedded deep within Earth.