Building a Habitable Earth

The Earth is the only world known, so far, to harbor life. There is nowhere else, at least in the near future, [to which] our species could migrate.

—Carl Sagan, Pale Blue Dot

Most of the Universe is too cold, too hot, too dense, too vacuous, too dark, too bright, or not composed of the right elements to support life. Only planets and moons with solid surface materials provide plausible oases for life as we know it. And even among planets with surfaces, most are highly undesirable. As we noted in the Introduction to this book, of all yet known celestial bodies, Earth is unique in both its physical properties and its proven ability to sustain life. The success of Earth in supporting life for billions of years is the result of a remarkable sequence of physical and biological processes; knowledge of these processes is our main source of insight into the possibilities of life elsewhere. In this chapter we will describe the formation and evolution of the planet Earth. Understanding how Earth attained its life-giving properties will provide a framework for understanding what is required for life and how likely it is to exist on other bodies.

Using Earth to generalize about what life requires is, of course, fraught with uncertainty. Lacking knowledge of any extraterrestrial life forms, we cannot be confident that we understand the optimal or even the minimal conditions necessary to support life beyond this planet. But our planet is an uncontested success in terms of the abundance and variety of life it sports, even though it was certainly sterile soon after its formation. How did that change come about, and what were the physical attributes of Earth that allowed it to become so rich with life?

Earth is the only location in the universe that is known to have life, but it is only one of perhaps millions of habitats in our galaxy, and trillions in the Universe, that might also harbor life. From the biased viewpoint of Earth-lings, however, it does appear that Earth is quite a charmed planet. It has the right properties for the only type of life we know, it formed in the right place in the solar system, and it underwent a most remarkable and unusual set of evolutionary processes. Several of its neighbors in the solar system even played highly fortuitous, supporting roles in making Earth a congenial habitat for life. The near-ideal nature of Earth as a cradle of life can be seen in its prehistory, its origin, its chemical composition, and its early evolution. What are the most important factors that allowed Earth to support advanced life? Earth has offered (1) at least trace amounts of carbon and other important life-forming elements, (2) water on or near the surface, (3) an appropriate atmosphere, (4) a very long period of stability during which the mean surface temperature has allowed liquid water to exist on its surface, and (5) a rich abundance of heavy elements in its core and sprinkled throughout its crust and mantle regions.

Earth is actually the final product of an elaborate sequence of events that occurred over a time span of some 15 billion years, three times the age of Earth itself. Some of these events have predictable outcomes, whereas others are more chaotic, with the final outcome controlled by chance. The evolutionary path that led to life included element formation in the Big Bang and in stars, explosions of stars, formation of interstellar clouds, formation of the solar system, assembly of Earth, and the complex evolution of the planet's interior, surface, oceans, and atmosphere. If some god-like being could be given the opportunity to plan a sequence of events with the express goal of duplicating our "Garden of Eden," that power would face a formidable task. With the best intentions, but limited by natural laws and materials, it is unlikely that Earth could ever be truly replicated. Too many processes in its formation involved sheer luck. Earth-like planets could certainly be made, but each outcome would differ in critical ways. This is well illustrated by the fantastic variety of planets and satellites that formed in the solar system. They all started with similar building materials, but the final products are vastly different from each other. Just as the more familiar evolution of animal life involved many evolutionary pathways with complex and seemingly random branch points, the physical events that led to the formation and evolution of the physical Earth also required an intricate set of nearly irreproducible circumstances.

Any construction project requires that building materials be on site before the actual construction begins. The formation of Earth was no different. Hence the first step is assembling the raw materials.

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