By Robert Hazen, Ph.D., George Mason University
Earth is, in many respects, like the other planets of the solar system. It orbits in the same plane and in the same direction. It’s composed of the same chemical elements and obeys the same physical laws of motion, energy, electricity, and magnetism. But the Earth is also unique; it has extensive surface deposits of liquid water, which is the essential medium for life as we know it.
The Earth Is Unique
The formation of the Earth happened 4.5 billion years ago. Many of the characteristics of Earth seem well suited to life, to a habitable surface. For instance, Earth is about 150 million kilometers from the Sun. That’s close enough to maintain liquid water but not so close as to boil it away.
Plus, the 24-hour rotation period is short enough to prevent extreme temperatures between night and day, but it’s slow enough to prevent extremely violent weather patterns that could disrupt the surface. Moreover, the Earth has a radius of about 7,000 kilometers; that’s large enough that the fast-moving water molecules can’t evaporate off the planet and escape into space, as they have done on Mars, which is somewhat smaller.
So here we have a planet with the ideal distance, rotation rate, and size; it also has liquid water—all characteristics that may be essential for life as we know it.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Formation of the Earth
Let’s turn our attention to Earth’s earliest period, the period called the Great Bombardment, when the Earth was accreting in its final stages. A proto-Earth had formed, a sphere, which was almost the size of the present-day Earth; but that sphere was molten, it was chaotic, it was a place of hellish violence because you had constant raining-down of asteroids and comets blasting the surface. You had volcanoes spewing molten lava and gases in great clouds and billowing fountains of red-hot molten rock onto the surface.
As that bombardment subsided, the red-hot molten sphere slowly cooled. A black crust appeared: the first rocks. Although that crust was constantly broken by volcanic activity, you gradually had areas of land forming. You gradually had an atmosphere forming from the volcanic gases that came from the inside. You soon had clouds forming; the first rains, the first oceans would then form on the surface.
Eventually, as the surface cooled, we began to see an arrangement of oceans and land, something analogous to what we see today. A late-stage impact probably formed the Moon. That great impact event was where a Mars-like object hit the Earth, ripped out a chunk of the mantle, and formed the Moon. Then the process of forming the ocean and the atmosphere started again, but eventually things settled down.
Learn more about the Big Bang theory.
So there is now a sphere of molten rock; but what happens in that sphere? You get the process called differentiation: a separation of different materials deep inside the Earth by gravity, by density. This process has given the Earth a radial structure, not unlike the layers of an onion.
The densest material was primarily metals: very dense iron metal, and nickel metal, which joined it; also probably most of the gold and the silver, the uranium, the lead, many other elements which would have sunk in that dense, metallic center.
Ultimately, we formed a metal core. That metal core increased in size until it was a sphere at the center of the Earth, about 3,470 kilometers in radius. The dense core represents only 17 percent of the Earth’s volume, but 33 percent of its mass because it’s so dense.
Learn more about the Earth’s topography.
By contrast, the lightest material, including many elements such as sodium, potassium, hydrogen, and lithium; these elements don’t form dense minerals at all, and so they basically float up to the surface.
They concentrate in a relatively thin surface layer called the crust, and that crust is typically less than a few tens of kilometers thick. That’s where we live, that’s where all the action that we see is—a very, very tiny fraction of the Earth’s volume, indeed.
The remaining material—that’s about 80 percent of the volume of the Earth and more or less two-thirds of the mass of the Earth— formed the Earth’s mantle. The mantle is composed primarily of elements like oxygen, silicon, magnesium, and iron. Almost 98 percent of the mantle is just those four elements. Of these four, oxygen is by far the most abundant, so most of the different minerals deep inside the Earth, in the mantle, are oxides: oxides of silicon, oxides of magnesium, oxides of iron.
Common Questions about the Formation of the Earth as a Unique Planet
Earth has unique features that have led to life thriving on the planet. Features like its distance from the Sun, its 24-hour rotation period, its radius, and availability of water on the surface make it unique.
The Great Bombardment was the period when the Earth was accreting in its final stages. A proto-Earth had formed, a sphere; but that sphere was molten and chaotic, with constant raining-down of asteroids and comets blasting the surface. There were volcanoes spewing molten lava and gases in great clouds and billowing fountains of red-hot molten rock onto the surface.
The differentiation process is the process of separation of different materials deep inside the Earth by gravity, by density. This process has given the Earth a radial structure, not unlike the layers of an onion.