Planetary Formation, Geologic Time, and Earth's Core Composition

Planetary Origins and Earth's Structure

1.1 The Formation of Earth

Our solar system was formed from a solar nebula, or cloud of gas and dust, that collapsed and condensed about 4.56 billion years ago.

Most of this matter compacted together to form the Sun, while the remainder formed planets, asteroids, and smaller bodies.

Terrestrial vs. Jovian Planets

The outer planets—Jupiter, Saturn, Uranus, and Neptune—condensed at cold temperatures and are made mostly of hydrogen and helium. In contrast, the terrestrial planets (Mercury, Venus, Earth, and Mars), formed closer to the Sun, contain large amounts of iron, silicates (silicon and oxygen), magnesium, and other heavier elements that condense at high temperatures.

About 4 billion years ago, conditions on Earth gradually began to moderate. The planet's surface cooled, allowing water vapor to condense in the atmosphere and fall back as rain. Evidence from ancient sediments indicates that oceans existed on Earth as long ago as 3.5 billion years.

The Paradox of Venus

Venus is sometimes referred to as our “Sister Planet.” However, Venus is a stifling inferno with an average surface temperature greater than 460°C. This effect is produced by Venus's dense atmosphere.

Paradoxically, if Venus had an atmosphere with the same composition as Earth's, Venus would be colder because of the albedo (reflectivity).

Mars: The Cold Neighbor

Mars is not much farther from the Sun than Earth, but it is much colder. Clouds of ice and frozen CO₂ (dry ice) drift over its surface. Frozen ice caps exist at the poles, visible from Earth with a telescope.

Although Mars's atmosphere consists mainly of CO₂, it is 100 times thinner than Earth's atmosphere, so it provides only a small warming effect. Today, all water on Mars is frozen.

Some scientists believe that Earth may not be the only environment in which life could develop.

1.2 Reading Geologic Records

Two Kinds of Time Scales

Geologists use two primary kinds of time scales:

• Relative time refers to sequences—whether one incident occurred before, after, or at the same time as another.
• The geologic time scale is based on observations from sedimentary rocks, which accrete from the bottom up. This scale is discontinuous and incomplete because plate tectonics are constantly reshaping Earth's crust.

2. Earth's Internal Structure

The interior structure of the Earth is layered in spherical shells, like an onion.

These layers can be defined by either their chemical properties.

In Earth's formation, melting would have caused denser substances to sink toward the center in a process called planetary differentiation, while less-dense materials would have migrated to the crust.

The core is thus believed to largely be composed of iron.