The Origin of Earth

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Printed: September 5, 2012

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www.ck12.org Concept 1. The Origin of Earth

CONCEPT 1 The Origin of Earth

Lesson Objectives

• Describe how the solar system formed more than 4 billion years ago.• Explain how Earth’s atmosphere has changed over time.• Explain the conditions that allowed the first forms of life to develop on Earth.

Vocabulary

• atmosphere• nuclear fusion• water vapor

Introduction

Imagine a giant camera in space. That camera has recorded pictures of Earth over the last 4.5 billion years. How doyou think Earth looked at different times? How do you think it changed?

Formation the Solar System

Our solar system began about 5 billion years ago. The Sun, planets and other solar system objects all formed atabout the same time.

The Solar Nebula

The Sun and planets formed from a giant cloud of gas and dust. This was the solar nebula. The cloud contractedand began to spin. As it contracted, its temperature and pressure increased. The cloud spun faster, and formed into adisk. Scientists think the solar system at that time looked like these disk-shaped objects in the Orion Nebula (Figure1.1). New stars are forming in the Orion Nebula today.

Solar System Bodies Form

Temperatures and pressures at the center of the cloud were extreme. It was so hot that nuclear fusion reactionsbegan. In these reactions hydrogen fuses to make helium. Extreme amounts of energy are released. Our Sun becamea star! Material in the disk surrounding the Sun collided. Small particles collided and became rocks. Rocks collidedand became boulders. Eventually planets formed from the material (Figure 1.2). Dwarf plants, comets, and asteroidsformed too (Figure 1.3).

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FIGURE 1.1The Orion Nebula is the birthplace of newstars.

FIGURE 1.2The Sun and Inner Planets.

Formation Earth and Moon

Material at a similar distances from the Sun collided together to form each of the planets. Earth grew from materialin its part of space. Moon’s origin was completely different from Earth’s.

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FIGURE 1.3The Kuiper Belt, a ring of icy debris inour solar system just beyond Neptune,contains many solar system bodies.

Earth Forms

Earth formed like the other planets. Different materials in its region of space collided. Eventually the material madea planet. All of the collisions caused Earth to heat up. Rock and metal melted. The molten material separatedinto layers. Gravity pulled the denser material into the center. The lighter elements rose to the surface (Figure 1.4).Because the material separated, Earth’s core is made mostly of iron. Earth’s crust is made mostly of lighter materials.In between the crust and the core is Earth’s mantle, made of solid rock.

FIGURE 1.4Earth’s layers.

Moon Forms

This model for how the moon formed is the best fit of all of the data scientists have about the moon.

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In the early solar system there was a lot of space debris. Asteroids flew around, sometimes striking the planets. Anasteroid the size of Mars smashed into Earth. The huge amount of energy from the impact melted most of Earth. Theasteroid melted too. Material from both Earth and the asteroid was thrown out into orbit. Over time, this materialsmashed together to form our Moon. The lunar surface Moon is about 4.5 billion years old. This means that thecollision happened about 70 million years after Earth formed.

Formation of the Atmosphere and Oceans

An atmosphere is the gases that surround a planet. The early Earth had no atmosphere. Conditions were so hot thatgases were not stable.

Earth’s First Atmosphere

Earth’s first atmosphere was different from the current one. The gases came from two sources. Volcanoes spewedgases into the air. Comets carried in ices from outer space. These ices warmed and became gases. Nitrogen, carbondioxide, hydrogen, and water vapor, or water in gas form, were in the first atmosphere (Figure 1.5). Take a look atthe list of gases. What’s missing? The early atmosphere had almost no oxygen.

FIGURE 1.5Gases from Earth’s interior came through volcanoes and into the atmo-sphere.

The Early Oceans

Earth’s atmosphere slowly cooled. Once it was cooler, water vapor could condense. It changed back to its liquidform. Liquid water could fall to Earth’s surface as rain.

Over millions of years water collected to form the oceans. Water began to cycle on Earth as water evaporated fromthe oceans and returned again as rainfall.

Lesson Summary

• Our solar system began about 5 billion years ago as a nebula contracted, forming our Sun and the planets.

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• Early Earth was a hostile world. The planet was continually bombarded by asteroids. Volcanoes eruptedcontinually, spewing lava and gases into the air.

• Early on the planet was too hot for liquid water or an atmosphere. Eventually both formed.

Lesson Review Questions

Recall

1. What was the solar nebula? Why was it important in the early solar system?

2. Describe how Earth formed?

Apply Concepts

3. Why was nuclear fusion important in the early solar system?

4. Why was the early atmosphere different from the atmosphere we have today?

Think Critically

5. Describe how the different layers of the Earth vary by density.

6. List three ways the Earth is different today from when it was first formed.

7. Suppose that the Earth had been much cooler when it first formed. How would the Earth’s interior be differentthan it is today?

Points to Consider

• How did life on Earth originate?• What were early landmasses like?• What happened when large amounts of oxygen entered the atmosphere?

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