12.1 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 4.6 billion years. How do you 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 at about 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 contracted and began to spin. As it contracted, its temperature and pressure increased. The cloud spun faster, and formed into a disk. Scientists think the solar system at that time looked like these disk-shaped objects in the Orion Nebula (Figure below). New stars are forming in the Orion Nebula today.

The Orion Nebula is the birthplace of new stars.

Solar System Bodies Form

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

The Inner Planets.

The Kuiper Belt, a ring of icy debris in our solar system just beyond Neptune, contains many solar system bodies.

Formation Earth and Moon

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

Earth Forms

Earth formed like the other planets. Different materials in its region of space collided. Eventually the material made a planet. All of the collisions caused Earth to heat up. Rock and metal melted. The molten material separated into layers. Gravity pulled the denser material into the center. The lighter elements rose to the surface (Figure below). 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.

Earth'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.

In the early solar system there was a lot of space debris. Asteroids flew around, sometimes striking the planets. An asteroid the size of Mars smashed into Earth. The huge amount of energy from the impact melted most of Earth. The asteroid melted too. Material from both Earth and the asteroid was thrown out into orbit. Over time, this material smashed together to form our Moon. The lunar surface is about 4.5 billion years old. This means that the collision 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 that gases were not stable.

Earth's First Atmosphere

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

Gases from Earth’s interior came through volcanoes and into the atmosphere.

The Early Oceans

Earth’s atmosphere slowly cooled. Once it was cooler, water vapor could condense. It changed back to its liquid form. 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 from the 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.
• Early Earth was a hostile world. The planet was continually bombarded by asteroids. Volcanoes erupted continually, 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 different than 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?