Lesson Objectives

• Explain how scientists think the Moon formed.
• Describe the features of the Moon.

Vocabulary

• crater
• lunar
• maria
• terrae

Introduction

On July 20, 1969, hundreds of millions of people all over the world witnessed something incredible. Never before had a human being walked on a planetary body other than Earth. But on that day, Neil Armstrong and Buzz Aldrin walked on the Moon (Figure below). More than 30 years later, the Moon remains the only place that humans have visited outside of our home planet.

Human explorations of the Moon, along with visits by rovers and satellites, have helped scientists learn a great deal about the geology of Earth’s only natural satellite. Much of what we know about the Moon was learned by astronauts visiting the Moon and from data collected by the Apollo missions.

Lunar Characteristics

The Moon is Earth’s only natural satellite, a body that moves around a larger body in space. The Moon orbits Earth for the same reason Earth orbits the Sun — gravity. The Moon is 3,476 km in diameter, about one-fourth the size of Earth. The satellite is also not as dense as the Earth; gravity on the Moon is only one-sixth as strong as it is on Earth. An astronaut can jump six times as high on the Moon as on Earth! (By the way, lunar means having to do with the Moon.)

The Moon makes one complete orbit around the Earth every 27.3 days, relative to the fixed stars. This is the Moon’s orbital period. The Moon also rotates on its axis once every 27.3 days. Do you know what this means? The same side of the Moon always faces Earth and so that side of the Moon is what we always see in the night sky (Figure below). The Moon makes no light of its own, but instead only reflects light from the Sun.

The Lunar Surface

The Moon has no atmosphere. Since an atmosphere moderates temperature, the Moon’s average surface temperature during the day is approximately 225°F but drops to minus 243°F at night. The coldest temperatures, around minus 397°F, occur in craters in the permanently shaded south polar basin. These are among the coldest temperatures recorded in the entire solar system.

Earth’s landscape is extremely varied with mountains, valleys, plains and hills. This landscape is always changing as plate tectonics builds new features and weathering and erosion destroys them.

The landscape of the Moon is very different. With no plate tectonics, features are not built. With no atmosphere, features are not destroyed. Still, the Moon has a unique surface. One major lunar surface feature is the bowl-shaped craters that are caused by meteorite impacts (Figure below). If Earth did not have plate tectonics or erosion, its surface would also be covered with meteorite craters.

Even from Earth, the Moon has visible dark areas and light areas. The dark areas are called maria, which means “seas” since that’s what the ancients thought they were. But the maria are not water but solid, flat areas of basaltic lava. From about 3.0 to 3.5 billion years ago the Moon was continually bombarded by meteorites. Some of these meteorites were so large that they broke through the Moon’s newly formed surface, then magma flowed out and filling the craters. Scientists estimate volcanic activity on the Moon ceased about 1.2 billion years ago, but most occurred long before that.

The lighter parts of the Moon are called terrae or highlands (Figure below). The terrae are higher than the maria and include several high mountain ranges. The terrae are the light silicate minerals that precipitated out of the ancient magma ocean and formed the early lunar crust.

There are no lakes, rivers, or even small puddles anywhere to be found on the Moon’s surface, but water ice has been found in the extremely cold craters and bound up in the lunar soil. Despite the possible presence of water, the lack of an atmosphere and the extreme temperatures make it no surprise to scientists that the Moon has absolutely no evidence of life.

Life from Earth has visited the Moon and there are footprints of astronauts on the lunar surface. With no wind, rain, or living thing to disturb them, these footprints will remain as long as the Moon exists. Only an impact with a meteorite could destroy them.

KQED: NASA Ames Rocket to the Moon

LCROSS crashed into the Moon in May 2009. This QUEST video describes the mission. After watching, look up the mission to see what they found! Learn more at: http://science.kqed.org/quest/video/nasa-ames-rocket-to-the-moon/.

Interior of the Moon

Like Earth, the Moon has a distinct crust, mantle, and core. What is known about the Moon’s interior was determined from the analysis of rock samples gathered by astronauts and from unpiloted spacecraft sent to the Moon (Figure below).

• The Moon’s small core, 600 to 800 kilometers in diameter is mostly iron with some sulfur and nickel.

• The mantle is composed of the minerals olivine and orthopyroxene. Analysis of Moon rocks indicates that there may also be high levels of iron and titanium in the lunar mantle.
• The crust is composed of igneous rock rich in the elements oxygen, silicon, magnesium, and aluminum. The crust is about 60 km thick on the near side of the Moon and about 100 km thick on the far side.

Lesson Summary

• The Moon makes one rotation on its axis in the time it takes for it to orbit the Earth.
• The Moon has dark areas, called maria, surrounded by lighter colored highland areas, called terrae.
• Because the Moon is geologically inactive and doesn’t have an atmosphere, it has many thousands of craters on its surface.
• The Moon is made of many materials similar to Earth and has a crust, mantle, and core, just like the Earth.

Review Questions

1. Compare the composition of the Moon’s surface with the composition of Earth’s surface.

2. Why is there no weather on the Moon?

3. Rusting is a process that happens when oxygen reacts chemically with iron, in the presence of water. Can rusting occur on the Moon? Explain your answer.

4. What is the difference between maria and terrea?

5. How does the Moon’s interior differ from Earth’s?

6. How much do landscape features on the Moon change over time compared to landscape features on Earth? Explain your answer.

7. Why is the force of gravity on your body weaker on the Moon than on the Earth?

Points to Consider

• What things would be different on Earth if Earth did not have a moon?
• If the Moon rotated on its axis once every 14 days, would we see anything different than we do now?
• How do we know that the Moon has been geologically inactive for billions of years?