22.13: Moon

Would you like to go on this field trip?

The most amazing "field trips" in human history took place in the Apollo program. Between 1969 and 1972, six manned spaceships landed on the Moon. Human astronauts walked on the Moon. They brought back soil and rock samples. No human has visited Earth's satellite since 1972. There is talk of someday returning, but there are no concrete plans.

Earth’s Moon

The Moon is Earth’s only natural satellite. The Moon is about one-fourth the size of Earth, 3,476 kilometers in diameter. Gravity on the Moon is only one-sixth as strong as it is on Earth. If you weigh 120 pounds on Earth, you would weigh only 20 pounds on the Moon. You can jump six times as high on the Moon as you can on Earth. The Moon makes no light of its own. Like every other body in the solar system, it only reflects light from the Sun.

Orbit

The Moon rotates on its axis once for every orbit it makes around Earth. What does this mean? This means that the same side of the Moon always faces Earth. The side of the Moon that always faces Earth is called the near side. The side of the Moon that always faces away from Earth is called the far side. From Earth, people have seen only the Moon's near side. The far side has been seen only by spacecraft and Apollo astronauts.

Atmosphere

The Moon has no atmosphere. With no atmosphere, the Moon is not protected from extreme temperatures. The average surface temperature during the day is approximately 107°C (225°F). Daytime temperatures can reach as high as 123°C (253°F). At night, the average temperature drops to -153°C (-243°F). The lowest temperatures measured are as low as -233°C (-397°F).

Lunar Surface

We all know what the Moon looks like. It's always looked the same during our lifetime. In fact, the Moon has looked the same to every person who has looked up at it for all time. Even the dinosaurs, trilobites, or cyanobacteria, should they have looked up at it, would have seen more-or-less the same thing. This is not true of Earth. Natural processes continually alter Earth’s surface. Without these processes, would Earth's surface resemble the Moon's? The Moon does have more craters than it did in the long distant past, of course.

Even though we can't see it from Earth, the Moon has changed recently too. Astronauts' footprints are now on the Moon. They will remain unchanged for thousands of years, because there is no wind, rain, or living thing to disturb them. A falling meteorite could destroy them. Tiny micrometeorites bombard the Moon's surface and smooth it over time too.

Craters

The surface of the Moon is very different from Earth. The lunar landscape is covered by craters caused by meteoroid impacts (Figure below). (Lunar means relating to the Moon.) The craters are bowl-shaped basins on the Moon’s surface. Because the Moon has no water, wind, or weather, the craters remain unchanged.

The Moon's coldest temperatures are found deep in the craters. The coldest craters are at the south pole on the Moon's far side. The Sun never shines on the bottoms of the deepest craters. These temperatures are amongst the coldest in our entire solar system.

Craters, like the one shown in this image, are found on the surface of the Moon.

Maria

When you look at the Moon from Earth, you notice dark and light areas. The maria are dark, solid, flat areas of lava (mostly basalt). Maria cover around 16% of the Moon’s surface, mostly on the near side. The maria formed about 3.0 to 4.0 billion years ago, when the Moon was continually bombarded by meteoroids (Figure below). Large meteorites broke through the Moon’s newly formed surface. This eventually caused magma to flow out and fill the craters. Scientists estimate volcanic activity on the Moon ended about 1.2 billion years ago.

Maria (the dark areas) and terrae (the light areas) cover the Moon.

Terrae

The lighter parts on the Moon are called terrae, or highlands (Figure above). They are higher than the maria and include several high mountain ranges. The rock that makes up the highlands is composed mostly of feldspar, which is lighter in color and crystallized more slowly than the maria. The rock looks light because it reflects more of the Sun’s light.

The figure below shows the near and far sides of the Moon (Figure below). They are different!

The Mare Moscoviense is one of the few maria, or dark, flat areas, on the far side.

The rocks of the terrae formed while the lunar magma ocean cooled just after the Moon formed. The maria formed later.

Water

There are no lakes, rivers, or even small puddles anywhere to be found on the Moon's surface. There is no running water and no atmosphere. This means that there is no erosion. Natural processes continually alter Earth’s surface. Without these processes, our planet’s surface would be covered with meteorite craters just like the Moon. Many moons in our solar system have cratered surfaces.

NASA scientists have discovered a large number of water molecules mixed in with lunar dirt. There is also surface water ice in some of the frigid places that are always in shadow. Even though there is a very small amount of water, there is no atmosphere. Temperatures are extreme. So it comes as no surprise that there has not been evidence of life on the Moon.

Lunar Interior

Like Earth, the Moon has a distinct crust, mantle, and core. The crust is composed of igneous rock. This rock is rich in the elements oxygen, silicon, magnesium, and aluminum. On the near side, the Moon’s crust is about 30 kilometers thick. On the far side, the crust is about 100 kilometers thick. The mantle is made of rock like Earth’s upper mantle. The Moon has a small metallic core, perhaps 300 to 500 kilometers in diameter. The composition of the core is probably mostly iron with some sulfur and nickel. We learned this both from the rock samples gathered by astronauts and from spacecraft sent to the Moon.

Exploration

Humans have walked on only two planetary bodies: Earth and the Moon. The footprints the men left behind are the first signs of life ever on the Moon. Scientists learned a great deal about the Moon from the Apollo missions. The samples that were brought back have been studied in laboratories. We know an incredible amount about the Moon because of these samples. Having these rocks radically changed our thinking about the Moon.

The Moon is easily seen from Earth. Early astronomers used telescopes to study and map its surface. The Moon has also seen a great number of satellites, rovers, and orbiters. After all, it is now relatively easy to get spacecraft to the satellite. Also, before humans could be safely sent to the Moon, many studies and experiments had to be completed.

Summary

• The Moon revolves around Earth as they orbit the Sun. The same side of Moon always faces Earth.
• The lunar surface has dark basalt maria. The light highlands are called terrae.
• The Moon has a crust, mantle, and core.
• The Moon has no atmosphere and only a very tiny amount of water.

Review

1. Why do we only see one side of the Moon from Earth?
2. What are the Moon's terrae like in structure and rock type?
3. How did the maria form?
4. In what form is water found on the Moon? In what form is it not found on the Moon?

Explore More

Use the resource below to answer the questions that follow.

1. What is the Moon?
2. How far is the Moon from Earth? Where will it be relative to its current location 100 years from now?
3. What did Galileo prove about the Moon? How did he do that?
4. Where did the Moon come from?
5. What is the Moon like relative to Earth?
6. Why do we always see the same side of the Moon?
7. Describe the Moon's structure.
8. What are maria?
9. What are terrae?
10. What created the craters on the Moon?
11. If a small meteor went toward the Moon's surface and one the same size went toward Earth's surface, which would actually reach the surface and why?
12. If there is water on the Moon, how did it get there and where is it?
13. What is an eclipse?
14. What is a lunar eclipse?
15. What is a total solar eclipse?
16. Why is the Moon unique?
17. Who was the first person on the Moon?