High School Earth Science/Earth's Moon

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On July 20, 1969 hundreds of millions of people all over the world excitedly sat in front of their televisions and witnessed something that had never happened before in the history of the world. On that day, two American astronauts named Neil Armstrong and Edwin "Buzz" Aldrin landed the Eagle on the surface of the Moon (Figure 24.12). Neil Armstrong was the commander of the mission, and he was the first human to ever step foot on the Moon. No other place in space, besides Earth, has been touched by humans. Even today, the Moon remains the only other body in space that humans have visited.

Figure 24.12: Astronaut Buzz Aldrin walks on the Moon on July 20, 1969.

Between 1969 and 1972, six piloted spaceships were sent to land on the Moon. They are often referred to as lunar expeditions, the word lunar meaning "related to the Moon". On some missions, the astronauts brought back soil and rock samples from the Moon. Once back at Earth, the samples were studied to help scientists learn about the surface features of the Moon. No astronauts have visited the Moon since 1972, but in 2004 the United States President George W. Bush called for a return to Moon exploration by the year 2020. Maybe you can be one of the astronauts to return to the Moon!

This lesson focuses on how the Moon was formed and gives a description of the features and characteristics of the Moon—many of which were investigated and discovered during the major years of lunar exploration in the 1960s and 1970s.

How the Moon Formed[edit]

Astronomers have carried out computer simulations showing that the collision of a Mars-sized object with the Earth could have resulted in the formation of the Moon. Additional data shows that the surface of the Moon dates to about 4.5 billion years ago, suggesting that the collision occurred during the heavy bombardment period, about 70 million years after the Earth formed. The Moon also has a relatively small core and appears to be largely comprised of the same basalt material found in the Earth mantle. Such a collision would have been incredibly powerful, producing oceans of liquid magma over much of the surface of the Earth.

The explosive impact that likely led to the formation of the Moon would have produced a huge amount of energy, leaving the surface of the Moon in an initially molten state. This means that its surface would have been hot and fluid, like magma inside the Earth today. The magma eventually cooled and hardened so that the Moon now has a solid surface.

Lunar Characteristics[edit]

The Moon is Earth's only natural satellite. A satellite is a body that moves around a larger body in space. The Moon orbits Earth in the same way that the Earth orbits the Sun, and the Moon remains close to Earth because of the strength of Earth's gravity. The Moon is 3,476 kilometers in diameter, about one-fourth the size of Earth. Because the Moon is not as dense as the Earth, gravity on the Moon is only one-sixth as strong as it is on Earth. You could jump six times as high on the Moon as you can on Earth.

If you watch the Earth and the Moon from space, the Moon makes one complete orbit around the Earth every 27.3 days. The Moon also rotates on its axis once every 27.3 days. Thus, the same side of the Moon always faces Earth. This means from Earth we always see the same side of the Moon. The side of the Moon that faces Earth is called the near side (Figure 24.13). The side of the Moon that faces away from Earth is called the far side (Figure 24.14). The Moon makes no light of its own, but instead only reflects light from the Sun.

Figure 24.13: The near side of the Moon, the one that we see, has a thinner crust with many more maria.
Figure 24.14: The far side of the Moon has a thicker crust and far fewer maria.

The Lunar Surface[edit]

The Moon has no atmosphere. The average surface temperature during the day is approximately 225°F and can reach temperatures as high as 253°F. At night the average temperature drops to -243°F and has been measured as low as -397°F. These extremely cold temperatures occur in craters in the permanently shaded south polar basin and are amongst the coldest temperatures recorded in our entire solar system.

There are no lakes, rivers, or even small puddles anywhere to be found on the Moon's surface. (However, it should be noted that in 2009, NASA scientists believe they discovered that in the top few millimeters of the Moon's surface, there is a large number of water molecules mixed in with dirt and rocks — you can stay up-to-date with their latest findings at http://www.nasa.gov). Yet, despite the possible presence of water, with a lack of atmosphere and extreme temperatures, it comes as no surprise to scientists that there has been zero evidence of life naturally occurring on the Moon.

Although there are no "naturally occurring" signs of life on the Moon, there are signs that life has encountered the Moon — that is, there are footprints of astronauts on the Moon's surface. It's likely that these footprints will remain unchanged for thousands of years, because there is no wind, rain, or living thing to disturb them. Only a falling meteorite or other matter from space could destroy them. A meteorite is a piece of rock that reaches the Moon from space. Meteorites also hit the Earth sometimes.

Figure 24.15: A crater on the surface of the Moon.

Earth has mountains, valleys, plains, and hills. This combination of all of the surface features of an area of land is called a landscape. The landscape of the Moon is very different from that of Earth. The lunar landscape is covered by craters caused by the impacts of asteroids and meteorites that crashed into the Moon from space (Figure 24.15). The craters are bowl-shaped basins on the Moon's surface. Because the Moon has no water, wind, or weather the craters remain unchanged. If Earth did not have plate tectonics, which continually alters the planet's surface, or an atmosphere, which makes erosion possible, our planet's surface would be at least as covered with meteorite craters as the Moon's. The surfaces of many other moons orbiting other planets have been shaped by asteroid impacts.

When you look at the Moon from Earth you notice dark areas and light areas. The dark areas are called maria. They are 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.

The lighter parts are the Moon is called terrae or highlands (Figure 24.16). They are higher than the maria and include several high mountain ranges. They are believed to be the rims of ancient impact craters.

Figure 24.16: A close-up of the Moon, showing maria (the dark areas) and terrae (the light areas); maria covers around 16% of the Moon’s surface, mostly on the side of the Moon we see.

Interior of the Moon[edit]

Like the Earth, the Moon has a distinct crust, mantle, and core. The crust is composed of igneous rock rich in the elements oxygen, silicon, magnesium, and aluminum. The Moon's crust is about 60 kilometers thick on the near side of the Moon and about 100 kilometers thick on the far side. 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 Moon has a small core, perhaps 600 to 800 kilometers in diameter. The composition of the Moon's core is not known, but it is probably made mostly of iron with some sulfur and nickel. This information is gathered both from rock samples gathered by astronauts and from unpiloted spacecraft sent to the Moon.

Lesson Summary[edit]

  • Many scientists believe the Moon formed when a Mars sized planet collided with Earth.
  • The Moon makes one rotation on its axis in the same number of days 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[edit]

  1. What is one piece of evidence that supports the idea that the Moon was formed by materials that were once part of the Earth?
  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 much do landscape features on the Moon change over time compared to landscape features on Earth? Explain your answer.
  6. Why is the force of gravity on your body weaker on the Moon than on the Earth?


Bowl-shaped depressions on the surface of the Moon caused by impact from meteorites.
The surface features of an area.
Related to the Moon.
The dark parts of the Moon’s surface, made up of ancient basaltic eruptions.
Piece of rock that hit the Moon, Earth, or another planet from space.
A body that orbits a larger body in space.
The light parts of the Moon's surface, composed of high crater rims.

Points to Consider[edit]

  • 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?

Earth's Motions · The Sun