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That's one small step for [a] man, one giant leap for mankind. — Neil Armstrong

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. The footprints the men left behind are the first signs of life ever on the Moon. Scientists have learned a great deal about the Moon from the Apollo missions and from rovers and satellites sent to the Moon for study.

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!

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. Do you know what this means? The same side of the Moon always faces Earth, 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.

(a) The near side of the Moon faces Earth continually. It has a thinner crust with many more maria (flat areas of basaltic rock). (b) The far side of the Moon has only been seen by spacecraft. It has a thicker crust and far fewer maria (flat areas of basaltic rock).

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 o F, but drops to -243 o F at night. The coldest temperatures, around -397 o 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. Lunar surface features include 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.

A crater on the surface of the Moon.

Even from Earth, the Moon has visible dark areas and light areas. The dark areas are called maria , which means “seas” because that’s what the ancients thought they were. In fact, 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 filled 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.

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.

There are no lakes, rivers, or even small puddles anywhere to be found on the Moon's surface, but water in the form of 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.

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 unmanned 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 Moon’s internal structure shows a small metallic core (yellow), a primitive mantle (orange), a depleted mantle (blue), and a crust (gray).

  • 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.

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 and light highlands called terrae.
  • The Moon has a crust, mantle, and core, but no water or atmosphere.

Making Connections

Review

1. Explain why one side of the Moon always faces toward Earth and the other side always faces away from Earth.

2. How did the Moon's terrae form?

3. Describe why the moon appears to be "lit up" in the night sky?

4. What is the temperature like on the moon?

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