Here on Earth, we often take the air that we breathe for granted. It is so abundant that we don’t even have to think about taking that next breath and getting the oxygen we need to survive.

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The Moon is our closest neighbor, but it seems worlds apart in terms of activity. Does the Moon even have an atmosphere to speak of?

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The Moon and Earth’s atmosphere as seen from the International Space Station. (Image credit: “The Moon and Earth’s atmosphere” by NASA Johnson on flickr CC BY-NC 2.0)

What Is an Atmosphere?

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Earth’s atmosphere and a setting sun. (Image credit: NASA)

In the celestial sense, an atmosphere represents the envelope of gases that surround a heavenly body. Since gases have mass, the gravity of that body needs to be able to hold those gases in place. It is easier for a celestial body to retain an atmosphere if the atmosphere’s temperature is low.

Every planet in our Solar System has an atmosphere, although Mercury’s is quite thin. Even the Sun has an atmosphere it holds in place with its massive gravity.

How Does an Atmosphere Form?

When a planetary object forms, it is incredibly hot. These hot gases spew forth from the forming surface up into the air. As the object cools, the gases also cool and become trapped by gravity, and an atmosphere is created.

A Look at the Moon

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The Moon is pictured above the Earth’s atmosphere. (Image credit: NASA)

Our Moon happens to be the fifth-largest object orbiting a planet in the entire Solar System, but still smaller than any of our planets. The Moon’s surface is very cold and inactive. It certainly seems like there’s nothing going on up there.

The astronauts that visited the moon had to wear massive spacesuits. Flags planted on the Moon’s surface required horizontal cross beams to give the illusion of waving. Even the footprints the astronauts made are still up there. So does this mean the Moon has no atmosphere to speak of?

The Moon actually does have a razor-thin atmosphere that holds in some gases. That being said, the air on the Moon is ten trillion times thinner than it is on Earth! Air particles rarely come in contact with each other as they move around within the atmosphere. Such atmospheres with collisionless gases are called exospheres.

Why Does the Moon Have Such a Thin Atmosphere?

The main reason for the Moon’s nearly non-existent atmosphere is due to its size. The Moon may be large, but at only one quarter the size of Earth, it has a lot of trouble holding onto all of those excited gases because of the relatively small amount of gravity it exerts.

When the Sun shines on the Moon’s surface, temperatures can easily clear 212° Fahrenheit, the boiling point of water. In these hot conditions, light gases like Helium often escape the Moon’s atmosphere and disappear into the vacuum of space.

In addition to low gravity, there is no plate tectonic activity on the Moon to speak of. Rumblings under the crust of our Earth stir up volcanic gases that escape into our atmosphere. The tiny bit of seismic activity the Moon sees sends minimal amounts of gas into the atmosphere. The rest comes from radioactive decay in the crust and mantle.

What Makes Up the Moon’s Atmosphere?

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The Far-Ultraviolet Spectrometer, one of the Apollo 17’s lunar orbital science experiments. (Image credit: NASA)

When Apollo 17 reached the moon, the team brought along a mass spectrometer to measure the composition of the Moon’s atmosphere. They discovered mainly neon, Helium, and hydrogen in roughly equal amounts, with bits of methane, carbon dioxide, ammonia, and water mixed in.

What Does This Mean for Humans?

This of course means that it would be impossible for humans to live on the Moon without having to wear complex spacesuits or live entirely indoors. Even if the Moon had a thick enough atmosphere to sustain enough air for life, there is almost no oxygen in the air for humans to breathe. We would have to generate our own oxygen while living there in order to survive.

While this may be a reality someday, it will undoubtedly take some time to get there.

What About the Other Moons in the Solar System?

We’ve taken a look at our Moon and its thin atmosphere, but what about the 170 plus other moons orbiting planets in our Solar System? Scientists have actually discovered that there are ten other moons with some form of atmosphere. Some are similar to our Moon, while others are a bit closer to Earth’s.

Moons With Thin Atmospheres

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Ganymede, Jupiter’s moon, has thin atmosphere. (Image credit: NASA)

Looking further out into the Solar System, we find six moons with atmospheres on the same level as our Moon. Ganymede and Europa of planet Jupiter, Rhea, Dione, and Enceladus of Saturn, and Titania of Uranus all hold a minimal amount of gas to create an atmosphere.

Just like our Moon, each of these moons has such a sparse amount of gas that the gases move around freely without colliding with one another. These moons either create gases through volcanic activity or when the Sun’s distant rays break up molecules.

Moons With More Significant Atmospheres

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Triton, Neptune’s moon, has an atmosphere which is 100,000 times thinner than Earth’s. (Image credit: NASA)

Callisto of the planet Jupiter was only recently discovered to have a dense enough atmosphere for gas molecules to actually collide with one another. While being 40 billion times less dense than Earth’s atmosphere, it is still dynamic and may even have weather.

Io, another of Jupiter’s Galilean moons, along with Triton of the planet Neptune, have substantial atmospheres. Each is still 100,000 times thinner than that of Earth’s, but are very similar to our planet versus all the other moons out there.

These moons actually have atmospheres thick enough to experience weather, seasons, and even clouds. Io’s atmosphere is primarily comprised of sulfur dioxide and salt. Triton is mostly nitrogen.

One Moon Like No Other

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Titan’s upper-most atmospheric hazes. (Image credit: NASA)

Titan, the second-largest moon in our Solar System (it barely loses out to Ganymede), is the only moon in our entire Solar System with a fully developed atmosphere. Its atmosphere is actually 50% denser than our Earth!

The smoggy moon has seasons, clouds, weather, and may even see lightning. Unfortunately, the atmosphere is nowhere near breathable, mainly being nitrogen and methane.

Final Thoughts

Even though the Moon has a very thin atmosphere, it is one of the Solar System’s few planetary satellites that does have one. The Moon isn’t able to support life, but it still has a significant impact on our lives here on Earth.

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Keep those motionless flags and eternal footprints in mind whenever you capture an image of our celestial neighbor.