It’s safe to assume you can’t breathe on the moon. You probably already guessed that. But not only is there no breathable air on the moon, there are practically no gasses at all — at least not enough for an astronaut to inhale. Nonetheless, the moon does have an atmosphere, albeit one that’s incredibly thin. Called the exosphere, the lunar atmosphere is about 100 trillion times thinner than Earth’s. In fact, it’s so incredibly thin that most of its neutral gas particles never interact with each other. Indeed, a gas particle on the moon is far more likely to get sniped by a charged particle from the sun and sent whizzing off into space than it is to collide with one of its kind.
The atmospheres of Earth and the moon have unique chemical compositions, as well. For instance, while the moon has familiar gases like oxygen, carbon dioxide, and nitrogen, the proportions and origins of those gases are different from those on Earth. Scientists have also detected radioactive isotopes in the moon’s exosphere that you definitely don’t want in your lungs, like argon-40, radon, and polonium.
The vast differences between the two bodies’ atmospheres has big implications for prospects of a lunar colony. The extreme thinness of the moon’s exosphere combined with its foreign composition means harvesting the meager air that’s there would be impractical. But even more damning for life on the moon is the lack of a magnetosphere to protect it from solar winds. That’s why “weather” on the moon is really dominated by solar storms that regularly blast the sparse lunar atmosphere into interplanetary space. To say the atmospheres of the Earth and the moon are different is an understatement.
Extreme differences in density
The moon’s atmosphere is classified as an exosphere due to its extremely low density. At 100 molecules per cubic centimeter, this layer of gases is nearly indistinguishable from empty space. In fact, the entire mass of the moon’s exosphere is only 55,000 pounds. That’s roughly the same mass as a standard dump truck filled to capacity.
Of course, the Earth has an exosphere as well, and it has the same density as that of the moon’s. However, Earth’s exoxphere lays above four other considerably thicker atmospheric layers. Starting from the surface, Earth’s major atmospheric layers are called the troposphere, the stratosphere, the mesosphere, the thermosphere, and, finally, the exosphere. The atmosphere in which we live, the troposphere, has about 100 billion, billion molecules per cubic centimeter at sea level. Thus, a tiny fraction of Earth’s atmosphere at sea level has the same mass as the entire atmosphere of the moon.
To put the incredible thinness of an exosphere into perspective, consider how high up Earth’s exosphere actually is from the surface. It starts 440 miles from sea level and extends 6,200 miles into space. That’s farther away than even the orbit of the International Space Station. The ISS orbits the Earth at around 250 miles from the surface, which is below the exosphere and inside of the thermosphere. Meanwhile, the moon’s exosphere starts and ends on the lunar surface. In other words, the ISS travels through an atmosphere denser than the moon’s.
A chemical composition you don’t want to breathe
The atmospheres of Earth and the moon are different, though not as drastically different in composition as you might assume. Earth’s atmosphere is composed of 78% nitrogen gas (N2) and 21% oxygen gas (O2). The final 1% is a mix of argon, carbon dioxide, neon, helium, methane, krypton, and many other gasses in minor quantities.
Surprisingly, almost all of the above-mentioned chemicals have been detected on the moon, as well. Using various methods, including spectrometers and detectors installed by the original moon-walking Apollo astronauts, scientists have detected a diversity of gasses in the lunar atmosphere. The list includes terrestrial classics like nitrogen, oxygen, argon, and carbon dioxide, but also gasses like helium-4, carbon monoxide, and isotopes of polonium and radon.
But while the molecules might overlap, their ratios do not. According to NASA’s Moon Fact Sheet, helium and neon are the most abundant in the lunar atmosphere. Measured in particles per cubic centimeter, helium-4 and neon are present at 40,000, hydrogen at 35,000, argon-40 at 30,000, and Argon-36 at just 2,000. After that, the rest of the detected gasses are reported in “trace” quantities. They’re so rare that their numerical estimates are admittedly tenuous at best. So, if you wanted to find breathable air on the moon, it’d be like searching for a needle in a haystack.
Why the moon’s atmosphere is so weak
The moon has a smaller mass than Earth, generating about ⅙ of the gravity. So why isn’t the moon’s atmosphere comparable? The answer has less to do with gravity and more to do with the magnetic field that Earth has and the moon doesn’t. Thanks to Earth’s liquid core, we enjoy a powerful magnetic shield that protects us against deadly solar winds. Our planet’s core is largely made of a liquid iron, which flows in a consistent rate beneath the tectonic plates and generates a sweeping magnetospheric shield. The magnetosphere isn’t part of the atmosphere, since it’s not composed of matter. Instead, it’s a charged field that deflects charged particles emitted by the sun during solar storms. Not only does this deflection protect organisms from radiation, but it also prevents solar winds from blowing Earth’s atmospheric gasses into space.
However, the moon doesn’t have a magnetosphere, at least not one powerful enough to make a difference. There’s some evidence that suggests the moon may have had a liquid core in the past, but these days, the moon gets blasted every time solar winds sweep through the solar system. These winds blow the moon’s meager atmospheric gasses into the interstellar medium where they escape the satellite’s gravitational pull. For comparison, solar winds are also partly responsible for the thin atmosphere of Mars. The red planet’s magnetosphere is significantly weaker than the Earth’s, allowing charged particles to penetrate its atmosphere and knock atoms into space like billiard balls. Consequently, while Mars has about ⅓ the gravity of the Earth, its atmospheric volume is less than 1% that of Earth’s.
Let’s be thankful for our liquid core.
