Many people are convinced the Moon lacks an atmosphere. They believe it is directly exposed to the vacuum of space. But this is incorrect. In fact, there is an atmosphere on the Moon. It is far less dense than Earth’s and wouldn’t be safe to breathe. However, it does exist and has been measured, by instruments carried to the Moon by Apollo astronauts and by reconnaissance satellites put into lunar orbit by NASA in subsequent years.

The atmosphere on the Moon is less than paper thin. On Earth, at sea level, the air we breathe contains about 100 billion billion molecules per cubic centimetre. This sounds incredibly thick, but of course we can’t see the air on Earth, because of the infinitesimal size of even the largest and most complex gaseous molecules.

In comparison, on the Moon there are only about one million molecules in a cubic centimetre of atmosphere. This is vanishingly empty, if we compare it to Earth. Yet the Moon’s atmosphere is very real, and is typical of the thin atmospheres found on other bodies in the solar system.

The gases in the Moon’s atmosphere are mostly found in Earth’s atmosphere as well. They include small quantities of nitrogen, helium, argon, neon, ammonia, methane and carbon dioxide. These gases were detected in samples taken by the Lunar Atmospheric Composition Experiment (LACE) , which was included in the last NASA mission to land astronauts on the Moon (Apollo 17).

Sodium and potassium were also found in the Moon’s atmosphere. These chemicals are not a part of the atmosphere on Earth, Mars or Venus. These substances were discovered by Earth-based researchers studying the Moon through special telescopes.

For the most part, these gases are produced as a result of interactions between the Moon and the Sun. Solar wind consists of charged particles including electrons and protons, and the bombardment of the lunar surface by these electrically interactive winds provokes the release of various chemical traces. The Sun’s heat and light are enough to spur further chemical reactions, adding even more content to the Moon’s atmosphere. The creation of the Moon’s atmosphere is a continuous process, as it must be since the molecules it contains will eventually leak out into space.

Outgassing from the interior is another likely contributor to the creation of the Moon’s atmosphere. This occurs as a side effect of seismic activity, the results of which are labeled appropriately as “moonquakes.”

These moonquakes may be caused by various triggers—the impact of large meteorites, the collapse of crater walls, dramatic changes in surface temperatures, the tidal (gravitational) pull of the Earth—and seem to happen quite regularly. Seismic measuring instruments left on the planet by Apollo 17 recorded 12,000 moonquakes between 1972 and 1977, after which the instruments stopped working.

The type of atmosphere that exists on the Moon is known as a “surface boundary exosphere,” or more simply as an exosphere. This term is reserved for atmospheres so sparse and so lacking in density that their molecules never come in contact.

If all the atmospheric gases on the Moon were condensed, they would weigh just 25,000 kilograms in total. This is extraordinarily light for a satellite that has a diameter of nearly 13,000 kilometres.

Exospheres can be found on Mercury, on some large asteroids and on several of the moons of Saturn and Jupiter. Bodies that are dense enough and large enough to surpass a certain gravitational threshold are able to keep a tenuous grip on thin atmospheres, assuming there are sources of atmospheric gases available to create those atmospheres in the first place.

In addition to the gases it contains, the Moon’s atmosphere is regularly contaminated by gigantic dust clouds rising from the surface. Ultraviolet rays in sunlight are responsible for kicking up these dust clouds, and they can rise quite high into the air because of the Moon’s low gravity. This atmospheric dust is responsible for a phenomenon known as the Lunar Horizon Glow , which can be seen from nearby space when the Sun is shining around the Moon’s edge. The dust rises during the day and then settles back to the Moon’s surface at night, repeating the same pattern on a daily basis.

This dust proved to be a real problem for the Apollo astronauts. Moon dust is gritty, clingy and abrasive. It caused significant wear and tear on the materials used in the construction of the Apollo astronaut space suits. This dust is also capable of damaging or clogging sensitive equipment, and could cause real headaches for future settlements on the Moon (if any are ever built).

An atmosphere is not the only interesting thing found on the Moon. It also has significant ice deposits at its poles, likely exceeding 600 million tonnes according to the latest estimates. Water molecules have been found in other latitudes in the lunar dust, suggesting a movement of water from the polar regions to lower latitudes through the activity of atmospheric circulation.

This water could be melted and used by future Moon colonies or scientific outposts, which might harvest its oxygen for breathing as well.

The Surprisingly Thick Atmosphere of Titan

Studies of other moons in the solar system have found conditions similar to those that exist on our Moon. Atmospheres can be found on several, and the majority are seriously lacking in content.

However, there are four moons in the solar system that contain much thicker atmospheres, which are classified as actual atmospheres and not as exospheres.

Three of these moons—Callisto (Jupiter), Triton (Neptune) and Io (Jupiter ) – have atmospheres much thicker than our Moon, but still much thinner than that found on Earth. But on Saturn’s largest moon, Titan, the situation is quite different. Titan’s atmosphere is 50 percent thicker than the Earth’s, which is remarkable considering this moon is less than half the size of Earth and less dense (therefore less gravitationally powerful) than even our moon.

Nearly 95 percent of Titan’s atmosphere is comprised of nitrogen, with methane making up most of the remainder. How Titan obtained its thick atmosphere remains a matter of speculation and investigation, since no other moon in the solar system comes anywhere close to matching this atmosphere in density or composition. It is possible that volcanic activity on Titan is largely responsible, but that will require more study before it can be confirmed.

The methane on Titan is particularly intriguing. While it has several possible sources, methane is often created as a byproduct of biological processes—in other words, it is emitted by living creatures. Planetary scientists have not ruled out the presence of lifeforms of some type on Titan, and they know organic chemicals could form there based on what is known about Titan’s chemical makeup.

Because of the nature of its atmosphere, Titan is covered by a thick, hazy, orange cloud. As a result it is impossible to get a close-up look at Titan’s surface from space, and the nature of its surface features remained a mystery until NASA’s Cassini-Huygens Saturn probe touched down on Titan in 2004. The probe revealed that Titan has liquid hydrocarbon lakes near its poles, and the overall results of this mission were intriguing enough that NASA plans to send another lander to Titan in 2034. This lander, known as Dragonfly, will set down on Titan’s surface and perform a number of tests to evaluate its capacity to support life.

A Solar System of Surprises

The discovery of atmospheres on Moons tells us a lot about how dynamic our solar system really is. Worlds that appear dead and inert from a distance are actually quite active, and have unexpected characteristics that have delighted planetary scientists and left them anxious to learn more.

Who knows what other amazing revelations await, once we begin to explore our solar system more deeply and in earnest?

By Nathan Falde