The Moon, the only natural satellite of the Earth, has fascinated humanity for centuries. Even if in most cases it is considered, due to practical reasons, that the Moon doesn’t have an atmosphere, the reality is that it has one. However, it is a so thin layer of gases that it can barely be considered an atmosphere. In fact, we refer to it as an exosphere (that to say, it is almost a vacuum).
In an exosphere, the gases are so separated that they rarely collide the ones with the others. The Moon’s atmosphere contains about 100,000 molecules per cubic centimetre. In comparison, at sea level, every cubic centimetre contains 10,000,000,000,000,000,000 molecules. The total mass of the Moon’s exosphere is only 25 tons, the same as a truck.
Composition of the atmosphere
The atmosphere of the Moon is composed of three main elements: argon, helium and neon. Other gases, such as sodium, potassium or hydrogen, exist in much smaller quantities. We know all this thanks to an instrument called Lunar Atmospheric Composition Experiment (LACE) that the Apollo 17 mission left on the surface of our satellite.
The atmosphere of the Moon isn’t evenly distributed around it. There are some zones that are denser than others, depending on factors such as the temperature of the surface, the magnetic field, or solar wind.
Origin and evolution of the gases in the atmosphere
One of the phenomena that create the thin atmosphere of the Moon is the release of gases from the interior of the Moon, due to the decay of some radioactive elements. After being released, most gases are ejected into space in little time. Only some of these gases end up forming part of the thin atmosphere.
Another factor that provides more gases to the atmosphere is the impact of the Sun’s light or the solar wind on the Moon’s surface. The photons and highly energetic particles of the solar wind can decompose materials on the surface of the Moon, releasing atoms and molecules to the dim atmosphere of the satellite. This same process can also be caused by the impact of meteorites, of small and big sizes, on the Moon.
When a face of the Moon isn’t exposed to the Sun’s light, the temperature of its surface drastically drops. This drop in temperature provokes the contrary effect to the one mentioned before. With such low temperatures, some gases of the atmosphere can condense, and deposit themselves on the surface of the Moon.