The lunar horizon glow, shown here as imaged by the Clementine Spacecraft in the 1990s, had actually been seen numerous times during the Apollo mission, but its existence was treated as dubious until an explanation for the lunar atmosphere was fully developed. This didn’t occur until 1998, when the sodium Moon spot, and a sodium tail extending from the Moon, were discovered. (Credit: NASA)

The “airless” Moon really does have an atmosphere, after all

We once thought the Moon was completely airless, but it turns out it has an atmosphere, after all. Even wilder: it has a tail of its own.

For a number of very good reasons, you wouldn’t expect the Moon to have an atmosphere at all. Compared to planets that do have substantial atmospheres — like Earth, Venus, and even Mars — the Moon is tremendously low in mass. At just 1.2% the mass of the Earth, it can still pull itself into a spheroidal shape, but its surface gravity is quite weak: just one-sixth of what it is here on our own planet. Similarly, the Moon has an escape velocity that’s much lower than Earth’s. Given its high daytime temperatures, as it receives the same amount of sunlight that the top of Earth’s atmosphere does, it’s extremely easy to “kick” any gaseous particles to unbound gravitational orbits.

Given this combination of factors, it’s no wonder that we’d assume the Moon was airless. In fact, the combination of radiation and particles from the Sun, known as the solar wind, are sufficiently energetic that if we were to bring a significant amount of Earth’s atmosphere onto the Moon, it would be less than a million years before they were entirely stripped away. All of Earth’s major atmospheric gases, including…