The planet Mercury, as imaged here with a special filter, has a detectable sodium tail. The Moon, despite being three times as far away from the Sun as Mercury and receiving only one-ninth the flux, has a similar, but much weaker, sodium tail as well. Despite their ‘airless’ appearance, both Mercury and the Moon have thin, tenuous atmospheres. (ANDREA ALESSANDRINI)

Surprise: The Moon Doesn’t Just Have An Atmosphere, But A Tail, Too

With sodium-sensitive eyes, we’d see it every new Moon.

With no detectable gases, the Moon appears to be atmosphere-free.

The Moon as seen from a view above the majority of Earth’s atmosphere. Whereas Earth’s atmosphere clearly affect the sunlight that passes close to our planet’s limb, the Moon exhibits no such observable effects. To the best of our measurement capabilities, we cannot optically detect an atmosphere. (NASA)

With its low mass, weak gravity, and high daytime temperatures, “airless” seems an excellent assumption.

The lunar lander can be seen returning to the orbiting module with the Earth and Moon in frame, from Apollo 11. The difference between the atmosphere-rich Earth and the atmosphere-free Moon provides a stark visual contrast. (MICHAEL COLLINS / NASA / APOLLO 11)

The radiation and solar wind fluxes are similar between the Earth and Moon.

As seen from the inner Solar System, the Earth and Moon are clearly identifiable and separable. However, this view of the Earth and Moon also showcase just how close together they are relative to the rest of the distances in the Solar System. A separation distance of ~380,000 km is negligible on interplanetary scales. (NASA / JOHN’S HOPKINS UNIVERSITY / CARNEGIE INSTITUTE OF WASHINGTON)

All of Earth’s atmospheric gases — nitrogen, oxygen, argon, carbon dioxide, methane, etc. — would quickly escape the Moon.

The unweathered, uneroded appearances of ancient craters, walls, and ridges supports an atmosphere-free Moon.