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.

Ethan Siegel
3 min readJul 12, 2021

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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.

The highest-resolution views of the entire lunar surface were taken recently by the Lunar Reconnaissance Orbiter. The maria (the younger, darker regions) are clearly less cratered that the lunar highlands, but the unchanging nature of the stacked craters over extremely long timescales indicate an atmospherically inactive world. (NASA/GSFC/ARIZONA STATE UNIVERSITY (COMPILED BY I. ANTONENKO))

So does crewed space activity.

Apollo 12 was the first precision landing of humans on the Moon, and we explored a much greater amount of the lunar surface than during the first landing. The dark grey markings on the surface are astronaut footprints, which have stood the test of time on the Moon, as the processes that erase them on Earth are absent on the Moon. (NASA / LRO / GSFC / ASU)

After more than 50 years, the Apollo landing sites, including astronaut footpaths, remain unchanged.

A photograph from Lunar Reconnaissance Orbiter of the landing site of Apollo 17. The tracks of the Lunar Roving Vehicle (LRV) can be clearly seen, as can the vehicle itself. Equipment and astronaut footpaths can be seen as well, if you know the proper places to look and the right features to seek. Similar photographs exist for each of the Apollo landing sites. (NASA / LRO / GSFC / ASU)

However, although it’s tenuous and temporary, the Moon actually possesses an atmosphere.

During the lunar eclipse of January 21, 2019, a meteorite struck the Moon. The bright flash, seen here at the upper left of the Moon’s limb, was extremely brief, but was captured by amateur and professional stargazers and photographers alike. These meteor strikes are responsible for creating a temporary, tenuous, but continuous atmosphere of thin atoms and ions on the Moon. (J. M. MADIEDO/MIDAS)

Meteoric impacts kick up particles from the Moon’s regiolith.

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Ethan Siegel
Starts With A Bang!

The Universe is: Expanding, cooling, and dark. It starts with a bang! #Cosmology Science writer, astrophysicist, science communicator & NASA columnist.