With sodium-sensitive eyes, we’d see it every new Moon.
With no detectable gases, the Moon appears to be atmosphere-free.
With its low mass, weak gravity, and high daytime temperatures, “airless” seems an excellent assumption.
The radiation and solar wind fluxes are similar between the Earth and Moon.
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.
So does crewed space activity.
After more than 50 years, the Apollo landing sites, including astronaut footpaths, remain unchanged.
However, although it’s tenuous and temporary, the Moon actually possesses an atmosphere.
Meteoric impacts kick up particles from the Moon’s regiolith.
Solar wind particles and ultraviolet radiation strike that airborne material.
Atoms can get ionized and/or accelerated, with the fastest escaping the Moon’s gravitational pull.
This creates a lunar “tail” of particles oriented away from the Sun.
Once-per-month, during the new Moon, Earth gains a 3° diameter feature: the Sodium Moon Spot.
It’s brightest ~5 hours after the new moon, and brighter during lunar perigee.
Earth’s gravity distorts this lunar tail during successful alignments.
Increased meteor activity brightens the Sodium Moon Spot.
Perhaps impacts indirectly drive this lunar tail.
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.
Starts With A Bang is written by Ethan Siegel, Ph.D., author of Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.