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Starts With A Bang

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

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)

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 lack of an atmosphere and low surface gravity of the Moon makes it easy to escape, as the Apollo 17 module does here. On Earth, we have to combat air resistance and accelerate to approximately ~25,000 mph (40,000 kph) to escape from our planet’s gravity. To escape from the Moon, there’s no air resistance to combat, and the escape velocity is only ~20% what it is on Earth. (KIPP TEAGUE, LUNAR SURFACE JOURNAL)

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.

Ever since humans first landed on the Moon, we realized what the lunar regiolith is like. This outermost layer of lunar material is somewhere between sand and dust, and even a small meteorite impact can kick up a very large number of particles of a wide variety of sizes. The relentless impacts create a small but measurable atmosphere on the Moon. (NASA / APOLLO 11)

Solar wind particles and ultraviolet radiation strike that airborne material.

Earth, at right, has a strong magnetic field to protect it from the Solar Wind. Worlds like Mars (left) or the Moon do not, and routinely get struck by the energetic particles emitted from the Sun, which continue to strip airborne particles off of those worlds. Even the Moon, which barely has an atmosphere at all, continues to lose it. (NASA / GSFC)

Atoms can get ionized and/or accelerated, with the fastest escaping the Moon’s gravitational pull.

When an atom gets struck by another particle, like a solar wind particle or an energetic photon, it can ionize and/or accelerate the atom. On the Moon, atoms struck by the light and particles from our Sun can easily impart them with escape velocity, while this hardly ever happens on Earth. (NICOLLE RAGER FULLER, NSF)

This creates a lunar “tail” of particles oriented away from the Sun.

Sodium atoms are knocked out of the Moon’s atmosphere by the Sun, creating a tail. When this tail interacts with Earth, which it does in the surrounding hours of the new Moon, we observe a Sodium Moon Spot about ~3 degrees in diameter. (JAMES O’DONAGHUE, BASED ON WORK BY JODY K WILSON)

Once-per-month, during the new Moon, Earth gains a 3° diameter feature: the Sodium Moon Spot.

At left, a view of the night sky with an all-sky camera from Earth during the new Moon. The stars and Milky Way are clearly visible. That same image, with the stars subtracted out (at right), clearly reveals the Sodium Moon Spot, which can then be seen in the left image where the yellow arrow points. This feature only appears during the new Moon. (J. BAUMGARDNER ET AL. (2021) JGR PLANETS, VOL. 126 ISSUE 3)

It’s brightest ~5 hours after the new moon, and brighter during lunar perigee.

A perigee full Moon compared with an apogee full Moon, where the former is 14% larger and the latter is 12% smaller than the other. The same size differences can also occur during the new Moon. The closer perigee occurs to the new Moon, the larger the signal from the Moon’s sodium tail becomes here on Earth. (WIKIMEDIA COMMONS USER TOMRUEN)

Earth’s gravity distorts this lunar tail during successful alignments.

When the Moon passes between the Earth and Sun, even if the alignment is too poor for an eclipse, the Moon’s sodium tail can interact with the Earth. Earth gravitationally disrupts the path of the tail, focusing and distorting it like a finger moving across the end of a rushing garden hose. (JAMES O’DONAGHUE, BASED ON WORK BY JODY K WILSON)

Increased meteor activity brightens the Sodium Moon Spot.

A view of many meteors striking Earth over a long period of time, shown all at once, from the ground (left) and space (right). The same debris streams that impact Earth throughout the course of the year also impact the Moon, and while they create mostly atmospheric phenomena on Earth, it’s suspected that these impacts create the majority of the Moon’s atmosphere itself. (ASTRONOMICAL AND GEOPHYSICAL OBSERVATORY, COMENIUS UNIVERSITY (L); NASA (FROM SPACE), VIA WIKIMEDIA COMMONS USER SVDMOLEN (R))

Perhaps impacts indirectly drive this lunar tail.

Models of the Moon’s sodium tail and how its brightness should appear to observers on Earth, bottom, compared with the observed brightness of sodium particles emitted from the Moon and observed at Earth’s location, top. The theoretical models and simulations line up spectacularly with what’s observed, pointing towards a successful model. (JODY K. WILSON / B.U. IMAGING SCIENCE)

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.


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