Carl Sagan, the American astronomer, has once mentioned that Earth is but a “mote of dust suspended in a sunbeam.” He would be thrilled to know that, actually, Earth is a speck of dust suspended in the tail of the moon.

Have you ever heard that the moon has a tail?

According to a study published on 3 March 2021 in the journal JGR Planets, the moon exhibits a comet-like tail, consisting of sodium atoms, which is too faint to be detected by naked human eyes. Once a month, at the time of the new moon a small, diffuse spot of light appears in the sky opposite the Sun. This spot is about five times the diameter of the full moon and is 50 times fainter than can be seen with the naked human eye. The spot was the reflected light from millions of sodium atoms that two days earlier were on the surface of the Moon. Following the detection of the tail of the moon, it has been also shown that the Earth passes directly through the tail once a month. For a few days of the month, when the new moon sits between the Sun and the Earth, gravity from the Earth drags the sodium tail of the moon so that it wraps around the atmosphere before blasting into space, like the Earth wearing the scarf. This phenomenon can be seen by high-powered telescopes that can detect the orange glow of sodium in the sky during those few new-moons days each month. 

Fig 1. How the moon’s tail appears from the Earth. (James O’Donoghue)

But, why does the moon have a sodium tail? As you ever heard before, the moon has a thin layer of atmosphere. This makes the surface of the moon to be perilous due to the sputtering of the solar wind, desorption of the solar-photons and impact from the meteoroids. This liberates sodium atoms to be released from the regolith of the surface and as they accelerate by solar photon pressure, it forms a long comet-like tail to the moon opposite the Sun. Overall, it can be expressed as the sodium tail of the moon is the escaping hot component of a coma-like exosphere from the impact on the surface of the moon.

Researchers have first detected this sodium tail in the 1990s. As the brightness of the sodium tail fluctuates wildly in the lunar circle, they have been using all-sky cameras, cameras used to parse the faint wavelengths of light given off by specific elements, situated at the El Leoncito Observatory (CASLEO) in Argentina to take more than 20,000 images of the moon from April 2006 to 2019. This allowed them to detect a few predictable patterns – the spot appeared to be brighter when the orbit of the moon was closer to the Earth, but also an unexpected one. The data has shown that the moon’s tail glowed brighter when the rate of the sporadic meteor, random occurrences not associated with any particular meteor shower, was higher over the Earth. This would be possibly explained by extending the reason for the sodium tail of the moon, as a sporadic meteor has the potential to be faster and larger than a normal meteor, which might exert more force, blasting larger amounts of sodium higher into the atmosphere. This creates a larger swarm of the atoms to be bombarded with the photons and push towards the Earth, displaying a brighter tail of the moon. According to James O’Donoghue, a planetary scientist at the Japan Aerospace Exploration Agency, if a large asteroid collides with the moon with enough force, it might produce an even brighter tail of the moon, which can be seen with our naked eyes.

The moon is one of the nearest alluring celestial bodies, close enough for the astronauts to be visited and studied. Also, nations around the world have dispatched a number of probes in order for research on our only natural satellite. However, still, there are numerous scientific questions and mysteries that have emerged in part of the moon. Even though this sodium tail of the moon does not impact any life on our Earth, this may be the chance for us to expand our knowledge of the moon and solve some of the key puzzles.

Reference

Amy Yeobin Han
Amy Yeobin Han

Student of NLCS Jeju
Chair of NLCS Jeju COSMOS Society

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