Carl Sagan once said that the earth is just “a speck of dust floating in a ray of sunshine”. He would probably be thrilled to know that at the time of a new moon, the earth is a speck of dust hanging in the tail of a moon.
The moon, which lacks an atmosphere to shield it, is constantly under attack. When meteorites bomb their volcanic surface, sodium atoms fly high into orbit. The sun’s photons collide with the sodium atoms, effectively pushing them away from the sun, creating a tail-like structure that flows downstream from the moon.
“It makes the moon look like a comet,” said Jeffrey Baumgardner, senior scientist at Boston University’s Center for Space Physics. “There is a stream of things coming out.”
For a few days a month, when the new moon moves between earth and sun, that comet-like tail dusts the side of our world that faces the sun. The gravity of our planet compresses this sodium stream and narrows it into a beam invisible to the naked eye that wraps around the earth’s atmosphere and shoots into space from the opposite side of the planet.
This moon beam can be seen by special cameras as a spot in the twilight. Sometimes it appears lighter, sometimes it appears darker. Ever since the tail and its beam were first seen in the late 1990s, scientists have wondered how to control the brightness of the beam. As reported Wednesday in a study published in the Journal of Geophysical Research: Planets, 14 years of observations suggest that meteors – especially larger, faster ones that randomly bombard the moon – might explain what controls its flicker.
“Does this have any practical application? Probably not, ”said Baumgardner, the study’s lead author. This research was driven by nothing more than curiosity, he said, a desire to simply learn more about this beautiful volcanic pearl in the sky and its mystifying moonbeam.
“I think it’s very cool,” said Sarah Luettgen, a student at Boston University and co-author of the study. “It almost seems like a magical thing.”
Boston University has installed several all-sky imaging cameras around the world – essentially fisheye lenses that see the entire visible sky. Originally designed to detect aurors, they can use a filter to see sodium in the earth’s atmosphere. They usually observe when meteors burn before they reach the surface of our planet.
In November 1998, during the height of the annual Leonid meteor shower, a team working with such a camera at the McDonald Observatory in Fort Davis, Texas hoped to see these sodium flares. They were confused when shortly after the summit a sodium stain remained in the sky for three nights. This speck, which appeared on the far side of the world from the sun, lightened as the new moon approached and then quickly faded.
After additional work, including models that simulated where the sodium stain might be coming from, the team concluded that this must be the result of a comet-like sodium tail stretching at least 500,000 miles from the moon.
The tail can spray the world with sodium, but it’s extremely diffuse, so there’s no chance moondust scales build up on our heads, said Luke Moore, a senior scientist at Boston University and co-author of the study.
The November 1998 moonspot appeared particularly bright after the height of the Leonid meteor shower. It was also seen during other new moons without simultaneous meteor showers, but it was weaker. The scientists therefore suspected that these meteor showers broke off enough sodium to fire a particularly bright spot.
However, the all-sky imager camera in the El Leoncito observatory in Argentina tells a slightly different story, which took 21,000 images of the moon between 2006 and 2019.
Annual meteor showers – like the Leonids, one of the most intense – can coincide with a lighter moonspot. This is not always the case, however, perhaps because its effects are not always forceful enough to throw lunar sodium far enough away from the moon that it can contribute to the comet-like tail and its moonspot.
Influences from sporadic meteors that do not occur in regular showers correlate more strongly with the brightness of the moonspot. This may be because they can be more massive, faster, and collide head-on with the moon, meaning they can push more sodium to a higher orbit.
If a suitably sized asteroid slams into the moon with enough momentum, it can potentially emit enough sodium to create a moon beam that anyone can see with the naked eye, said James O’Donoghue, a planetary scientist with the Japan Aerospace Exploration Agency, who made it was not involved in the research. And if you could watch it, “it would be a blurred patch of light the size of Orion’s belt-stars,” said Baumgardner.
But even invisibly, it is satisfying enough to know that the earth has a meteor-powered moonbeam – a reminder of the moon’s dynamics.
“I think we definitely take it for granted,” said Dr. O’Donoghue.