Carl Sagan once said that the earth is just a “substance that hangs in a sunbeam.” He will probably rejoice to know that around the time of a new moon, the earth hangs a speck of dust in a moontail.
The moon, which does not have an atmosphere to protect it, is constantly attacked. When meteorites bombard its volcanic surface, sodium atoms fly high in orbit. The sun’s photons collide with the sodium atoms, effectively pushing them away from the sun and creating a tail-like structure that flows downstream from the moon.
“It makes the moon look like a comet,” said Jeffrey Baumgardner, a senior researcher at Boston University’s Center for Space Physics. “There’s a stream of things coming out of it.”
For a few days each month, as the new moon moves between the earth and the sun, this comet-like tail dusts off the side of our world facing the sun. The gravity of our planet pinches the sodium stream, narrows it into a beam, invisible to the naked eye, which revolves around the Earth’s atmosphere and shoots from space across the planet.
This moonbeam can be seen by special cameras as a spot in the twilight sky. Sometimes it looks brighter, sometimes duller. Since the tail and its beam were first seen in the late 1990s, scientists have been wondering who controls 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 bomb the moon – may explain what controls their flicker.
‘Does it have a practical application? Probably not, “said Mr. Baumgardner, the lead author of the study, said. This research was driven by nothing but curiosity, he said, a desire to simply learn more about the beautiful volcanic pearl in the sky and the mystifying moonbeam.
“I find it very cool,” said Sarah Luettgen, an undergraduate student at Boston University and co-author of the study. “It almost looks like a magic thing.”
The University of Boston has placed several cameras for the sky – in the fish fish lenses that see the entire visible sky – around the world. Originally designed to detect auras, they can see sodium in the earth’s atmosphere with a filter. They usually observe when meteors burn up before reaching the planet’s surface.
In November 1998, during the height of the annual Leonid meteor shower, a team with one such camera at the McDonald Observatory in Fort Davis, Texas, hoped to see the sodium flares. They were surprised when, just after the peak, a spot of sodium persisted in the air for three nights. This place, which appears on the side of the world looking from the sun, brightens as the new moon approaches, and then disappears quickly.
After additional work, including models that simulated where the sodium spot could come from, the team concluded that it should be the result of a comet-like tail of sodium extending at least 500,000 miles from the moon.
The tail may be sprinkling the world with sodium, but it is extremely diffuse, so there is no chance that lunar dust rose will collect on our heads, says Luke Moore, a senior research scientist at Boston University and co-author of the study.
The lunar spot in November 1998 looked particularly bright after the peak of the Leonid meteor shower. It was also seen during other new moons without simultaneous meteoric showers, but it was fainter. The scientists therefore suspected that this meteorite impact cut off enough sodium to evoke a particularly bright spot.
But the all-sky camera in Argentina’s El Leoncito Observatory, which took 21,000 photos of the moon from 2006 to 2019, tells a slightly different story.
Annual meteorite showers – such as the Leonids, one of the most intense – can coincide with a brighter moon spot. But this is not always the case, perhaps because its consequences are not always energetic enough to throw lunar sodium far enough away from the moon so that it can contribute to the comet’s tail and its moon spot.
The impact by sporadic meteors, which do not occur in frequent moods, is more strongly related to the brightness of the moon spot. This is possible because they can be more massive, faster and collide with the moon right in front, which means they can emit more sodium into a higher orbit.
James O’Donoghue, a planetary scientist at the Japanese space agency, James O’Donoghue, as an appropriate substantial asteroid with enough momentum in the moon, was not involved in the research. And if you could observe it, ‘it would be a vague column of light the size of the belt stars of Orion’, said Mr. Baumgardner said.
But even invisible, the satisfaction is enough to know that the earth has a meteor-irradiated moonbeam – a reminder of the moon’s dynamics.
“I think we definitely take it for granted,” he said. O’Donoghue said.