Lopsided cloud hanging hundreds of miles over the moon may finally have an explanation
A strange, unbalanced cloud of dust envelops the Earth. moonalways tilted towards the side facing the sun. Now, a new study could finally explain how the asymmetric cloud took its shape.
Most of the Moon’s surface is covered in a layer of gray dust and loose rocks. This layer, called regolith, appears because the lunar surface is constantly bombarded by micrometeoroids — tiny space rocks created by collisions of asteroids and comets. Without a protective atmosphere — which, in EarthIn the case of this planet, micrometeoroids burn as “shooting stars” – the moon is hit daily by several tons of micrometeoroids. These impacts, in turn, grind the regolith rocks into dust.
Micrometeoroids also kick up lunar dust. In 2015, researchers find that this rising dust creates a massive cloud that extends several hundred kilometers above the lunar surface. The cloud is not very thick and is not visible to the naked eye, Sébastien Verkerckepostdoctoral researcher at the Center National d’Etudes Spatiales (the French national space agency) in Paris and first author of the new study, told Live Science in an email.
“The maximum density measured was only 0.004 particles per cubic meter (the equivalent of 4 grains of dust in a grain silo),” he said. However, the cloud has the particularity of being asymmetrical, with more dust present on the day side of the moon (the side facing the sun at a given moment) than on its night side. In fact, the cloud is “densest near the surface, near the dawn terminator” added Verkercke, referring to the sharp line that separates sunlight from darkness on the moon’s surface.
The cloud’s discoverers had attributed this imbalance to specific meteoroid groups whose trajectories cause meteoroids to strike the daytime surface more frequently. But the obvious difference between the day and night sides of the moon – the temperature – struck Verkercke.
While the moon’s surface is often grill during the daywith temperatures well above those of the hottest place on Earth, the lunar night is four times colder than the average temperature of Antarctica. This enormous temperature swing of up to 545 degrees Fahrenheit (285 degrees Celsius) led Verkercke and his co-authors to wonder whether it might be responsible for the cloud’s asymmetrical appearance.
To test this hypothesis, Verkercke and his colleagues (researchers from American and European universities) turned to computer models. The team simulated tiny meteoroids – each the width of a human hair – crashing into lunar dust at two temperatures, 233 degrees Fahrenheit (112 degrees Celsius) and minus 297 degrees Fahrenheit (minus 183 degrees Celsius), corresponding to the moon’s average daytime and predawn temperatures, respectively.
“The ejected dust grains are then tracked individually to monitor their distribution in space,” Verkercke said. The researchers also repeated the simulations while varying the compactness of the packaging.
They found that meteoroids that hit “softer” surfaces shed smaller amounts of dust, because the fluffiness of the surface cushions the impacts. In contrast, meteoroids that strike more compact surfaces produce larger quantities of dust particles at low speeds. Researchers believe this difference means that dust clouds may be an indicator of the compactness of the lunar surface.
Additionally, daytime meteoroids kick up 6-8% more dust than nighttime meteoroids. And a larger fraction of these high-temperature dust particles (compared to those formed at lower temperatures) have enough energy to reach the height of orbiting satellites that can detect them. The larger amounts of dust kicked up and the larger fractions of dust reaching satellites could explain the excess daytime dust, the researchers explain in the study published October 15 in the journal. Geophysical Research Journal: Planets.
The team plans to extend its analysis to other organisms in the solar system which are impacted by small meteoroids. Verkercke noted that a particularly interesting case is that Mercurywhich has a much warmer temperature than the moon’s daytime surface and therefore a greater day-night temperature difference. This in turn should create an even more asymmetrical dust cloud.
The researchers hope to virtually replicate this hypothetical observation, that the BepiColombo the mission to Mercury will also investigate, Verkercke added.
