New Study Explains Why the Moon’s Dust Cloud Is Heavier on Its Sunlit Side

New Study Explains Why the Moon’s Dust Cloud Is Heavier on Its Sunlit Side

A diffuse cloud of dust follows the Moon as it orbits Earth, but observations have long shown this cloud to be uneven — consistently denser over whichever hemisphere is sunlit. A new computer-modelling study offers a straightforward explanation: temperature-dependent dust ejection from micrometeoroid impacts.

Regolith and micrometeoroids. Most of the lunar surface is covered by regolith, a layer of gray dust and loose rock produced when micrometeoroids — tiny fragments generated by asteroid collisions and comet activity — continually strike the surface. Without an atmosphere to burn them up, the Moon is hit by several tons of micrometeoroids every day; those impacts grind surface rock into fine dust and can loft grains into space.

A sparse but asymmetric cloud. Since about 2015 researchers have known that these lofted grains form a vast, very tenuous cloud extending several hundred miles above the surface. It is far too thin to see with the naked eye: peak densities are only about 0.004 particles per cubic meter (roughly the equivalent of four dust grains in a large grain silo). Yet the cloud is asymmetric, with more dust over the daytime hemisphere and especially concentrated near the dawn terminator — the sharp line separating night from day on the lunar surface.

“The maximum density measured was only 0.004 particles per cubic meter,” said Sébastien Verkercke, the study’s lead author and a postdoctoral researcher at the Centre National d’Etudes Spatiales (CNES) in Paris.

Earlier explanations attributed the asymmetry to particular meteoroid streams whose incoming trajectories preferentially strike the Sun-facing side. Verkercke and colleagues instead considered a simpler large-scale difference: temperature. Lunar daytime temperatures can far exceed the hottest places on Earth, while lunar nights drop to temperatures many times colder than Antarctic averages — a swing of up to about 545°F (285°C).

Computer simulations. To test whether temperature controls how much dust is lofted, the team ran detailed numerical simulations of hair-width meteoroids striking regolith at two representative thermal states: roughly 233°F (112°C) for typical daytime surfaces and about −297°F (−183°C) for pre-dawn conditions. The model tracked each ejected grain individually to map how impacts distribute dust into space.

The simulations also varied surface packing from loosely packed, “fluffy” regolith to more compacted layers. Results showed that fluffy surfaces cushion impacts and produce less ejecta, while more compact surfaces release larger numbers of slow-moving dust grains.

Key findings. Daytime (warmer) impacts produced about 6%–8% more dust than equivalent cold-side impacts. In addition, a larger fraction of grains ejected from the warmer surface had enough energy to reach orbital altitudes where satellites can detect them. Those two effects — modestly greater ejecta mass plus a higher fraction reaching detector heights — plausibly account for the observed daytime excess in the lunar dust cloud.

The study was published Oct. 15 in the Journal of Geophysical Research: Planets and includes researchers from CNES and several U.S. and European universities.

Broader implications. The authors plan to apply the same modelling approach to other airless bodies bombarded by micrometeoroids. Mercury is a particularly intriguing case because its daytime temperatures are much higher than the Moon’s and its day–night temperature contrast is larger; the team predicts an even stronger dust-cloud asymmetry there. The ESA–JAXA BepiColombo mission to Mercury may supply data to test that prediction.