The Moon is the most studied object in space. But our nearest neighbour still holds a few mysteries. One of those mysteries is the lunar swirls. These strange serpentine features are brighter than their surroundings and are much younger. They’re not associated with any specific composition of lunar rock, and they appear to overlay other surface features like craters and ejecta.
Scientists have been puzzling over the swirls for decades, and with lunar outposts looming as a real possibility, understanding these swirls takes on new importance. Now a new study finds a link between lunar topography and the swirls.
Dust transport on the Moon is a poorly understood phenomenon. There’s no atmosphere, so there’s no wind to move dust around. Yet dust must be moving to form the swirls.
We’ve known about the dust movement on Mars since the earlier days of space exploration. Some of NASA’s Lunar Surveyors found evidence of it, as did Apollo 17’s Lunar Ejecta And Meteorite (LEAM) experiment.
The Moon is geologically dead or close to it. Whatever’s causing the lunar swirls, it’s not geology. The Moon has a plasma environment involving the solar wind, the terrestrial magnetotail and other factors. Scientists invoke them as the engine of dust transport, and they’ve even pointed to comet impacts as a cause. These effects are powerful enough to shift tiny particles around the surface in the absence of wind. But much of the detail around the phenomenon is obscured.
A new study found a link between the swirls and the Moon’s topography. While the connection between the swirls and topography may be the beginning of an explanation for the swirls, it also leads to more questions.
The new study is “Topographic Correlations Within Lunar Swirls in Mare Ingenii.” The lead author is Deborah Domingue, a Senior Scientist at the Planetary Science Institute. The study is available online at the journal AGU Geophysical Research Letters.
There are three potential explanations for lunar swirls. The first is the cometary impact model. In this model, a comet struck the Moon, and the coma’s turbulence exposed the surface by transporting fine dust. The heat from comet impacts magnetized some materially locally, and the magnetized material responded to remnant magnetism in the lunar crust. This chain of events could’ve created the swirled morphology.
The second model is the solar wind shielding model. This model says that magnetic features on the Moon protected some material from the wind while allowing material with specific albedoes to move around with the solar wind. This formed the swirls over time.
The third is the dust transport model. This model says that interactions between crustal magnetic anomalies and the solar wind attracted or repelled fine lunar dust.
These explanations have to account for magnetism and the swirls’ youthful appearance. But none of them hint at topography, which sets this new research apart.
“This is the first time there has been a demonstrated correlation between the swirl albedo patterns and topography,” said lead author Deborah Domingue.
“Until now, the swirls were thought to overlay the topography, which has been cited as part of the evidence that they are created through shielding of the surface from the solar wind by the magnetic fields present at swirls. This correlation argues that there is more than just shielding from space weathering that goes into their creation,” Domingue said.
The swirl region within Mare Ingenii is one of the areas examined in the study. It’s one of the lunar swirls that displays a correlation between the swirls and topography. Image Credit: LROMare Ingenii is one of the Moon’s basaltic plains. Ancient volcanic eruptions created the lunar mares, appearing dark because of their higher iron content. Mare Ingenii has two swirl features, and they both show a correlation between topography and albedo. The bulk elevation in the bright areas of the swirl is lower than in the dark regions. The different heights affect dust transport.
“For swirls, dust transport is the process most affected by elevation changes, and we now re-examine the role of dust mobility across the lunar surface in the context of this new discovery,” Domingue said.
The elevation differences between brighter on-swirl and darker off-swirl areas are not huge. The differences range from about 2.3 meters to 4.4 meters. But they’re consistent, and scientists can’t ignore them.
This figure presents results for Study Area B, one of two study areas in the research. The high-resolution DEM (Digital Elevation Model) is from the Lunar Reconnaissance Orbiter Camera (LROC) and the Narrow-Angle Cameras (NAC) on NASA’s LRO. Image Credit: Domingue et al. 2022.Magnetic anomalies on the lunar surface play a role too. The Moon has a weak magnetic field, but it doesn’t come from an internal dynamo as Earth’s does. Instead, it comes from different mineral concentrations in the crust and could be related to giant impacts.
The brighter sections of the swirls are areas shielded by magnetic anomalies, and spectral analysis shows they’re younger than the dark areas. The lighter regions also show “… a decreased abundance of implanted solar wind hydrogen, which forms OH (hydroxide.).” Spectral analysis of the darker areas shows that they’re older. According to the research, this indicates that shielding of the solar wind helps form the swirls.
This figure from the study presents the off-swirl and on-swirl heights in the lunar swirls in both study areas. Note that the height is along the horizontal axis. The decreasing mean heights from off-to on-swirl are apparent. Image Credit: Domingue et al. 2022When Apollo astronauts set foot on the Moon, they were surprised by the fine dust. When they entered and exited the lunar module, dust came along for the ride. NASA says the dust “… clogged mechanisms, interfered with instruments, caused radiators to overheat and even tore up their spacesuits.”
“We learned from Apollo that lunar dust can be less than 20 microns (about 0.00078 inches) in size,” said Sharon Miller, the passive dust shedding material program’s principal investigator at NASA Glenn. “The dust is very fine, abrasive and sharp, like tiny pieces of glass, making it more of a dangerous threat than just a simple nuisance.”
This picture shows Apollo 17 astronaut Harrison Schmitt collecting a soil sample. Notice how dust coats his spacesuit. Credit: NASALunar dust is highly abrasive. All kinds of atmospheric and geological activity grinds away at the dust on Earth, making it smoother. But that’s not the case on the Moon.
The lunar swirls are more than just a curiosity. With lunar exploration ramping up in the next years, those swirls indicate one of the puzzles yet to be solved. And if the dust is dangerous to equipment, then it’s a hazard that must be understood and mitigated.
“Experiments on dust transport have not yet been conducted at the km scales of our subregions and at the relatively shallow depth differences between on-swirl and off-swirl regions, but there is support for electrostatic dust collection in topographical lows on airless surfaces,” the authors write in their conclusion. Some laboratory experiments show particles can be transported into depressions, and simulations show that dust can be deposited into craters as shallow as one meter deep.
Lunar swirls imaged by SMART-1, the ESA’s first Moon mission. Image credit: ESAThis all leads to more questions about dust transportation on the Moon, and answers to some of those questions will help missions like Artemis. But at least scientists are making some progress. The team behind this study only examined two swirl regions, and they say other regions should be studied to strengthen the link between lunar dust movement and topography.
“The topographically low, higher albedo, on-swirl regions observed in Mare Ingenii support the process of dust migration across the lunar surface, contributing to swirl formation,” the authors conclude. “Future examination of other swirl regions will potentially clarify the correlation of regional topographic differences within swirls, especially in cases where swirls appear on much steeper slopes and drape over impact ejecta.”
