At first glance, these formations exhibit an atmospheric quality: nebulous swirls, clouds, and loops that stand out brightly against the surrounding, darker lunar regions. The often kilometer-long swirls are practically ubiquitous in all types of terrain but are concentrated on the moon’s far side. They appear to frequently traverse independently of the surrounding geological structures.
The origin of these formations is uncertain. A 2015 study attributed the “lunar swirls” to comet impacts. However, another investigation contradicts this by noting the absence of impact traces at the appropriate positions for the largest lunar swirls. Alternative hypotheses involve the influence of magnetic fields or electric fields, affecting the orientation of fine dust particles.
Now, topographic maps of the moon contribute new elements to the enigmatic puzzle, raising additional questions. Detailed elevation data reveal a correlation between lunar swirls and topography: the bright structures, on average, appear to reside at a different elevation than their darker surroundings.
“For a long time, the prevailing view was that the topography had little influence on the location or shape of lunar swirls,” stated John Weirich of the Planetary Science Institute. However, in 2022, data from NASA’s Lunar Reconnaissance Orbiter indicated, for the first time, that there were indeed overlooked connections: in a lunar swirl in the Mare Ingenii region on the far side of the Moon, the bright areas averaged three meters below the surrounding dark zones.
Weirich and his team now report in the Planetary Science Journal on similar observations in another swirl. In the structure known as Reiner Gamma, the bright zones are about four meters deeper than the darker regions. “Finding a relationship with topography in one swirl location could just be a fluke, but finding it in two vastly separate swirl regions is harder to ignore. It is especially hard to ignore because Reiner Gamma is the archetypical lunar swirl,” says Weirich. “It is especially hard to ignore because Reiner Gamma is the archetypical lunar swirl.”
The matter is even more complex, Weirich continued. “it is not as simple as the bright areas are uniformly lower than the dark areas. If that was the case this relationship between topography and swirl would be easy to demonstrate by comparing an elevation map to a picture of the swirl.” Instead, this correlation only became apparent when the researchers compared the average height of the light areas with the average height of the dark zones. The mechanism behind this remains a mystery.
For their study, Weirich and his colleagues utilized not only the topographic data from images captured by the Lunar Reconnaissance Orbiter Camera (LROC) but also the Stereophotoclinometry software of the probe, which uses stereo images to determine the height of the lunar surface. A machine learning program ultimately assisted the team in identifying the transitional areas between the two regions, referred to as “diffuse-swirl unit.”
How could these observations be related to the causes of lunar swirls? Wierich does not have a ready answer. Ultimately, there is still a significant lack of understanding: “Forming them could involve a combination of well- understood processes interacting together, or a currently unknown process. Unusual objects or phenomenon are sometimes the key to obtaining deeper knowledge, and for this reason lunar swirls are very intriguing. And the fact that they look really cool.“