Formation of lunar swirls: Implications from Chang’E-1 Interference Imaging Spectrometer data

Lunar swirls remain one of the most enigmatic geological features on the Moon's surface. They appear as sinuous, high albedo patterns that are interwoven with “dark lanes” and stand out against the low-albedo background. Their unique spectral properties and strong correlation with lunar magnetic anomalies have attracted widespread scientific interest. The origin of lunar swirls is still debated. The prevailing solar wind deflection model suggests that pre-existing magnetic anomalies deflect incoming solar wind particles, leading to different degree of space weathering inside and outside the swirls and resulting in their distinctive spectral characteristics. As a key product of space weathering, nanophase iron (npFe⁰) directly reflects this differences inside and outside the swirls. In this study, we investigated the npFe⁰ content distribution of the swirl regions, offering a new perspective on the origin of lunar swirls.

In this study, we developed a model to estimate npFe⁰ content in lunar highland and maria soils using band ratio of remote sensing data based on laboratory-measured spectral data and npFe⁰ content of returned Apollo lunar samples. Then, this model was employed to the hyperspectral data acquired by Chang’E-1 Interference Imaging Spectrometer (IIM) to map the npFe⁰ content across five typical lunar swirl regions including Reiner Gamma, Mare Ingenii, Rima Sirsalis, Airy, and Firsov. Our results showed that npFe⁰ content in on-swirl regions is lower than that in off-swirl regions, indicating a suppressed space weathering effect within the swirl regions. Moreover, the relative npFe⁰ abundance between swirl dark lanes and surrounding off-swirl regions seems to be linked to different stages of space weathering. The distinct difference in npFe⁰ abundance between on-swirl regions and off-swirl fresh craters could be due to their different weathering processes. Additionally, we found a correlation between npFe⁰ abundance and the intensity of lunar magnetic anomalies in swirl regions. This indicates that the shielding effect of magnetic anomalies against solar wind particles may be influenced by the strength of the magnetic field. A potential relationship between npFe⁰ and OH^-/H₂O distributions within swirl regions also offer valuable insights into the solar wind-induced formation of lunar surface water. These findings support the hypothesis that incoming solar wind particles are deflected in swirl regions, leading to reduced space weathering on their surfaces.