Lunar swirls influenced by soil structure

Lunar swirls are distinct high- and low-albedo patterns found on the Moon's surface, both in the dark basalt plains known as mare and the bright highland regions. The origins of these features remain a subject of scientific debate.

"The scientific community has long been examining the differences between the bright and dark regions in these distinctive albedo markings," said Deborah Domingue, Senior Scientist at the Planetary Science Institute and lead author of "Photometric Properties Within Reiner Gamma Swirl - Constraining Formation Mechanisms" in The Planetary Science Journal.

PSI scientists John Weirich, Frank Chuang, Samuel Courville, Roger Clark, Amanda Sickafoose, Eric Palmer, and Robert Gaskell contributed to the study.

"Reiner Gamma is the archetype lunar swirl, associated with magnetic anomalies. Previously, these swirls were thought to traverse the surface without being influenced by topography," Domingue said. "Recent research by a PSI team has shown that bright 'on-swirl' areas are typically a few meters lower than dark 'off-swirl' areas. This study examines a region within the Reiner Gamma swirl, investigating if there are variations in the regolith (soil) structure between on- and off-swirl areas."

The study reveals that intrinsic brightness differences, defining these features, are due to variations in composition, porosity, and grain size of the regolith. The research indicates that compositional differences primarily drive these brightness changes, rather than differences in soil compaction. Grain size variations might also play a role, but compaction differences were not observed.

"Grain size differences might also contribute to the brightness variations, but we do not see differences in compaction within our study area. We also note differences in grain types and structures between bright and dark area soils. This indicates that there is more than one process at work to form these features," Domingue said.

Three main hypotheses exist for the formation of lunar swirls: 1) solar wind shielding by magnetic anomalies prevents space weathering, 2) dust is levitated and sorted by magnetic fields, and 3) the swirls are remnants of comet impacts. The paper evaluates evidence for and against each hypothesis, suggesting that multiple processes likely contribute to the formation of these features.

"There are a couple of processes that have been put forth for explaining the presence of these features on the Moon, yet the evidence is contradictory. This paper is important because it adds another set of properties - regolith structure - to the evidence. Interpretation of this new evidence, in context of past work, all indicate that these features are the result of a complex interaction of multiple processes, and that laboratory studies will be needed to deconvolve the relative roles of these processes in the formation of lunar swirls," Domingue said.