Lunar Swirl Morphology Constrains the Geometry, Magnetization, and Origins of Lunar Magnetic Anomalies

• Published in: Journal of geophysical research. Planets 2018-08, Vol.123 (8), p.2223-2241
• Main Authors: Hemingway, Douglas J., Tikoo, Sonia M.
• Format: Article
• Language: English
• Subjects: Basalt; Carrying capacity; Dikes; Geometric constraints; Lava; lava tubes; lunar magnetic anomalies; Lunar maria; Lunar orbits; Lunar soil; Lunar surface; lunar swirls; Magnetic anomalies; magnetic field topology; Magnetic fields; Magnetism; Magnetization; Mathematical morphology; Metal content; Moon; Origins; Plasma interactions; Rocks; Solar magnetic field; Solar wind; Tubes
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1029/2018JE005604

Lunar swirls are collections of finely structured bright and dark surface markings, alternating over length scales of typically 1–5 km. If swirls are the result of plasma interactions with crustal magnetic anomalies or electrostatic or magnetic sorting of fine materials, the magnetic field orientation must vary over similar length scales. This requires that the associated source bodies be both shallow and narrow in horizontal extent. The correspondingly restricted volume of the source bodies in turn implies strong rock magnetization. Here we show that if ∼300‐nT surface fields are necessary to produce observable swirl markings, the required rock magnetization must be >0.5 A/m, even for very shallow sources and likely closer to ∼2 A/m or more. This strong source rock magnetization, together with the geometric constraints that favor magmatic structures such as dikes or lava tubes, requires a mechanism to enhance the magnetic carrying capacity of the rocks. We propose that heating associated with magmatic activity could thermochemically alter host rocks and impart them with magnetizations an order of magnitude stronger than is typical of lunar mare basalts. Our results both place constraints on the geometry and magnetization of the source bodies and provide clues about the possible origins of the Moon's crustal magnetic anomalies. Plain Language Summary Lunar swirls are collections of finely structured, bright and dark surface markings, alternating over length scales of typically 1–5 km. Swirls are thought to form where local magnetic fields shield parts of the lunar surface from exposure to the solar wind or where those magnetic fields lead to sorting of some of the finest lunar soils. In either case, the length scales of swirls are effectively telling us about the structure of near‐surface magnetic fields on scales that are finer than what can be measured from lunar orbit. Here we use this length scale information, along with estimates of the strength of those magnetic fields, to obtain new constraints on the underlying magnetized rocks, showing that they must be shallow, narrow, and strongly magnetized. This result helps us to better understand the origin of these magnetized rocks and the history of lunar magnetism more generally. In particular, we suggest that these rocks were likely injected into the crust in the form of dikes or subsurface channels of flowing lava and that they cooled slowly, leading to enhancement of their metal content and enabling th