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

Abstract

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 SummaryLunar 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 the rocks to capture a stable record of the Moon's ancient global magnetic field. Key Points:• Lunar swirl morphology suggests finely structured near-surface magnetic fields, requiring narrow, shallow magnetic source bodies • The source bodies underlying swirls must be much more strongly magnetized than is typical of lunar mare basalts • The inferred source geometry and magnetization could be explained both by magmatic intrusions and associated thermochemical alteration

Investigating Sources of Mercury's Crustal Magnetic Field: Further Mapping of MESSENGER Magnetometer Data

“…Both the SPA ejecta and the younger basin ejecta models have an advantage in that the susceptibility of metallic iron‐enriched ejecta to TRM is orders of magnitude larger than that of typical lunar igneous rocks such as mare basalt (Weiss & Tikoo, ). On the other hand, Hemingway and Tikoo () have recently proposed that, under favorable conditions (high‐temperature magmas in a highly reducing environment), dikes or lava tubes with thicknesses >1 km could acquire magnetization intensities consistent with observed magnetic anomaly amplitudes.…”

“…Some groups have suggested igneous sources of lunar anomalies consisting of vertical tabular (dike) intrusions (Hemingway & Tikoo, 2018;Purucker et al, 2012;Tsunakawa et al, 2014Tsunakawa et al, , 2015 while other groups have suggested sources in the form of impact basin ejecta deposits (Hood et al, 2013;Wieczorek et al, 2012). Analyses of Apollo samples found that impact melt rocks originating from large basin-forming impacts contain enhanced metallic iron abundances (1-2 wt %) that were apparently derived from the impactors that formed the basins (Haskin et al, 1998;Korotev, 1994Korotev, , 2000.…”

Reiner Gamma: A Magnetized Elliptical Disk on the Moon

“…Furthermore, the disk model is more physically plausible than linear structures. Intrusive bodies or lava tubes, which are candidates for producing linear magnetic anomalies (Hemingway & Tikoo, 2018), would not typically have the high degree of symmetry and ellipticity observed in the optical pattern. In contrast, two-dimensional circular or elliptical features are ubiquitous on the Moon in the form of craters.…”

“…In particular, researchers have suggested that the Reiner Gamma anomaly may be ejecta from the Cavalerius (Hood et al, 1979a) or Imbrium impacts ; the latter supported by the possible orientation of Reiner Gamma optical pattern toward Imbrium), although they did not model these processes in detail. Other studies suggest that volcanic processes may be responsible (Hemingway & Tikoo, 2018;Purucker et al, 2012), and there is still the possibility that some were magnetized by plasma-amplified magnetic fields (Hood & Artemieva, 2008).…”

“…They proposed that two linear magnetic source bodies could produce these cusp structures. Linear sources for lunar swirls were also proposed by Hemingway and Tikoo (2018). However, it is not clear what physical process would produce a highly symmetric pair of line sources at Reiner Gamma.…”