Strength, Depth, and Geometry of Magnetic Sources in the Crust of the Moon From Localized Power Spectrum Analysis

Abstract

Spacecraft observations show that weak magnetic fields of crustal origin are ubiquitous across the surface of the Moon. To investigate the origin of these magnetic anomalies, a model was developed for the magnetic power spectrum that consists of ensembles of randomly magnetized sills or prisms. Localized spectrum analyses constrained how the parameters of this model vary with position, including the size of the sources, a quantity proportional to their mean‐squared dipole moment, and the depth to the top and bottom of the magnetized region. The depth to the top of the magnetized region varies from the surface to about 25 km. The magnetic carriers in the deep crust likely formed at the same time as the crust itself, implying that a core‐generated dynamo field must have existed when the crust was cooling during the first 100 Myr of lunar evolution. The parameter related to the strength of magnetization shows the existence of a prominent region on the nearside hemisphere that is largely unmagnetized and that correlates with a region of extremely low surface field strengths. This region lies entirely within a geological province that is highly enriched in heat‐producing elements (the Procellarum KREEP Terrane), suggesting that this region escaped being magnetized because of prolonged high crustal temperatures. The nearside magnetic low may be representative of the size of that portion of the crust that is highly enriched in heat‐producing elements, which is almost one third the size of the Procellarum KREEP Terrane based on surface thorium abundances.

Distribution of Radioactive Heat Sources and Thermal History of the Moon

“…As explained in detail in Wieczorek (2018), a region in the central portion of the PKT is found to have significantly lower magnetic field strengths and magnetizations than does the rest of the PKT. The low-field strengths of this region were ascribed to locally higher concentrations of heat-producing elements that could have caused this region to have temperatures above the Curie point of iron metal either when the core dynamo was operating or when the strongest dynamo field strengths were present.…”

“…Thus, as noted by Wieczorek (2018), if the weak magnetic fields that are observed throughout the highlands crust are the result of thermoremanent magnetization acquired in the presence of a dynamo field, a dynamo must have been operating during the first 100 Myr of lunar evolution. The average thickness of the Moon's crust is somewhere between 34 and 43 km and could be locally as high as about 80 km for some regions on the farside hemisphere.…”

“…The observed concentration of heat-producing elements on the nearside hemisphere of the Moon in a region that encompasses the Imbrium impact basin and Oceanus Procellarum, the Procellarum KREEP Terrane (PKT; Figure 2. Strength of a parameter that is proportional to crustal magnetization (M) over the nearside hemisphere (Wieczorek, 2018), which shows the size of the low-magnetism region (blue) in comparison to the traditional Procellarum KREEP Terrane boundary defined by 4-ppm Th (white contour). The black contour represents the size of the Imbrium impact basin, the image is presented using a Lambert azimuthal equal projection, and grid lines are spaced every 30 ∘ in latitude and longitude.…”

“…In an analysis of the Moon's magnetic field by Wieczorek (2018), it was noted that the magnetic field strength is extremely weak within the interior portion of the PKT (see Figure 2). In that study, the inversion for a quantity proportional to the crustal magnetization showed that this quantity was about an order of magnitude smaller than elsewhere.…”

Reiner Gamma: A Magnetized Elliptical Disk on the Moon

“…The present consensus is that many, if not all, of the Moon's crustal magnetic anomalies formed in the presence of an ancient dynamo field (e.g., Arkani-Hamed & Boutin, 2017;Hood et al, 2017;Nayak et al, 2017;Oliveira & Wieczorek, 2017;Takahashi et al, 2014;Weiss & Tikoo, 2014;Wieczorek, 2018). However, the geologic formation mechanism for any of these anomalies has yet to be established.…”