Magnetization in the South Pole-Aitken basin: Implications for the lunar dynamo and true polar wander

Abstract

A number of magnetic anomalies are present along the northern edge of the lunar South Pole-Aitken (SPA) basin. A variety of hypotheses for their formation have been proposed, but an in-depth study of their properties has not been performed. Here we use two different methods to invert for their source body characteristics: one that completely searches a small parameter space of less than ten uniform-strength dipoles per anomaly, and another that uses grids of hundreds of dipoles with variable magnetization strengths. Both methods assume uniform magnetization directions at each anomaly and with one exception, produce nearly the same results. We introduce new Monte Carlo methods to quantify errors in our inversions arising from Gaussian time-dependent changes in the external field and the uncertain geometry of the source bodies. We find the errors from uncertainty in source body geometry are almost always higher. We also find a diverse set of magnetization directions around SPA, which we combine with other physical arguments to conclude that the source bodies were likely magnetized in a dynamo field. Igneous intrusions are a reasonable explanation (Purucker et al., 2012) for the directional variability, since they could be intruded over different magnetic epochs. However the directional variability also implies either surprisingly large amounts of true polar wander or non-axially aligned dynamo fields. We also explore the possibility that true polar wander caused by the SPA impact could allow iron-rich SPA ejecta to record a diverse set of magnetic field directions. Some of this material may have also become "sesquinary" ejecta and re-impacted across the Moon on 10 4-10 6 year timescales to capture such changes. No completely satisfactory answer emerges, except that the dipoleaxis of the lunar dynamo may have been variable in direction. the vector magnetic field at up to 18 Hz and transmitted its measurements at a reduced 87 resolution of 9 Hz. Level 1B LP-MAG data are obtained from the NASA Planetary Data 88 System (ppi.pds.nasa.gov). The SELENE/Kaguya spacecraft used a tri-axial fluxgate 89 magnetometer with a sampling rate up to 32 Hz. K-MAG magnetometer data are obtained 90 from the SELENE data archives (l2db.selene.darts.isas.jaxa.jp). The cadence of 91 measurements used in this study is 0.2 Hz for LP-MAG and 0.25 Hz for K-MAG. 92 Topography data are from the Lunar Orbiter Laser Altimeter (LOLA) (Smith et al., 2010) 93 (pds-geosciences.wustl.edu). 94 95 To best capture the Moon's crustal field, all data used for analysis were collected in either 96 the lunar wake or while the Moon was in the Earth's magnetotail (wake/tail), avoiding 97 distortions caused by the solar wind noted by (Halekas et al., 2008; Kurata et al., 2005). 98 Tail datasets specifically exclude epochs during which plasma sheet disturbances were 99 noted (Halekas et al., 2012). Consecutive orbits are ~1° in longitude apart. At 0.2 Hz, 100 successive magnetometer measurements are separated by ~8 km in the latitudinal 101 direction. All data used...

The Lunar Paleo‐Magnetosphere: Implications for the Accumulation of Polar Volatile Deposits

“…Here, we explore equatorial surface field values of 0.5, 2, and 5 μT and ultimately will show that paleomagnetic fields of these strengths have considerable effects on the solar wind H + surface flux. As for the orientation of the ancient dynamo, numerous recent studies have shown that the dynamo axis may not have been steady in time (Arkani-Hamed & Boutin, 2017;Baek et al, 2019;Nayak et al, 2017;Oliveira & Wieczorek, 2017;Takahashi & Tsunakawa, 2009). Hence, we model the field as a centered dipole oriented in the +Z SSE direction (parallel to the lunar spin axis) and also as a dipole perpendicular to the spin axis, in the +X/+Y SSE plane.…”

“…For example, if the paleofield was weak enough, compressions of the magnetopause to the lunar surface under high solar wind pressure conditions could distort the local magnetic field at the lunar surface. This could confound efforts to infer the dynamo field orientation from Apollo samples (Cournède et al, 2012) and crustal magnetic anomalies (Nayak et al, 2017). The frequency and efficiency of such compression events may have been larger if the ancient solar wind density was higher than it is today (Airapetian & Usmanov, 2016) and if the sun produced more frequent coronal mass ejections (Airapetian et al, 2014).…”

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.…”

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

“…Though a few of these anomalies are associated with impact basins, the vast majority have no known correlation with any lunar geologic process. Many investigations have investigated the strength and direction of magnetization of the strongest and most isolated of these anomalies (e.g., Arkani‐Hamed & Boutin, ; Blewett et al, ; Halekas et al, , ; Hemingway & Garrick‐Bethell, ; Hood, ; Hood et al, , ; Mitchell et al, ; Nayak et al, ; Nicholas et al, ; Oliveira & Wieczorek, ; Oliveira et al, ; Purucker et al, ; Richmond et al, , ; Takahashi et al, ; Wieczorek et al, ).…”