Constraining the Early History of Mercury and Its Core Dynamo by Studying the Crustal Magnetic Field

Abstract

Low‐altitude magnetic field data acquired by MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) over a small portion of Mercury's surface revealed weak crustal magnetic field signatures. Here we study the crustal magnetic anomalies associated with impact craters on Mercury. We assume that the sources of these anomalies consist of impact melt, enriched in impactor iron. We assume that the subsurfaces of Mercury's impact craters have cooled in the presence of a constant global magnetic field, thus becoming thermoremanently magnetized. We invert for the crustal magnetization direction within five craters using a unidirectional magnetization model which assumes that the melt impact rocks recorded the constant core magnetic field present when the crater was formed and that the crater's magnetization has not been altered since its formation. From the best fitting magnetization direction we then obtain the corresponding north magnetic paleopole position assuming a centered core dipolar field. Results show that all five magnetic paleopoles lie in the southern hemisphere but are not required to be located near the present‐day magnetic pole, which lies near the south geographic pole. Accounting for the uncertainties, we show that our results all agree in a common small region that excludes the current magnetic pole. This strongly suggests that the dynamo has evolved with time. Our results represent valuable information for understanding the evolution of Mercury and emphasize the importance of including more anomaly analyses to complete and refine our conclusions.

The BepiColombo Planetary Magnetometer MPO-MAG: What Can We Learn from the Hermean Magnetic Field?

“…They concluded that this is very unlikely, confirming the results of at the global scale. More recently, Oliveira et al (2019) examined five magnetic field anomalies located above impact craters, and focused on magnetization directions, using a method previously used on the Moon and on Mars (Oliveira and Wieczorek 2017;Thomas et al 2018). This can indeed be used to infer properties about the magnetic field which was present when the magnetization was acquired, when the rock cooled down.…”

Evidence for an Ancient Near‐Equatorial Lunar Dipole From Higher Precision Inversions of Crustal Magnetization

“…How to choose the maximum misfit is an open question. Several authors have chosen a misfit equal to the RMS of the background field, using the assumption that the background field is statistically similar to the unmodeled magnetic field, that is the portion of the magnetization that is not a part of the unidirectionally magnetized anomaly (Oliveira & Wieczorek, 2017; Oliveira et al., 2019; Lee et al., 2019; Thomas et al., 2018). The instrument noise from magnetometers on Lunar Prospector and Kaguya is less than 0.1 nT (Lin et al., 1998; Tsunakawa et al., 2010).…”

“…One major concern of magnetic paleopole analyses is the uncertainty associated with the best‐fit magnetization direction. Large uncertainties (near 90° in some cases, e.g., Oliveira et al., 2019; Oliveira & Wieczorek, 2017; Thomas et al., 2018) make it difficult to determine if paleopoles found distant from the present pole truly represent deviation of the dipole axis from the spin axis. Unfortunately, there currently exists no consistent method of describing direction uncertainties across inversion methods (Maxwell et al., 2017).…”

The BepiColombo–Mio Magnetometer en Route to Mercury

“…Moreover, both spacecraft (Mio and MPO) have nearly symmetric orbits between the northern and southern hemispheres, ideal to separate the magnetic field of internal origin from that of external origin and to perform a spherical harmonic analysis on the planetary internal field. The internal sources include the magnetic field generated by the dynamo mechanism (Wicht and Heyner 2014), the remanent crustal field Oliveira et al 2019), and time-varying induction field in response to changes in the solar wind and magnetospheric conditions (Grosser et al 2004;Johnson et al 2016). The external fields originate in the currents flowing in the magnetosphere, e.g., magnetopause current on the dayside and in the tail, plasma sheet current, field-aligned current, and possibly a partial ring current (Müller et al 2012;.…”