Mercury’s Ancient Crustal Magnetization: A Stronger Dynamo can be Confirmed by Future BepiColombo Measurements

Introduction:  Along with the discovery of Mercury’s dynamo-generated dipolar magnetic field (~200 nT at the equator), measurements from the MESSENGER spacecraft indicate the presence of crustal remanent magnetization [1, 2]. Mercury’s dipole field is anomalously weak (given its large core), which has elicited a variety of dynamo mechanisms (e.g., stable layers, iron-snow, double-diffusive convection, solar wind feedback, asymmetric heat flux, core growth) to explain this oddity [3]. Global measurements of crustal magnetization can give insights into the time-evolution of Mercury’s dynamo and interior.

MESSENGER measured the region spanning 40°-70° N and 120° E - 120° W, showing peak crustal field strengths of ~20 nT measured at ~20 km altitude, with no measurements made <30° N [1]. These crustal fields are co-located with ~3.9-3.7 Gyr old volcanic smooth plains [4, 5] and are inferred to be generated from thermoremanent magnetization (TRM) acquired in the presence of the ancient Hermean dynamo field.  Assuming uniform magnetization, paleointensities of 10¹-10⁴ nT are compatible with the measurements, with large uncertainty due to the poorly constrained TRM efficiency,  χ_TRM, and source thickness (<70 km) [1].

Here, we aim to answer the following: (1) How strong must the Mercury dynamo have been to account for the crustal magnetization observed by MESSENGER? To answer this, we determine the minimum field strength needed to stand off the solar wind and produce a steady field that could magnetize the crust. In answering this, we show that future BepiColombo low-altitude (<100 km) measurements of crustal fields and surface composition (especially latitudes <30° N) can further constrain the strength of the ancient dynamo.

Objectives and Methods: First, we estimate the solar wind and interplanetary magnetic field (IMF) conditions for Mercury ~3.9-3.7 Gyr ago. We next use magnetohydrodynamic (MHD) simulations of the ambient surface magnetic field conditions. We then use these MHD simulations to create surface magnetic field timeseries, which we couple with thermal cooling and magnetization models to match with MESSENGER measurements.

Ancient solar wind, IMF conditions, and surface field timeseries. We use estimates of the ~0.5-0.7 Gyr-old Sun’s mass-loss and rotation rate to calculate solar wind speed, density, and IMF values at Mercury, assuming ancient Mercury had the same semi-major axis of 0.387 AU as today. We obtain solar wind speeds of 600–1000 km-s^-1, densities of 200–3,500 amu-cm^-3, and IMF strengths ≳300–600 nT, 1.5–3x stronger than Mercury’s modern dipole equatorial field [6].

To create the surface magnetic field timeseries for our magnetization models, we perform MHD simulations of the solar wind interacting with the Hermean dynamo field, varying the IMF and solar wind density for a fixed dipole field strength. We quantify the magnetic field history experienced at a given surface latitude by taking the surface magnetic field over all longitudes and mapping this magnetic field to a timeseries over the Hermean day (using longitude as a proxy for the time in the fixed surface point reference frame, given the axisymmetry of the dipole field [1]).

Crustal field calculations. From the MHD simulated surface magnetic field timeseries, we then estimate the TRM recorded in cooling volcanic deposits and compare the generated crustal fields to those measured by MESSENGER. We simulate cooling and TRM acquisition for three different structures: a cylindrical magmatic intrusion of diameter 2 km and length 8 km, a thin effusive volcanic disk of diameter 20 km and thickness 1 km, and a large cylindrical intrusion/lava flow of diameter 50 km and thickness 20 km. We then calculate the generated crustal fields from these sources with  χ_TRM values of various magnetic minerals. The main constraints on the candidate surface magnetic minerology are the MESSENGER measured average ~1.75 wt% Fe and likely extreme reducing conditions during formation like those for aubrite meteorites [7]. As such, we consider  χ_TRM values from aubrites [8] and Fe metal, FeNi alloys [1]. We note that magnetic carriers like suessite, schreibersite, and greigite could be stable in the reducing Hermean environment, but extensive TRM measurements are lacking [7]. Future BepiColombo measurements could provide further constraints on the surface magnetic minerology [9].

Results: We find that the Hermean dynamo field must have been >2,000 nT (10 stronger than at present) to magnetize the surface to MESSENGER measured anomalies at around 40° N latitude  [1]. (Fig. 1) We further find that crustal field measurements at low latitudes from BepiColombo could further constrain the ancient Hermean dynamo’s intensity. For example, the ancient dynamo intensity lower limit can be constrained to stronger values (possibly 20,000 nT) if BepiColombo measures similarly strong crustal fields (as did MESSENGER) at lower latitudes (e.g., 10° N).

Conclusions: In this study, we demonstrate that Mercury’s dynamo field at ~3.9-3.7 Gyr ago was very likely ≳2,000 nT to stand off the solar wind and magnetize the surface to the level inferred from MESSENGER data. The crustal magnetization may even have required a ~20,000 nT dynamo field if the Hermean surface magnetic mineralogy is dominated by Fe-metal or FeNi alloys.

Numerous dynamo models have been developed to explain Mercury’s modern planetary field. Our findings of a heightened, ancient dynamo field might point to different interior conditions ~3.9-3.7 Gyr ago, such that these stably stratified layers or Fe-snow zones were not present in that period. Furthermore, evidence for a stronger surface field might imply an enhanced heat flux or a larger inner core to power this ancient dynamo [10, 11]. Future low-altitude (<100 km) measurements of the lower latitude crustal field environment and surface mineralogy would help constrain the strength and time-evolution of the Hermean dynamo.

 

References:

[1]Johnson+2018,Mercury’s Internal Magnetic Field.[2]Hood+2018,JGR,2647-2666.[3]Heyner+2021,SSR,217.[4] Denevi+2013,JGR,891-907.[5]Wang+ 2021,GRL,e2021GL094503.[6]Vidotto+2021,LivingRevSolPhys,3.[7]Strauss+2016,JGR,2225-2238.[8]Rochette+2010,M&PS,405-427.[9]Rothery+2020,SSR 66.[10]Takahashi+2019,NatCommun,208.[11]Hauck+2018,Mercury’s Global Evolution,516-543.

Fig. 1: Crustal magnetic field parameter space for generating strongest MESSENGER-level magnetic field (B_Altitude) measurements (blue and red markers) for (top) 200 nT dipole field and (bottom) 2000 nT dipole field at 10° N latitude. The horizontal axis shows three magnetized volumes with different diameters (D) and heights (H) and the vertical axis is TRM efficiency, χ_TRM. For the proposed magnetic materials (iron-metal or similar composition to Aubrites), the ancient Hermean dynamo needed to be >2000 nT, possibly even 20,000 nT, to magnetize the strongest measured crustal anomalies.