Crustal Magnetization Research Articles

The Dayside Ionopause of Mars: Solar Wind Interaction, Pressure Balance, and Comparisons With Venus

AbstractDue to the lower ionospheric thermal pressure and existence of the crustal magnetism at Mars, the Martian ionopause is expected to behave differently from the ionopause at Venus. We study the solar wind interaction and pressure balance at the ionopause of Mars using both in situ and remote sounding measurements from the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument on the Mars Express orbiter. We show that the magnetic pressure usually dominates the thermal pressure to hold off the solar wind at the ionopause at Mars, with only 13% of the cases where the ionospheric thermal pressure plays a more important role in pressure balance. This percentage at Venus, however, is up to 65%. We also find that the ionopause altitude at Mars decreases as the normal component of the solar wind dynamic pressure increases, similar to the altitude variation of the ionopauses at Venus. Moreover, our results show that the ionopause thickness at Mars and Venus is mainly determined by the ion gyromotion and is equivalent to about five ion gyroradii.

A Spherical Harmonic Martian Crustal Magnetic Field Model Combining Data Sets of MAVEN and MGS

AbstractThis study presents a new spherical harmonic (SH) model of the crustal magnetic field of Mars, based on the magnetic field data set measured by the Mars Global Surveyor (MGS) and the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. To minimize the influence of external fields due to solar wind interaction with Mars, we rejected data that were observed dayside and above an altitude of 500 km. The data points of MAVEN were reduced by using a proxy of solar wind activity that identified and rejected any data measured during magnetically disturbed intervals. We used a conventional least squares technique to estimate the Gauss coefficients fitted to the reduced data set and made a compromise between model misfit and model roughness by truncating the SH model at degree 110. This model is capable of representing crustal fields with a spatial resolution approaching ∼200 km at 120 km altitude and ∼260 km at the Martian surface. Since our model fits MAVEN's observational data better than previous models, especially the data obtained during MAVEN's low altitude periapsis passes, we conclude that it may more accurately approximate the low‐altitude crustal field. We calculate the crustal field power spectrum of various models and find that small‐scale fields at low altitudes were underestimated by most previous models. This new model could benefit future studies associated with the Martian crustal field and its interaction with the solar wind.

On the horizontal currents over the Martian magnetic cusp

Localized crustal magnetization over heavily cratered southern hemisphere at Mars gives rise to open magnetic field configurations which interact with the solar wind magnetic field to form magnetic cusp. The downward acceleration of energetic electrons in these cusps can produce aurora and an extended topside ionospheric structure over regions of magnetic anomalies. We report plasma collisions with the neutral atmosphere at one of the Martian cusps located at 82 o S and 180 o E, where the crustal field is strong with a radial component ~35° from the local zenith. We find that the dynamo region in the upper ionosphere of Mars is located between altitudes of 102 km and 210 km. The electrons in this region are constrained to gyrate along magnetic field lines while ions are dragged by neutrals and move along the direction of applied force. In the absence of the electric field, the horizontal current in the Martian dynamo is generated by the differential motion of ions and electrons. We find that the bulk of the current density is equatorward and confined within the Martian dynamo near the ionospheric peak with a magnitude of ~3.5 µA/m 2 . We also find that the eastward current density of magnitude ~0.4 µA/m 2 peaking near the upper boundary of the Martian dynamo is generated by magnetized ions in the F × B direction.

Crustal structure in the Campanian region (Southern Apennines, Italy) from potential field modelling

We present a 3D model of the main crustal boundaries beneath the Campanian region and the onshore and offshore surrounding areas, based on high-resolution potential field data. Our main objective is the definition of the main structural interfaces in the whole Campanian region from gravity and magnetic data, thanks to their ability to define them on a regional and continuous way. The complex morphology of the Mesozoic carbonate platform, which is fundamental to constrain the top of geothermal reservoir, was reconstructed by inverting the vertical gradient of gravity. We assumed local information from seismic models and boreholes to improve the model. We modeled the deep crustal structures by spectral analysis of Bouguer gravity and magnetic data. The inferred depth estimates indicate a shallow crystalline basement below the Tyrrhenian crust and the Apulian foreland and a significant depression beneath the Bradanic foredeep. The map of the Moho boundary shows a NE-SE verging trough below the Southern Apennine chain and two pronounced uplifts beneath the foreland and the Tyrrhenian crust. We also estimated the depth to the magnetic bottom, showing a thick magnetic crust below the mountain chain and shallow depths where the crustal heat flow is high. The models were compared with seismic sections along selected profiles; a good agreement was observed, despite of some inherent lower resolution for the gravity modelling from spectral methods. The regional covering and the continuity of our estimated crustal interfaces make it a new and valid reference for further geological, geophysical and geothermal studies, especially in areas such as northern and eastern Campania, where there is an incomplete geophysical and geological information.

Paleointensity and Rock Magnetism of Martian Nakhlite Meteorite Miller Range 03346: Evidence for Intense Small‐Scale Crustal Magnetization on Mars

AbstractThe martian dynamo’s strength and duration are essential for understanding Mars' habitability and deep interior dynamics. Although most northern volcanic terranes were likely emplaced after the martian dynamo ceased, recent data from the InSight mission show stronger than predicted crustal fields. Studying young volcanic martian meteorites offers a precise, complementary method to characterize the strength of the martian crustal field and examine its implications for past dynamo activity. We present the first rock and paleomagnetic study of nine mutually oriented samples from the martian Nakhlite meteorite Miller Range (MIL) 03346, which is well‐suited for paleomagnetic analysis due to its well‐known age (1,368 ± 83 Ma) and lack of significant aqueous, thermal, and shock overprinting. Rock magnetic analysis, including quantum diamond microscope imaging, showed that the natural remanent magnetization (NRM) is carried by Ti‐magnetite crystals containing µm‐scale ilmenite exsolution lamellae, which can accurately record ancient magnetic fields. Demagnetization of the NRM revealed a high coercivity magnetization interpreted to date from the age of eruption based on its intensity, unidirectionality, and a passing fusion crust baked contact test. Paleointensities of four samples reveal a 5.1 ± 1.5 µT paleofield, representing the most reliable martian paleointensity estimates to‐date and stronger than the 2 µT surface fields measured by InSight. Modeling shows that the observed fields can be explained by an older subsurface magnetized layer without a late, active dynamo and support a deeply buried, highly magnetized crust in the northern hemisphere of Mars. These results provide corroborating evidence for strong, small‐scale crustal fields on Mars.

Compositional Enhancement of Crustal Magnetization on Mars

AbstractMartian orbital and lander measurements revealed strong (∼1–2 orders of magnitude greater than Earth) crustal magnetic anomalies and the lack of an active detectable core dynamo. This strong crustal magnetization remains unexplained given that models of an ancient core dynamo on Mars predict surface field strengths comparable to modern Earth. We explored the relationship between Mars' crustal magnetization and its composition in multivariate space. We identified that 530 and 1,000 nm absorptions (from orbital spectrometers) have unique correlations with crustal magnetization in the Terra Sirenum‐Terra Cimmeria region and ∼13% of the variance of the magnetization can be attributed to these correlations. Because we do not expect the topmost material, detectable by the orbital spectrometers, to retain magnetization from an ancient core dynamo, we propose this material is compositionally similar to the bulk rock below it, which is more likely to retain magnetization. Therefore, the observed variance is a lower limit.

Mapping and Modeling Sources of Natural Remanent Magnetization in the Microcline–Sillimanite Gneiss, Northwest Adirondack Mountains: Implications for Crustal Magnetism

AbstractDistinct crustal remanent magnetic anomalies are a strong indicator of rocks with stable natural remanent magnetization (NRM) carriers. The latter are able to store information on the history of a rock over long geological periods, and can therefore be used for a variety of applications in the field of paleomagnetism and rock magnetism. Typically, paleomagnetic and rock‐magnetic studies rely on rocks bulk magnetic properties. With the advent of high‐resolution magnetometric scanning techniques, it is now possible to map magnetic sources at the mineral scale, identify the different magnetic carriers, and analyze the effect of geometry, microstructures, and composition on their magnetic response. We investigate the stability of discrete remanent magnetization sources of a microcline–sillimanite gneiss sample from Russell Belt with a strong NRM, by scanning the sample before, and after alternating field demagnetization to 100 mT. We quantified changes in the magnetization intensity and direction by inverting the magnetic scan data. Here, we confirm that the exsolved titanohematite with ilmenite lamellae is the major source of magnetization, and the coexisting multidomain hematite grains contribute little to the NRM, or the magnetic anomalies. The microstructures in the titanohematite control local magnetic properties at the mineral scale. Magnetic modeling results suggest a consistent average magnetization direction before and after demagnetization at both the grain and thin section scale, with a decrease in the magnetization intensity of ≈30%. Results are consistent with previous bulk magnetic measurements and the likelihood to use high‐resolution magnetometric techniques in future magnetic studies is high.

Was the moon magnetized by impact plasmas?

The crusts of the Moon, Mercury, and many meteorite parent bodies are magnetized. Although the magnetizing field is commonly attributed to that of an ancient core dynamo, a longstanding hypothesized alternative is amplification of the interplanetary magnetic field and induced crustal field by plasmas generated by meteoroid impacts. Here, we use magnetohydrodynamic and impact simulations and analytic relationships to demonstrate that although impact plasmas can transiently enhance the field inside the Moon, the resulting fields are at least three orders of magnitude too weak to explain lunar crustal magnetic anomalies. This leaves a core dynamo as the only plausible source of most magnetization on the Moon.

Magnetic Reconnection in the Ionosphere of Mars: The Role of Collisions

AbstractThis paper evaluates the role of magnetic reconnection in the dayside ionosphere of Mars in the collisional regime and presents some relevant data from the MAVEN (Mars Atmosphere and Volatile EvolutioN) mission. Magnetic reconnection is an important process operating in the solar corona, planetary magnetospheres, and astrophysical plasmas, but most of the literature focuses on collisionless plasmas. However, at Mars the reconnection should often occur in regions where collisions are important. Mars does not have a large‐scale global magnetic field; however, Mars has locally large magnetic fields associated with remnant crustal magnetization, particularly in the highland regions of the southern hemisphere. The crustal fields provide a “target” for reconnection both in the ionosphere and in the magnetotail of Mars. The current paper emphasizes the dayside ionosphere. Magnetic reconnection is associated with topological changes in the magnetic field and is also a source of energy for the plasma and can thus affect the ionospheric dynamics and ion loss at Mars. Both theoretical concepts and MAVEN particle and field data from several instruments are considered in this paper.

Geophysical evidence of first stages of inflation in Domuyo volcanic center in northwestern Neuquén province, Argentina

Cerro Domuyo, in northwestern Neuquén province of Argentina, shows notable geothermal activity, although it is located at a considerable distance from the actual volcanic arc. Many studies have been developed in this area with the aim of investigating its geothermal field. Despite these efforts, the characterization of the dynamic activity in the area is still poorly known. This work shows the results of a network of seismological stations, which registered volcano-tectonic, long-period and hybrid events. A large number of volcano-tectonic events concentrated mostly in the Cerro Domuyo geothermal area were detected. These events can be divided into two groups, those that occurred at shallow depths below the geothermal area and those at greater depths below the high region of Cerro Domuyo. Shallow events were located around the Manchana Covunco fault and many of them were particularly clustered at its intersection with the Humazo fault. Seismicity in the area shows that these faults are continually active due to fluid movement, as was evidenced by the hydrothermal manifestation of the Humazo in 2003. Moreover, new studies provide evidence that the Cerro Domuyo is experiencing an important inflation caused by a magmatic body. Deep volcano-tectonic events are evidencing this activity. Considering the large distance between the actual volcanic arc and the study region, and the high density of shallow volcano-tectonic events, it is highly likely that this magmatic body is increasing the geothermal activity. Additionally, the aeromagnetic anomaly over Cerro Domuyo is showing a thin magnetic crust of less than 6 km, suggesting a clear link between the geophysical results and the inflation in Cerro Domuyo.

Integrated geophysical investigation of recent earth tremors in Nigeria using aeromagnetic and gravity data.

Understanding the tectonics at active sites of recent seismic activities in Nigeria is fundamental towards disaster mitigation and emergency planning. We apply geophysical techniques of gravity and magnetic methods to investigate crustal depths, subsurface geologic faults and fractures, and the overall subsurface interaction at Mpape region and environs. Estimated depths to the bottom of magnetic crust (basal depths) range between 11.0 and 11.4km at the Mpape region and decrease further southward towards Guabe town. This signifies the depth range of the active crust within the region. Comparative deeper basal depths (15.0-16.2km) were obtained at locations farther from Mpape-Guabe towns at Nasarawa, Rubochi, and Fuka regions, showing a more stable region away from Mpape region. Computed Moho depths from gravity data show deeper depths at the Mpape region (~ 34.1km) suggesting that the active crust exists in the upper crust. Two-dimensional modeling analysis along a profile taken across the Mpape region shows a conspicuous subsurface basement intrusion at the Mpape region with deep faults and fractures reaching depths of 7-14km. Shallow basal depths at the Mpape region resulting from significant subsurface intrusion and concentrated subsurface faults at the intruded region may be responsible for the instability of the Mpape region. The most affected area is located within the Mpape-Guabe towns. We recommend the establishment of seismic monitoring facilities in this area for effective monitoring and evaluation.

The evolution of scientific goals for Mars exploration and future prospects

Mars is currently in the spotlight of solar system exploration and planetary science study. The scientific questions of Mars are closely integrated with big enigmas of the solar system, highlighting the centrality of Mars in understanding the formation and evolution of our solar system. After nearly six decades of exploration, Mars is easily the most studied planet in the solar system besides Earth, with a profusion of research across space environment, atmosphere, surface-subsurface compositions, topography and structure, impact history, glaciers and cryosphere, climate change, and internal structure. Martian meteorite and laboratory simulation studies (experimental and numerical) are also developing rapidly. Key discoveries in the past 20 years include evidence of past and current aqueous activity, geological environment diversity, modern geological processes, methane emissions and preserved organics, atmospheric composition and evolution, current and recent climate change, gravity fields and surface radiation environments, etc. Such scientific achievements, underpinned by peer-reviewed research goals developed by the planetary community, in turn shape future goals and targets. We review how the Mars exploration goals and targets (e.g., life, climate, geology, preparation for human exploration) have evolved in the past 20 years, and show priorities and focii for future international Mars exploration. For example, the Mars exploration strategy evolved thematically from “follow the water” to “understand Mars as a system”, “understand the long-term evolution of habitability on Mars”, and “exploration by humans on Mars”. In the next 10−20 years, Mars exploration will further characterize the internal structure of Mars, start a new era of life detection, and return samples from Mars and its satellites. China’s first Mars mission will contribute to the international Mars science community with an orbiter and a rover exploring the northern lowlands with advanced payloads. New findings such as the structure of the Martian critical zone (i.e., vertical zone of interaction between the atmosphere and crust with immediate relevance for habitability), local and global characteristics of the residual crustal magnetism, volcanic-geothermal evolution and the cyclicity of glaciations will clarify the still poorly known provenance and evolution of the topographic dichotomy. Hypotheses such as the formation of the global dichotomy by an oblique impact or an ancient ocean-sustaining climate may be tested and constrained. Internationally planned sample-return from Mars and its satellites, combined with study of Martian meteorites, experimental and numerical simulation of Martian processes, and study of terrestrial analog sites and samples will build absolute geochronology of Mars and constrain the timing and duration of critical events, such as cessation of the global magnetic field, quiescence of volcanism across the Noachian-Hesperian temporal boundary, transition to a single-plate planet, aqueous and sedimentary processes, and global climate change.

InSight’s first look inside Mars

The first results reported from NASA’s InSight mission on Mars include evidence for locally strong crustal magnetization, unexpected atmospheric processes, and marsquakes from distant, enigmatic sources. Some of the marsquakes detected by Insight’s seismometer can be traced to Cerberus Fossae, a region that may be tectonically active. Together, InSight’s geophysical measurements provide information about Mars’s interior structure and evolution.

Formation and Change of Planetary Magnetic Field

The influential theories about the origin of planetary magnetic field hold that the planetary magnetic field is produced by the flow of conductive fluid in the core. But these hypotheses have serious defects, unable to explain the inhomogeneity of the spatial distribution of planetary magnetic field and its changing characteristics with time. Thus, the author analyzed the formation and evolution of solar system planets as well as their internal structures and external environments, and has found the formation mechanism and change law of various planetary magnetic fields. The polar vortices at Earth’s North and South Poles can produce spiral currents, which then form a magnetic dipole at Earth’s North and South Poles respectively. Mercury is about 70% metal and 30% silicate, so it has been magnetized by the Sun's magnetic field. Venus’ rotation speed is too slow to form polar vortices needed to produce dipole magnetic field, and Venus is far away from the Sun, causing the solar magnetic field has little effect on the magnetization of Venus, so Venus’ magnetic field is extremely weak. During the first 500 million to1 billion years of Mars formation, polar vortices existed for a long time. The dipole magnetic field produced by the polar vortices has a long-term magnetization effect on the Mars' surface, therefore a magnetized crust on the surface of Mars has been formed. But with the heat inside the Mars accumulated to a certain extent, a large part of Mars Polar ice sheet melt into water. The melting of the Martian polar ice sheet greatly weakened the polar vortex and therefore the magnetic field. Especially, in the northern part of Mars, there are large-scale lava activities in the lowlands or volcanic areas, the ice sheet melted a lot there, therefore no polar vortex could be formed, causing the dipolar field disappeared. During Jupiter's rapid rotation, a series of strong polar vortices are produced at the poles of Jupiter. These vortices contain a series of strong spiral currents, which can form a series of strong dipole magnetic fields. The superposition of these dipole magnetic fields form the original magnetic field of Jupiter. But some of Jupiter's massive moons can induce some sub cyclones from the Jupiter's vortices, these sub cyclones form powerful cyclones by absorbing dense clouds and generate some new magnetic fields, which are superimposed on the original magnetic field to form more complex magnetic field of Jupiter.

Hydrothermal Activity at the Ultraslow‐Spreading Mohns Ridge: New Insights From Near‐Seafloor Magnetics

AbstractHydrothermal circulation is a process fundamental to all types of mid‐ocean ridges that largely impacts the chemical and physical balance of the World Ocean. However, diversity of geological settings hosting hydrothermal fields complicates the exploration and requires thorough investigation of each individual case study before effective criteria can be established. Analysis of high‐resolution bathymetric and magnetic data, coupled with video and rock samples material, furthers our knowledge about mid‐ocean‐ridge‐hosted venting sites and aid in the interpretation of the interplay between magmatic and tectonic processes along the axial volcanic ridges. The rock‐magnetic data provide constraints on the interpretation of the observed contrasts in crustal magnetization. We map the areal extent of the previously discovered active basalt‐hosted Loki's Castle and inactive sediment‐hosted Mohn's Treasure massive sulfide deposits and infer their subsurface extent. Remarkably, extinct hydrothermal sites have enhanced magnetizations and display clear magnetic signatures allowing their confident identification and delineation. Identified magnetic signatures exert two new fossil hydrothermal deposits, MT‐2 and MT‐3. The Loki's Castle site coincides with negative magnetic anomaly observed in the 2‐D magnetic profile data crossing the deposit. First geophysical investigations in this area reveal the complexity of the geological setting and the variation of the physical properties in the subsurface.

Hydrogenation of the Martian Core by Hydrated Mantle Minerals With Implications for the Early Dynamo

AbstractMars lacks an internally generated magnetic field today. Crustal remanent magnetism and meteorites indicate that a dynamo existed after accretion but died roughly four billion years ago. Standard models rely on core/mantle heat flow dropping below the adiabatic limit for thermal convection in the core. However, rapid core cooling after the Noachian is favored instead to produce long‐lived mantle plumes and magmatism at volcanic provinces such as Tharsis and Elysium. Hydrogenation of the core could resolve this apparent contradiction by impeding the dynamo while core/mantle heat flow is superadiabatic. Here we present parameterized models for the rate at which mantle convection delivers hydrogen into the core. Our models suggest that most of the water that the mantle initially contained was effectively lost to the core. We predict that the mantle became increasingly ironrich over time and a stratified layer awaits detection in the uppermost core—analogous to the E′ layer atop Earth's core but likely thicker than alternative sources of stratification in the Martian core such as iron snow. Entraining buoyant, hydrogen‐rich fluid downward in the core subtracts gravitational energy from the total dissipation budget for the dynamo. The calculated fluxes of hydrogen are high enough to potentially reduce the lifetime of the dynamo by several hundred million years or longer relative to conventional model predictions. Future work should address the complicated interactions between the stratified, hydrogen‐rich layer and convection in the underlying core.

Misbehaving High‐Energy Electrons: Evidence in Support of Ubiquitous Wave‐Particle Interactions on Dayside Martian Closed Crustal Magnetic Fields

AbstractMultiple studies have reported either isotropic or trapped pitch angle distributions of high‐energy (>100 eV) electrons on closed crustal field lines on the dayside of Mars. These pitch angle distributions are not to be expected from collisional scattering and conservation of adiabatic invariants alone. We use 2 years of data from the Mars Atmosphere and Volatile EvolutioN mission to analyze the pitch angle distributions of superthermal electrons on dayside‐closed crustal magnetic fields and compare to results from an electron transport model. Low‐energy electrons (10–60 eV) have pitch angle distributions in agreement with modeling results, while high‐energy electrons (100–500 eV) do not. High‐energy electrons have a flux peak at perpendicular pitch angles which suggests there is a ubiquitous energization process occurring on crustal fields. Wave‐particle interactions seem to be the most likely candidate. Trapping of high‐energy electrons may impact the nightside ionosphere dynamics.

Geophysical Investigation of the Geothermal Potential Under the Largest Volcanic Cover in Anatolia: Kars Plateau, NE Turkey

In this study, Curie-point depth (CPD), geothermal gradient, radiogenic heat production, and heat flow maps were constructed based on different thermal conductivity coefficients using magnetic anomaly data for the Kars Plateau, which has the largest volcanic cover in Turkey. The bottom depths of the magnetic crust in the research area were revealed by the CPD map for the first time in this investigation. There are two apparent magnetic anomaly trends in the study area: the first is the Horasan–Senkaya–Sarikamis–Selim–Arpacay trend in the NE–SW direction, and the other is the Hanak–Ardahan–Arpacay trend in the NW–SE direction. Two other prominent elongations extend into the Ardahan–Gole–Senkaya and Kars–Digor axes. All these trends represent mountain chains and/or stratovolcanoes in the region, and no anomalies are observed around the non-volcanic outcrops. Curie depths are shallow, up to 14 km between Horasan and Kagizman towns, and 12 km in the northwestern part of the study area. Gradient values can reach 50 °C km−1 in the northwestern sector, together with the high heat flows represented by the 150 Wm−1 K−1 contours. The deepest CPD region lies between Gole and Susuz towns, where the geothermal gradient decreases to 27 °C km−1. Heat flows decrease 60 Wm−1 K−1 in the same area. An apparent gap around the Kars Plateau was observed in previous regional heat flow maps of Turkey by other authors (who used the bottom hole temperatures of boreholes and hot springs temperatures). This gap has been accurately filled from the results of this study, and geothermal exploration areas and the geothermal potential of the Kars Plateau have thus been determined for future exploration activity on the basis of the tectonic elements and earthquake data.

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

AbstractLow‐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 history of the core dynamos of Mars and the Moon inferred from their crustal magnetization: a brief review

The core dynamos of Mars and the Moon have distinctly different histories. Mars had no core dynamo at the end of accretion. It took ∼100 Myr for the core to create a strong dynamo that magnetized the martian crust. Giant impacts during 4.2–4.0 Ga crippled the core dynamo intermittently until a thick stagnant lithosphere developed on the surface and reduced the heat flux at the core–mantle boundary, killing the dynamo at ∼3.8 Ga. On the other hand, the Moon had a strong core dynamo at the end of accretion that lasted ∼100 Myr and magnetized its primordial crust. Either precession of the core or thermochemical convection in the mantle or chemical convection in the core created a strong core dynamo that magnetized the sources of the isolated magnetic anomalies in later times. Mars and the Moon indicate dynamo reversals and true polar wander. The polar wander of the Moon is easier to explain compared to that of Mars. It was initiated by the mass deficiency at South Pole Aitken basin, which moved the basin southward by ∼68° relative to the dipole axis of the core field. The formation of mascon maria at later times introduced positive mass anomalies at the surface, forcing the Moon to make an additional ∼52° degree polar wander. Interaction of multiple impact shock waves with the dynamo, the abrupt angular momentum transfer to the mantle by the impactors, and the global overturn of the core after each impact were probably the factors causing the dynamo reversal.