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The Red Planet is a magnetic planet. The Martian crust contains strong magnetization from a core dynamo that likely was active during the Noachian period when the surface may have been habitable. The evolution of the dynamo may have played a central role in the evolution of the early atmosphere and the planets transition to the current cold and dry state. However, the nature and history of the dynamo and crustal magnetization are poorly understood given the lack of well-preserved, oriented, ancient samples with geologic context available for laboratory study. Here, we describe how magnetic measurements of returned samples could transform our understanding of six key unknowns about Mars planetary evolution and habitability. Such measurements could i) determine the history of the Martian dynamo fields intensity; ii) determine the history of the Martian dynamo fields direction; iii) test the hypothesis that Mars experienced plate tectonics or true polar wander; iv) constrain the thermal and aqueous alteration history of the samples; v) identify sources of Martian crustal magnetization and vi) characterize sedimentary and magmatic processes on Mars. We discuss how these goals can be achieved using future laboratory analyses of samples acquired by the Perseverance rover.

Interpretation of palaeointensity measurements is difficult since natural samples are made up of magnetic grains of different size, shape and chemical composition. Néels single-domain (SD) model is the main theoretical tool used to understand paleomagnetic measurements, but it is limited since it only applies to uniformly magnetized grain assemblies.Recent work has shown that the single-vortex (SV) state is not only significantly more common than SD as it occupies a much larger range of grain sizes but is also surprisingly thermally stable [1]. As these grains likely account for the majority of the signal measured in the laboratory, they must also be responsible for the range of observations that lead to large inaccuracies in measurement, for example, pTRM tails. Additionally, if we have a clear understanding of the SV state, can we account for the true cooling rate dependence of grain assemblies, and also find a physical link between magnetic coercivity and blocking temperature.We present a framework of tools for paleomagnetists to interpret measurements in the light of advances in numerical modelling and the increase in speed of modern computers. We hope that these tools will advance our understanding of the basic mechanisms by which samples record and preserve the Earth’s magnetic field and so will allow for the quantification of errors observed in real samples.[1] L.  Nagy,  W.  Williams,  A.  R.  Muxworthy,  K.  Fabian,  T.  P.  Almeida, P. Ó Conbhuí, and V. P. Shcherbakov. Stability of equidimensional pseudo–single-domain magnetite over billion-year timescales. PNAS 114(39):10356–10360, 2017

Interpretation of palaeointensity measurements is difficult since natural samples are made up of magnetic grains of different size, shape and chemical composition. Néels single-domain (SD) model is the main theoretical tool used to understand paleomagnetic measurements, but it is limited since it only applies to uniformly magnetized grain assemblies.Recent work has shown that the single-vortex (SV) state is not only significantly more common than SD as it occupies a much larger range of grain sizes but is also surprisingly thermally stable [1]. As these grains likely account for the majority of the signal measured in the laboratory, they must also be responsible for the range of observations that lead to large inaccuracies in measurement, for example, pTRM tails. Additionally, if we have a clear understanding of the SV state, can we account for the true cooling rate dependence of grain assemblies, and also find a physical link between magnetic coercivity and blocking temperature.We present a framework of tools for paleomagnetists to interpret measurements in the light of advances in numerical modelling and the increase in speed of modern computers. We hope that these tools will advance our understanding of the basic mechanisms by which samples record and preserve the Earth’s magnetic field and so will allow for the quantification of errors observed in real samples.[1] L.  Nagy,  W.  Williams,  A.  R.  Muxworthy,  K.  Fabian,  T.  P.  Almeida, P. Ó Conbhuí, and V. P. Shcherbakov. Stability of equidimensional pseudo–single-domain magnetite over billion-year timescales. PNAS 114(39):10356–10360, 2017

Paleomagnetic observations provide valuable evidence of the strength of magnetic fields present during evolution of the Solar System. Such information provides important constraints on physical processes responsible for rapid accretion of the protoplanetesimal disk. For this purpose, magnetic recordings must be stable and resist magnetic overprints from thermal events and viscous acquisition over many billions of years. A lack of comprehensive understanding of magnetic domain structures carrying remanence has, until now, prevented accurate estimates of the uncertainty of recording fidelity in almost all paleomagnetic samples. Recent computational advances allow detailed analysis of magnetic domain structures in iron particles as a function of grain morphology, size, and temperature. Our results show that uniformly magnetized equidimensional iron particles do not provide stable recordings, but instead larger grains containing single-vortex domain structures have very large remanences and high thermal stability-both increasing rapidly with grain size. We derive curves relating magnetic thermal and temporal stability demonstrating that cubes (>35 nm) and spheres (>55 nm) are likely capable of preserving magnetic recordings from the formation of the Solar System. Additionally, we model paleomagnetic demagnetization curves for a variety of grain size distributions and find that unless a sample is dominated by grains at the superparamagnetic size boundary, the majority of remanence will block at high temperatures ([Formula: see text]C of Curie point). We conclude that iron and kamacite (low Ni content FeNi) particles are almost ideal natural recorders, assuming that there is no chemical or magnetic alteration during sampling, storage, or laboratory measurement.

The Neoproterozoic carbonate rocks of the Araras Group (Amazon Craton) and the Sete‐Lagoas and Salitre Formations (São Francisco Craton) share a statistically indistinguishable single‐polarity (reversed) characteristic direction. This direction is associated with paleomagnetic poles that do not align with the expected directions for primary detrital remanence. We employ a combination of classical rock magnetic properties and micro imaging/chemical analysis (in thin sections) using synchrotron radiation to examine these remagnetized carbonate rocks. Magnetic data indicate that most samples lack the anomalous hysteresis properties typically associated with carbonate remagnetization (except for distorted loops). Through a combination of Scanning Electron Microscopy with Energy Dispersive X‐ray Spectroscopy (SEM‐EDS), X‐ray Fluorescence (XRF), and X‐ray Absorption Spectroscopy (XAS), we identified subhedral/anhedral magnetite, or spherical grains with a core‐shell structure of magnetite surrounded by maghemite. These grains are within the pseudo‐single domain size range, as do most of the iron sulfides, and are spatially associated with potassium‐bearing aluminosilicates. While fluid percolation and organic matter maturation play a role, smectite‐illitization appears to be crucial for the growth of these phases. X‐ray diffraction analysis, in addition, identifies these silicates as predominantly highly crystalline illite, suggesting exposure to epizone temperatures. These temperatures were likely reached during the final stages of the Gondwana assembly (Cambrian), but remanence was only locked in afterward, in successive cooling events during the Early Middle Ordovician. This is supported by the carbonates' paleomagnetic pole positions compared to Gondwana's apparent polar wander path, and the absence of reversals, contrasting with the high reversal frequency of the Late Ediacaran/Cambrian. Plain Language Summary: Carbonate rocks serve as important records of ancient climates and host magnetic minerals capable of documenting the evolution of Earth's magnetic field. Nevertheless, their original magnetization, acquired during sedimentation, is frequently supplanted by a secondary magnetization, originating from various geological processes altering local thermochemical stability. For carbonate rocks, this remagnetization process is often associated with a "magnetic fingerprint." In South America, carbonate rocks from different sedimentary basins (hundreds of kilometers apart) exhibit statistically similar magnetic components that deviate from their formation origin, indicating secondary processes. This multidisciplinary study integrates classical paleomagnetic analysis with micro‐chemical and imaging analysis to comprehend the geological phenomena responsible for remagnetizing such an extensive continental area. We demonstrate that not all remagnetized samples display the anticipated magnetic fingerprint. Highly detailed chemical analysis confirms the presence of nanoscopic magnetic minerals spatially correlated with clay minerals indicating that while organic matter transformation may play a significant role, clay mineral transformation is a key phenomenon governing remagnetization in these rocks. However, our data also supports the notion that these rocks underwent heating during the final assembly of the ancient continental landmass, Gondwana, and a sequential cooling event is suggested as locking their magnetization in the state observed today. Key Points: Remagnetized Neoproteorozoic carbonates in Brazil may not consistently display anomalous hysteresis parametersSynchrotron‐based analysis revealed pseudo‐single domain‐sized magnetite spatially correlated with aluminosilicates (smectite‐illite)The Late Cambrian assembly of Gondwana thermally reset the carbonates' remanence, eventually blocking it (470–460 Ma) during gradual cooling [ABSTRACT FROM AUTHOR]

Magnetic hysteresis measurements are routinely made in the Earth and planetary sciences to identify geologically meaningful magnetic recorders, and to study variations in present and past environments. Interpreting magnetic hysteresis data in terms of domain state and paleomagnetic stability are major motivations behind undertaking these measurements, but the interpretations remain fraught with challenges and ambiguities. To shed new light on these ambiguities, we have undertaken a systematic micromagnetic study to quantify the magnetic hysteresis behavior of room‐temperature magnetite as a function of particle size (45–195 nm; equivalent spherical volume diameter) and shape (oblate, prolate and equant); our models span uniformly magnetized single domain (SD) to non‐uniformly magnetized single vortex (SV) states. Within our models the reduced magnetization associated with SV particles marks a clear boundary between SD (≥0.5) and SV (<0.5) magnetite. We further identify particle sizes and shapes with unexpectedly low coercivity and coercivity of remanence. These low coercivity regions correspond to magnetite particles that typically have multiple possible magnetic domain state configurations, which have been previously linked to a zone of unstable magnetic recorders. Of all the hysteresis parameters investigated, transient hysteresis is most sensitive to particles that exhibit such domain state multiplicity. When experimental transient hysteresis is compared to paleointensity behavior, we show that increasing transience corresponds to more curved Arai plots and less accurate paleointensity results. We therefore strongly suggest that transient behavior should be more routinely measured during rock magnetic investigations. Plain Language Summary: Characterizing the magnetic properties and behavior of natural materials in the Earth and planetary sciences is key to identifying reliable magnetic recorders and variations in the environment. One standard method for achieving this is through room‐temperature measurements of magnetic hysteresis. However, the interpretation of magnetic hysteresis data remains one of the most challenging aspects of rock magnetism. To improve our understanding of magnetic hysteresis data, we have systematically investigated how the hysteresis properties of distributions of randomly oriented magnetite change as a function of particle size and shape and how this can help us quantify the contents of natural materials and identify rocks that may give unreliable magnetic signals. We model prolate, oblate and equant magnetite particles in the size range 45–195 nm. We show that magnetic hysteresis defines a clear boundary between simple uniform magnetic structures and more complex non‐uniform magnetic structures. We also identify the sizes and morphologies of magnetic particles that are likely to have unstable remanent magnetizations. These unstable particles are associated with distinctive hysteresis behavior, suggesting that hysteresis data can be used to identify rock samples dominated by such behavior. Key Points: Magnetic hysteresis is micromagnetically modeled for oblate, prolate and equant magnetite particles (45–195 nm)Hysteresis loop shape is highly dependent on particle size/shape due to complex switching behavior of single vortex statesTransient magnetic hysteresis is a powerful tool for identifying stable remanent magnetizations but is currently infrequently reported [ABSTRACT FROM AUTHOR]

The ability of rocks to hold a reliable record of the ancient geomagnetic field depends on the structure and stability of magnetic domain‐states contained within constituent particles. In paleomagnetic studies, the Day plot is an easily constructed graph of magnetic hysteresis parameters that is frequently used to estimate the likely magnetic recording stability of samples. Often samples plot in the region of the Day plot attributed to so‐called pseudo‐single‐domain particles with little understanding of the implications for domain‐states or recording fidelity. Here we use micromagnetic models to explore the hysteresis parameters of magnetite particles with idealized prolate and oblate truncated‐octahedral geometries containing single domain (SD), single‐vortex and occasionally multi‐vortex states. We show that these domain states exhibit a well‐defined trend in the Day plot that extends from the SD region well into the multi‐domain region, all of which are likely to be stable remanence carriers. We suggest that although the interpretation of the Day plot and its variants might be subject to ambiguities, if the magnetic mineralogy is known, it can still provide some useful insights about paleomagnetic specimens' dominant domain state, average particle sizes and, consequently, their paleomagnetic stability. Plain Language Summary: Ancient magnetic field recordings from rocks, provide information about the early habitability of Earth and formation of the Solar System. Key to understanding the reliability of these magnetic recordings is knowing the particle size and shape of a rock's constituent magnetic minerals. Small magnetite particles (⪅100 nm) are magnetically uniform, but as particle size increases the magnetic structures become non‐uniform and increasingly complex. These different types of structures are termed domain states, and yield different magnetic hysteresis responses, often summarized on a so‐called "Day" diagram—a commonly used diagnostic of domain state (or particle size). The position of particles in the size 100–1,000 nm on the Day plot is poorly quantified. This is a problem, as it has been shown in the last 5 years, that this particle size range carries the most stable magnetic recordings, lasting potentially longer than the age of the universe. These particles contain vortex‐like magnetic structures. Using a numerical micromagnetic algorithm, this is the first comprehensive study to quantify the magnetic response of vortex structures on the Day plot. We show that careful use of the Day plot can provide insight into the ability of a sample to retain recordings of the ancient geomagnetic field. Key Points: Vortex domain states for magnetite predominantly have pseudo‐single‐domain Day plot characteristicsIf the mineralogy is known, the Day plot can provide an estimate on the dominance of stable remanence carriers [ABSTRACT FROM AUTHOR]

Micromagnetic modeling allows the systematic study of the effects of particle size and shape on the first‐order reversal curve (FORC) magnetic hysteresis response for magnetite particles in the single‐domain (SD) and pseudo‐single domain (PSD) particle size range. The interpretation of FORCs, though widely used, has been highly subjective. Here, we use micromagnetics to model randomly oriented distributions of particles to allow more physically meaningful interpretations. We show that one commonly found type of PSD particle—namely the single vortex (SV) particle—has far more complex signals than SD particles, with multiple peaks and troughs in the FORC distribution, where the peaks have higher switching fields for larger SV particles. Particles in the SD to SV transition zone have the lowest switching fields. Symmetrical and prolate particles display similar behavior, with distinctive peaks forming near the vertical axis of the FORC diagram. In contrast, highly oblate particles produce "butterfly" structures, suggesting that these are potentially diagnostic of particle morphology. We also consider FORC diagrams for distributions of particle sizes and shapes and produce an online application that users can use to build their own FORC distributions. There is good agreement between the model predictions for distributions of particle sizes and shapes, and the published experimental literature. Plain Language Summary: In Earth, planetary and environmental sciences, magnetic methods are often used to rapidly measure the properties of rocks, sediments, meteorites, and soils. These magnetic properties can be proxies for environmental change or indicators of magnetic recording fidelity. The magnetic properties are dependent on the magnetic mineralogy and particle size and shape of the minerals within a sample. There is no single measurement that provides a unique interpretation; instead, a range of methods are applied, one of which is magnetic hysteresis analysis. First‐order reversal curves (FORCs) are an advanced type of hysteresis analysis, and the FORC response of a mineral depends on magnetic domain state, which is strongly controlled by both the size and shape of a particle. We understand the FORC response of very big (> ${ >} $1,000 nm) and very small particles (< ${< } $100 nm), but interpretation for particles in between, the so called pseudo‐single domain (PSD) state, has remained elusive. This is a problem as the magnetic signature of many rocks is dominated by PSD signals. In this paper we use numerical micromagnetic modeling to systematically study the effect of particle size and shape on particles that support a particular type of PSD FORC response—the single‐vortex. Key Points: We have modeled first‐order reversal curve (FORC) diagrams for single‐domain and single‐vortex sized magnetite for a range of particle sizes and morphologiesSingle‐vortex particles display complex structures in the FORC distribution with multiple peaks and troughsThe main peaks of the single‐vortex FORC distributions have higher coercivities than the single‐domain peak [ABSTRACT FROM AUTHOR]

Magnetotactic bacteria produce chains of nanoscopic iron minerals used for navigation, which can be preserved over geological timescales in the form of magnetofossils. Micrometer-sized magnetite crystals with unusual shapes suggesting a biologically controlled mineralization have been found in the geological record and termed giant magnetofossils. The biological origin and function of giant magnetofossils remains unclear, due to the lack of modern analogues to giant magnetofossils. Using distinctive Ptychographic nanotomography data of Precambrian (1.88 Ga) rocks, we recovered the morphology of micrometric cuboid grains of iron oxides embedded in an organic filamentous fossil to construct synthetic magnetosomes. Their morphology is different from that of previously found giant magnetofossils, but their occurrence in filamentous microfossils and micromagnetic simulations support the hypothesis that they could have functioned as a navigation aid, akin to modern magnetosomes. [ABSTRACT FROM AUTHOR]

An increasing amount of evidence suggests that the tetrataenite‐bearing cloudy zones (CZ) in iron and stony‐iron meteorites can preserve magnetic records of ancient magnetic activity of their parent bodies over solar system timescales. Tetrataenite islands in the CZ are nanometer‐sized (<200 nm) crystals that usually form through ordering from precursor taenite islands upon extremely slow cooling through 320°C. Recent micromagnetic models have shown that such precursor taenite islands form highly thermally stable single‐domain (SD) or single‐vortex states (SV). In this work we employ a 3D finite element multi‐phase micromagnetic modeling to show that tetrataenite inherits the magnetic remanence of taenite precursor when it forms over underlying SD states. When taenite forms SV states, however, tetrataenite resets the precursor magnetization and records a new remanence through chemical ordering at 320°C. We further assess the thermal stability of tetrataenite islands. We show that in cases where tetrataenite inherits the domain states of its precursor taenite, the origin of the remanence can be up to ∼105 years older than previously thought in fast‐cooled meteorites, and ∼1–≳6 Myr in slowly cooled meteorites. It indicates, therefore, that different regions across slowly cooled CZ record distinct stages of planetary formation. Plain Language Summary: Meteorites are fragments of other planetary bodies. They can give us unique insights about how extraterrestrial bodies formed and evolved, and if these bodies have once had a liquid core capable of generating magnetic fields. Nanometer‐sized FeNi particles formed within meteorite metal can retain a magnetic memory of planetary fields. By assessing a meteorite's magnetic memory we can then understand the internal conditions prevailing within their parent bodies. In this work we use numerical models to show that meteorites can retain magnetic signals of different periods of their parent body's formation and evolution. Inspired by our findings, we also point toward potential pathways to optimize FeNi‐based nanoparticles for technological applications. Key Points: Single‐domain states formed in taenite precursor are preserved through tetrataenite chemical orderingInheritance of remanence in fast‐cooled Iron meteorites impacts both the timing and intensity of the recorded paleomagnetic fieldTime‐resolved records in slowly cooled Iron meteorites are possible through different recording mechanisms in different grain sizes [ABSTRACT FROM AUTHOR]

The minerals carrying the magnetic remanence in geological samples are commonly a solid solution series of iron‐titanium spinels known as titanomagnetites. Despite the range of possible compositions within this series, micromagnetic studies that characterize the magnetic domain structures present in these minerals have typically focused on magnetite. No studies systematically comparing the domain‐states present in titanomagnetites have been undertaken since the discovery of the single vortex (SV) structure and the advent of modern micromagnetism. The magnetic properties of the titanomagnetite series are known to vary strongly with composition, which may influence the domain states present in these minerals, and therefore the magnetic stability of the samples bearing them. We present results from micromagnetic simulations of titanomagnetite ellipsoids of varying shape and composition to find the size ranges of the single domain (SD) and SV structures. These size ranges overlap, allowing for regions where the SD and SV structures are both available. These regions are of interest as they may lead to magnetic instability and "partial thermal remanent magnetization (pTRM) tails" in paleointensity experiments. We find that although this SD + SV zone occupies a narrow range of sizes for equidimensional magnetite, it is widest for intermediate (TM30‐40) titanomagnetite compositions, and increases for both oblate and prolate particles, with some compositions and sizes having an SD + SV zone up to 100s of nm wide. Our results help to explain the prevalence of pTRM tail‐like behavior in paleointensity experiments. They also highlight regions of particles with unusual domain states to target for further investigation into the definitive mechanism behind paleointensity failure. Plain Language Summary: Rocks that record Earth's magnetic field often contain the mineral magnetite. The crystal structure of magnetite allows titanium atoms to substitute for iron, giving rise to a range of minerals known as titanomagnetites. The internal magnetic structure of titanomagnetite particles in rocks, known as the "domain structure," controls the ability of that particle to record magnetic fields. Particles with certain kinds of domain structure are unstable magnetic recorders, which can cause problems for experiments trying to determine Earth's magnetic field strength in the past (paleointensity experiments). Although the domain structures in magnetite are well understood, there are no recent studies which describe them in titanomagnetites. In this paper, we simulate the domain structures in small titanomagnetite particles and map these out as a function of size, shape and chemical composition. In doing so, we identify types of magnetic particles with multiple possible domain structures that may give rise to unstable magnetizations. Our results indicate that some titanomagnetite particles may have unstable magnetizations over a much larger range of sizes than has previously been seen in magnetite. This wide range of sizes could explain the high failure rates of paleointensity experiments. Key Points: We systematically map out the domain states in titanomagnetite as a function of shape and compositionOur results highlight ranges of compositions, shapes and sizes which contain unreliable paleomagnetic recordersFor certain shapes and compositions, these regions span hundreds of nanometers, representing a significant proportion of remanence carriers [ABSTRACT FROM AUTHOR]

A dynamic micromagnetic algorithm is used to address systematically the contributions of magnetostatic interaction fields in magnetic systems during first-order-reversal-curve (FORC) simulations. Details of the interaction field distributions (IFD) that display a nonlinear dependency on the field history and intergrain spacing are disdcussed.