Lunar Core Dynamo Research Articles

A lunar core dynamo limited to the Moon’s first ~140 million years

Single crystal paleointensity (SCP) reveals that the Moon lacked a long-lived core dynamo, though mysteries remain. An episodic dynamo, seemingly recorded by some Apollo basalts, is temporally and energetically problematic. We evaluate this enigma through study of ~3.7 billion-year-old (Ga) Apollo basalts 70035 and 75035. Whole rock analyses show unrealistically high nominal magnetizations, whereas SCP indicate null fields, illustrating that the former do not record an episodic dynamo. However, deep crustal magnetic anomalies might record an early lunar dynamo. SCP studies of 3.97 Ga Apollo breccia 61016 and 4.36 Ga ferroan anorthosite 60025 also yield null values, constraining any core dynamo to the Moon’s first 140 million years. These findings suggest that traces of Earth’s Hadean atmosphere, transferred to the Moon lacking a magnetosphere, could be trapped in the buried lunar regolith, presenting an exceptional target for future exploration.

Dataset of replicate Apollo sample magnetizations bearing on impacts and absence of a long-lived lunar dynamo

The absence or presence of a lunar paleomagnetosphere is important because it bears directly on the volatile content of the regolith and exploration targets for Artemis and other missions to the Moon. Recent paleointensity study of samples from the Apollo missions has readdressed this question. Multiple specimens from a young 2-million-year-old glass shows a strong magnetization compatible with that induced by charge-separation in an impact plasma, whereas paleointensities of single crystals yield evidence for null magnetizations spanning 3.9 to 3.2 Ga. Together, these data are consistent with an impact mechanism for the magnetization of some lunar samples, and absence of a long-lived lunar core dynamo and paleomagnetosphere recorded in other samples. Here, we present a dataset that allows researchers to examine replicates of these measurements. For the glass, we present data from specimens that fail standard paleointensity selection criteria but nevertheless imply a complex, changing magnetic field environment. For the single crystals, the replicate measurements further illustrate the initial zero magnetization state of these materials.

Long-lived lunar volcanism sustained by precession-driven core-mantle friction.

Core-mantle friction induced by the precession of the Moon's spin axis is a strong heat source in the deep lunar mantle during the early phase of a satellite's evolution, but its influence on the long-term thermal evolution still remains poorly explored. Using a one-dimensional thermal evolution model, we show that core-mantle friction can sustain global-scale partial melting in the upper lunar mantle until ∼3.1 Ga, thus accounting for the intense volcanic activity on the Moon before ∼3.0 Ga. Besides, core-mantle friction tends to suppress the secular cooling of the lunar core and is unlikely to be an energy source for the long-lived lunar core dynamo. Our model also favours the transition of the Cassini state before the end of the lunar magma ocean phase (∼4.2 Ga), which implies a decreasing lunar obliquity over time after the solidification of the lunar magma ocean. Such a trend of lunar obliquity evolution may allow volcanically released water to be buried in the lunar regolith of the polar regions. As a consequence, local water ice could be more abundant than previously thought when considering only its accumulation caused by solar wind and comet spreading.

Size and density of lunar core estimated using simulated annealing algorithm

This study focuses on the size of composition of lunar core. In this study, we consider the lunar mean density and mean moment of inertia factor in our inversion. We use the degree-2 coefficients of lunar gravity field model GL990D and the lunar physical liberation parameters to compute mean moment of inertia factor, which is treated as an observed value. We also compute the observed value of the mean density according to the total mass of the Moon. Based on the interior structure with various layers, we deduce the modeled expressions for the lunar mean density and mean moment of inertia factor. Summing the squares of the difference between the observed value and modeled value as an inversion criterion, we estimate the multi-parameters based on the simulated annealing algorithm. By considering the lunar interior structure with three layers, the estimated size of the lunar core is around 470 km, and the density of the core is close to 5486 kg·m<sup>–3</sup>. The computed size and density of the lunar core are close to other reported values, thereby validating our algorithm. We then consider the scenarios that the lunar core differentiates between a solid inner core and a liquid outer core. The good-inversed outer core is close to 385 km, while the inner core approaches to 350 km. By using the good-inversed sizes as fixed parameters, it is found that the inner core reaches 7879 kg⋅m<sup>–</sup>³, quite denser than the outer core, which is estimated at 4618 kg⋅m<sup>–</sup>³. Our result indicates that the outer core is composed of ferrous sulfide (FeS), while the inner core is comprised of ferrous or ferro-nickel, formed 3.56 billion years ago when the lunar core dynamo ended.

Simulations of magnetic fields produced by asteroid impact: Possible implications for planetary paleomagnetism

The origin and evolution of the Moon's magnetic field, specifically whether it has an endogenic or exogenic origin, has been a major question in lunar science. The lunar field today is a patchwork of magnetic anomalies carried as remnant magnetization in the rocks of the lunar crust. Recent publications suggest some magnetic anomalies may be associated with magnetized impact melt sheets of some large lunar basins cooling in the presence of an early lunar core dynamo or associated with portions of the impactor that formed the South Pole-Aitken Basin. By performing CTH simulations of asteroid impacts incorporating magnetic field generation and thermoremanence magnetization models, this paper helps explain the origin of lunar magnetic anomalies by showing: 1) transient magnetic fields produced by impact events will increase to substantial magnitude at the scale of large lunar basins and 2) magnetization of lunar rocks can occur during crater formation at nearly all scales. If occurring in pristine feldspathic lunar highlands rocks with relatively low thermoremanence susceptibility, the magnetic anomalies resulting from (1) and (2) produce magnetic fields observable in orbital magnetic field data only for large and relatively rare impact craters - large lunar basins greater than ~200 km diameter. If occurring in thick units with higher thermoremanence susceptibility, craters as small as 50–100 km may produce orbital anomalies. Craters at nearly all scales may leave behind remnant magnetic fields observable at the surface or in samples. This paper concludes magnetic fields produced by impacts may be an important contributor to the present state of the Moon's magnetic field.

Phase equilibria of a low S and C lunar core: Implications for an early lunar dynamo and physical state of the current core

Multiple lines of geochemical and geophysical evidence suggest the Moon has a small metallic core, yet the composition of the core is poorly constrained. The physical state of the core (now or in the past) depends on detailed knowledge of its composition, and unfortunately, there is little available data on relevant multicomponent systems (i.e., Fe–Ni–S–C) at lunar interior conditions. In particular, there is a dearth of phase equilibrium data to elucidate whether a specific core composition could help to explain an early lunar geodynamo and magnetic field intensities, or current solid inner core/liquid outer core states. We utilize geochemical information to estimate the Ni, S and C contents of the lunar core, and then carry out phase equilibria experiments on several possible core compositions at the pressure and temperature conditions relevant to the lunar interior. The first composition is 0.5 wt% S and 0.375 wt% C, based on S and C contents of Apollo glasses. A second composition contains 1 wt% each of S and C, and assumes that the lunar mantle experienced degassing of up to 50% of its S and C. Finally a third composition contains C as the dominant light element. Phase equilibrium experiments were completed at 1, 3 and 5 GPa, using piston cylinder and multi-anvil techniques. The first composition has a liquidus near 1550 °C and solidus near 1250 °C. The second composition has a narrower liquidus and solidus temperatures of 1400 and 1270 °C, respectively, while the third composition is molten down to 1150 °C. As the composition crystallizes, the residual liquid becomes enriched in S and C, but S enrichment is greater due to the incorporation of C (but not S) into solid metallic FeNi. Comparison of these results to thermal models for the Moon allow an evaluation of which composition is consistent with the geophysical data of an early dynamo and a currently solid inner and liquid outer core. Composition 1 has a high enough liquidus to start crystallizing early in lunar history (4.3 Ga), consistent with the possible core dynamo initiated by crystallization of a solid inner core. Composition 1 also stays partially molten throughout lunar history, and could easily explain the seismic data. Composition 2, on the other hand, can satisfy one or the other set of geophysical data, but not both and thus seems like a poor candidate for a lunar core composition. Composition 3 remains molten to temperatures that are lower than current estimates for the lunar core, thus ruling out the possibility of a C-rich (and S-poor) lunar core. The S- and C-poor core composition studied here (composition 1) is consistent with all available geochemical and geophysical data and provides a simple heat source and mechanism for a lunar core dynamo (core crystallization) that would obviate the need for other primary mechanisms such as impacts, core–mantle coupling, or unusual thermal histories.

Carbon as the dominant light element in the lunar core

Geophysical and geochemical observations point to the presence of a light element in the lunar core, but the exact abundance and type of light element are poorly constrained. Accurate constraints on lunar core composition are vital for models of lunar core dynamo onset and demise, core formation conditions (e.g., depth of the lunar magma ocean or LMO) and therefore formation conditions, as well as the volatile inventory of the Moon. A wide range of previous studies considered S as the dominant light element in the lunar core. Here, we present new constraints on the composition of the lunar core, using mass-balance calculations, combined with previously published models that predict the metal-silicate partitioning behavior of C, S, Ni, and recently proposed new bulk silicate Moon (BSM) abundances of S and C. We also use the bulk Moon abundance of C and S to assess the extent of their devolatilization. We observe that the Ni content of the lunar core becomes unrealistically high if shallow ( 3 GPa) LMO scenarios are considered for S and C. The moderately siderophile metal-silicate partitioning behavior of S during lunar core formation, combined with the low BSM abundance of S, yields only <0.16 wt% S in the core, virtually independent of the pressure (P) and temperature (T) conditions during core formation. Instead, our analysis suggests that C is the dominant light element in the lunar core. The siderophile behavior of C during lunar core formation results in a core C content of ~0.6-4.8 wt%, with the exact amount depending on the core formation conditions. A C-rich lunar core could explain (1) the existence of a present-day molten outer core, (2) the estimated density of the lunar outer core, and (3) the existence of an early lunar core dynamo driven by compositional buoyancy due to core crystallization. Finally, our calculations suggest the C content of the bulk Moon is close to its estimated abundance in the bulk silicate Earth (BSE), suggesting more limited volatile loss during the Moon-forming event than previously thought.

Decline of the lunar core dynamo

Recent analyses of Apollo samples have demonstrated that a core dynamo existed on the Moon between at least 4.25 and 3.56 billion years ago (Ga) with surface field intensities reaching ∼70 μT. However, it is unknown when the Moon's magnetic field declined. Determining the temporal evolution of the dynamo is important because it constrains secular changes in power at the lunar core–mantle boundary and, by implication, the Moon's thermal and orbital evolution and the field generation mechanism. Here we present paleomagnetic data from several younger mare basalts which demonstrate that the surface magnetic field had declined precipitously to <∼4 μT by 3.19 Ga. It is currently unclear whether such a rapid decrease in field strength reflects either the cessation of the dynamo during this period or its persistence beyond 3.19 Ga in a drastically weakened state.

A long-lived lunar dynamo powered by core crystallization

The Moon does not possess an internally generated magnetic field at the present day, but extensive evidence shows that such a field existed between at least 4.2 and 3.56 Ga ago. The existence of a metallic lunar core is now firmly established, and we investigate the influence of inner core growth on generating a lunar core dynamo. We couple the results of a 3-D spherical thermochemical convection model of the lunar mantle to a 1-D thermodynamic model of its core. The energy and entropy budget of the core are computed to determine the inner core growth rate and its efficiency to power a dynamo. Sulfur is considered to be the main alloying element and we investigate how different sulfur abundances and initial core temperatures affect the model outcomes. For reasonable initial conditions, a solid inner core between 100 and 200 km is always produced. During its growth, a surface magnetic field of about 0.3 μT is generated and is predicted to last several billion years. Though most simulations predict the existence of a core dynamo at the present day, one way to stop magnetic field generation when the inner core is growing is by a transition between a bottom–up and top–down core crystallization scheme when the sulfur content becomes high enough in the outer core. According to this hypothesis, a model with about 6 to 8 wt.% sulfur in the core would produce a 120–160 km inner core and explain the timing of the lunar dynamo as constrained by paleomagnetic data.

A wet, heterogeneous lunar interior: Lower mantle and core dynamo evolution

While recent analyses of lunar samples indicate the Moon had a core dynamo from at least 4.2–3.56 Ga, mantle convection models of the Moon yield inadequate heat flux at the core-mantle boundary to sustain thermal core convection for such a long time. Past investigations of lunar dynamos have focused on a generally homogeneous, relatively dry Moon, while an initial compositionally stratified mantle is the expected consequence of a postaccretionary lunar magma ocean. Furthermore, recent re-examination of Apollo samples and geophysical data suggests that the Moon contains at least some regions with high water content. Using a finite element model, we investigate the possible consequences of a heterogeneously wet, compositionally stratified interior for the evolution of the Moon. We find that a postoverturn model of mantle cumulates could result in a core heat flux sufficiently high to sustain a dynamo through 2.5 Ga and a maximum surface, dipolar magnetic field strength of less than 1 μT for a 350-km core and near ∼2 μT for a 450-km core. We find that if water was transported or retained preferentially in the deep interior, it would have played a significant role in transporting heat out of the deep interior and reducing the lower mantle temperature. Thus, water, if enriched in the lower mantle, could have influenced core dynamo timing by over 1.0 Gyr and enhanced the vigor of a lunar core dynamo. Our results demonstrate the plausibility of a convective lunar core dynamo even beyond the period currently indicated by the Apollo samples.

Persistence and origin of the lunar core dynamo

The lifetime of the ancient lunar core dynamo has implications for its power source and the mechanism of field generation. Here, we report analyses of two 3.56-Gy-old mare basalts demonstrating that they were magnetized in a stable and surprisingly intense dynamo magnetic field of at least ~13 μT. These data extend the known lifetime of the lunar dynamo by ~160 My and indicate that the field was likely continuously active until well after the final large basin-forming impact. This likely excludes impact-driven changes in rotation rate as the source of the dynamo at this time in lunar history. Rather, our results require a persistent power source like precession of the lunar mantle or a compositional convection dynamo.

Geology, geochemistry, and geophysics of the Moon: Status of current understanding

The Moon is key to understanding both Earth and our Solar System in terms of planetary processes and has been a witness of the Solar System history for more than 4.5Ga. Building on earlier telescopic observations, our knowledge about the Moon was transformed by the wealth of information provided by Apollo and other space missions. These demonstrated the value of the Moon for understanding the fundamental processes that drive planetary formation and evolution. The Moon was understood as an inert body with its geology mainly restricted to impact and volcanism with associated tectonics, and a relative simple composition. Unlike Earth, an absence of plate tectonics has preserved a well-defined accretion and geological evolution record. However recent missions to the Moon show that this traditional view of the lunar surface is certainly an over simplification. For example, although it has long been suspected that ice might be preserved in cold traps at the lunar poles, recent results also indicate the formation and retention of OH− and H2O outside of polar regions. These volatiles are likely to be formed as a result of hydration processes operating at the lunar surface including the production of H2O and OH by solar wind protons interacting with oxygen-rich rock surfaces produced during micrometeorite impact on lunar soil particles. Moreover, on the basis of Lunar Prospector gamma-ray data, the lunar crust and underlying mantle has been found to be divided into distinct terranes that possess unique geochemical, geophysical, and geological characteristics. The concentration of heat producing elements on the nearside hemisphere of the Moon in the Procellarum KREEP Terrane has apparently led to the nearside being more volcanically active than the farside. Recent dating of basalts has shown that lunar volcanism was active for almost 3Ga, starting at about 3.9–4.0Ga and ceasing at ∼1.2Ga. A recent re-processing of the seismic data supports the presence of a partially molten layer at the base of the mantle and shows not only the presence of a 330km liquid core, but also a small solid inner core. Today, the Moon does not have a dynamo-generated magnetic field like that of the Earth. However, remnant magnetization of the lunar crust and the paleomagnetic record of some lunar samples suggest that magnetization was acquired, possibly from an intrinsic magnetic field caused by an early lunar core dynamo. In summary, the Moon is a complex differentiated planetary object and much remains to be explored and discovered, especially regarding the origin of the Moon, the history of the Earth–Moon system, and processes that have operated in the inner Solar System over the last 4.5Ga. Returning to the Moon is therefore the critical next stepping-stone to further exploration and understanding of our planetary neighborhood.

Magnetic fidelity of lunar samples and implications for an ancient core dynamo

Some lunar rocks have stable magnetizations that provide compelling evidence for an ancient lunar core dynamo. However, a longstanding problem has been interpreting the unstable alternating field (AF) demagnetization behavior observed in the remaining majority of the Apollo lunar sample suite. Similar unstable behavior has also been observed for many meteorites. It is unclear whether the behavior of these samples indicates that they formed in the absence of an ancient magnetizing field or whether they simply have poor magnetic recording properties. It is necessary to distinguish between these two possibilities in order to determine whether paleomagnetic fields were present on a sample's parent body. To address this issue, we analyzed five samples whose rock magnetic properties span the full suite of observed demagnetization behaviors: mare basalts 15556, 15016, 12017, 10020, and troctolite 76535. We demonstrate that the effects of spurious anhysteretic remanent magnetization (ARM) during AF demagnetization, in combination with multidomain magnetic carriers and magnetic remanence anisotropy, are likely responsible for the apparently poor magnetic behavior of many lunar samples. Therefore, the unstable AF demagnetization behavior observed for many lunar samples is not alone sufficient for ruling out the presence of an ancient lunar core dynamo. Nevertheless, spurious ARM may explain the observation of surprisingly high (≥1μT) Apollo-era paleointensity measurements for samples thought to have formed after the cessation of a lunar core dynamo (<1.5Ga).

A Long-Lived Lunar Core Dynamo

Paleomagnetic measurements indicate that a core dynamo probably existed on the Moon 4.2 billion years ago. However, the subsequent history of the lunar core dynamo is unknown. Here we report paleomagnetic, petrologic, and (40)Ar/(39)Ar thermochronometry measurements on the 3.7-billion-year-old mare basalt sample 10020. This sample contains a high-coercivity magnetization acquired in a stable field of at least ~12 microteslas. These data extend the known lifetime of the lunar dynamo by 500 million years. Such a long-lived lunar dynamo probably required a power source other than thermochemical convection from secular cooling of the lunar interior. The inferred strong intensity of the lunar paleofield presents a challenge to current dynamo theory.

Early Lunar Magnetism

It is uncertain whether the Moon ever formed a metallic core or generated a core dynamo. The lunar crust and returned samples are magnetized, but the source of this magnetization could be meteoroid impacts rather than a dynamo. Here, we report magnetic measurements and 40Ar/39Ar thermochronological calculations for the oldest known unshocked lunar rock, troctolite 76535. These data imply that there was a long-lived field on the Moon of at least 1 microtesla approximately 4.2 billion years ago. The early age, substantial intensity, and long lifetime of this field support the hypothesis of an ancient lunar core dynamo.

Magnetic fields of lunar multi‐ring impact basins

Abstract— We survey the magnetic fields of lunar multi‐ring impact basins using data from the electron reflectometer instrument on the Lunar Prospector spacecraft. As for smaller lunar craters, the primary signature is a magnetic low that extends to ˜1.5–2 basin radii, suggesting shock demagnetization of relatively soft crustal magnetization. A secondary signature, as for large terrestrial basins, is the presence of central magnetic anomalies, which may be due to thermal remanence in impact melt rocks and/or shock remanence in the central uplift. The radial extent of the anomalies may argue for the former possibility, but the latter or a combination of the two are also possible. Central anomaly fields are absent for the oldest pre‐Nectarian basins, increase to a peak in early Nectarian basins, and decrease to a low level for Imbrian basins. If basin‐associated anomalies provide a good indication of ambient magnetic fields when the basins formed, this suggests the existence of a “magnetic era” (possibly due to a lunar core dynamo) similar to that implied by paleointensity results from returned lunar samples. However, the central basin anomalies suggest that the fields peaked in early Nectarian times and were low in Imbrian times, while samples provide evidence for high fields in Nectarian and early Imbrian times.

An early lunar core dynamo driven by thermochemical mantle convection.

Although the Moon currently has no internally generated magnetic field, palaeomagnetic data, combined with radiometric ages of Apollo samples, provide evidence for such a magnetic field from approximately 3.9 to 3.6 billion years (Gyr) ago, possibly owing to an ancient lunar dynamo. But the presence of a lunar dynamo during this time period is difficult to explain, because thermal evolution models for the Moon yield insufficient core heat flux to power a dynamo after approximately 4.2 Gyr ago. Here we show that a transient increase in core heat flux after an overturn of an initially stratified lunar mantle might explain the existence and timing of an early lunar dynamo. Using a three-dimensional spherical convection model, we show that a dense layer, enriched in radioactive elements (a 'thermal blanket'), at the base of the lunar mantle can initially prevent core cooling, thereby inhibiting core convection and magnetic field generation. Subsequent radioactive heating progressively increases the buoyancy of the thermal blanket, ultimately causing it to rise back into the mantle. The removal of the thermal blanket, proposed to explain the eruption of thorium- and titanium-rich lunar mare basalts, plausibly results in a core heat flux sufficient to power a short-lived lunar dynamo.

Magnetism of the Moon - a lunar core dynamo or impact magnetization?

Surface and satellite observations of lunar crustal magnetization and the remanent magnetization of the lunar samples returned by the Apollo missions of 1969–72 provide evidence for past magnetic fields on the Moon. During the more than twenty years in which research has been carried out there has been controversy concerning the origin of the field, with two possible processes of major interest. Dynamo generation of the field in a molten, electrically-conducting core is consistent with most of the observations, but encounters theoretical difficulties associated with the deduced magnitude of the ancient field and lack of positive evidence for a lunar core. The most likely alternative process is the generation of a transient magnetic field during meteorite impacts followed by thermoremanent or shock magnetization of debris and adjacent crust. This paper reviews the evidence and compares the observations with characteristics of lunar magnetism expected as a result of each of the two possible processes. It is concluded that the evidence very strongly favours the past existence of a dynamo-generated lunar magnetic field, with impact magnetization playing a minor role.

Fission tracks in chondritic meteorites and the search for siderophile superheavy elements

ABSTRACT The absence of excesses of fission tracks along the boundaries between olivine crystals and metal grains indicates that the concentration of fissioning superheavy elements in the metal must have been less than about 2.5 × 10−15 kg kg−1 and 2.5 × 10−13 kg kg−1 in the chondritic meteorites Wold Cottage and Alta'ameem respectively. The abundance of 244Pu fission tracks in phosphates from each meteorite suggests that track retention could not have commenced later than about 4 × 109 y ago. Superheavy element concentrations of such magnitudes would not provide a sufficient heat source to drive a lunar core dynamo.

A search for evidence of a fissionable nuclide in iron meteorites and implications on heat sources in planetary cores

It is shown that nuclear fission could not have significantly contributed as a heat source to drive an early lunar core dynamo. Furthermore, contributions of any fissionable nuclide to the energetics of the earth's core today are very unlikely in comparison to other sources of energy. These conclusions are inferred from searches for records of fission events in the metal phase of IA and IVB meteorites. We obtain strict upper limits for the concentration level of Nd and Sm in the Canyon Diablo (IA), Pin˜on (anom.), and Santa Clara (IVB) meteorites which are not distinguishable from the levels of the procedural blank and exhibit a normal isotopic composition. No contribution from fission to the unshielded isotopes could be detected. Upper limits for contributions by fission are 10 8 atoms g −1 per isobar. Assuming a low fission yield of 10 −4, we obtain 10 12 fission events g −1. It is reasonable to assume no losses of REE fission products due to late stage recrystallization or outgassing. This limit would correspond to a maximum deposition of 30 J g −1 in the earth's core due to fission. From earlier experiments on Kr and Xe in other IA iron meteorites, upper limits for contributions by fission of 3 × 10 5 and 2 × 10 6 fission events g −1 per isobar can also be estimated for the unshielded isotopes. Recent studies on nuclear tracks in silicate inclusions of IA iron meteorites set upper limits of 10 9–10 10 fission events g −1, the detected tracks being most plausibly explained by spontaneous fission of 244Pu. We can find no evidence in support of a fissionable, siderophile superheavy element and all limits for fission events are too low to support fission as a heat source in planetary cores. Furthermore, by Nd and Sm studies on two IVB iron meteorites, we demonstrate that Ag anomalies found earlier in meteorites of this class cannot have been produced by in situ fission of any known nuclide. This further supports the interpretation that extinct 107Pd is the source of the 107Ag anomalies.