A new view of lunar magnetism

One of the enduring mysteries about the Moon is whether it ever had an internal-generated magnetic field, and if so, when the core dynamo ceased. New research by Tinghong Zhou, John Tarduno, Rory Cottrell, and Eric Blackman at the University of Rochester and collaborators from the University of Notre Dame, UC Santa Cruz, and the University of Arizona, in a study supported by NSF and NASA, have provided new insights into this lunar puzzle, narrowing down the potential lifespan of the Moon’s dynamo to its first ~140 million years. The new study focused on analyzing magnetic field intensity (called paleointensity) recorded in Apollo samples that are between 4.36 to 3.7 billion years old. Using an advanced technique known as single-crystal paleointensity analysis, the researchers were able to obtain accurate measurements of the Moon’s ancient ambient surface magnetic field environment – which indicated negligible field strengths. This evidence for the absence of a dynamo resolves the long-lasting paradox between the previously hypothesized long-lived lunar dynamo and energy considerations, namely that the tiny lunar core would have been unable to power a strong, sustained magnetic field.

Figure 1: Astronaut John Young stands on the rim of the Plum Crater on the Moon. Image source: NASA, Apollo Lunar Surface Journal, Apollo Image Library Hasselblad Magazine, 109/G (B&W), AS16-109-17804.

A key part of this new understanding of lunar magnetic history comes from a focus on magnetic carriers meeting the demanding requirements of paleointensity theory. Single-domain magnetic grains, which are very small, some 20 to 200 nm in size, are required. In contrast, larger magnetic grains are far less reliable because their internal domain walls can move with time and during laboratory treatments, corrupting any original magnetic signal. Magnetic minerals in lunar rocks are dominated by these problematic multidomain grains, making paleointensity analysis very challenging. The single-crystal paleointensity technique used in the study by Zhou and others builds on an earlier study led by the University of Rochester and focuses on silicate crystals that contain single-domain magnetic grain inclusions to meet the paleointensity recording requirement. The authors tested the fidelity of their records by CO2 laser heating in different fields and in the presence or absence of an applied field. These tests exclude thermal alteration and provide a measure of recording efficiency. The authors found high recording efficiencies, indicating that if surface fields had been present, they would have been recorded. Hence, the absence of a paleointensity indicates absence of a surface field.

In addition to the single crystal paleointensity, the study also employed whole rock paleointensity on 3.7-billion-year-old Apollo basalts using a non-thermal technique. Unlike thermal methods that measure magnetization acquired from natural cooling, non-thermal methods rely on additional assumptions and empirical calibrations. The results from the non-thermal technique showed abnormally high and inconsistent paleointensities. These anomalies could indicate shock magnetization from lunar impacts or issues with the multidomain grains and/or the applied non-thermal method. Because non-thermal analysis of whole rocks is the basis for some calls for an episodic lunar dynamo, the researchers conclude there is no robust evidence for such a phenomenon from Apollo samples.

Figure 2: Lunar magnetic history indicated by paleointensity data. Single crystals suggest a null lunar magnetic field since 4.36 Ga, while some whole rock data obtained by non-thermal methods yield abnormally high values that might be related to large multidomain magnetic grains and/or impact induced magnetic field. Figure modified from Tarduno et al., 2021 and Zhou et al., 2024.

If the Moon did not have a dynamo for most of its history, the early Earth’s (for example, during the Archean and Hadean eons) atmosphere can be transferred to the Moon, which would be unshielded by an intrinsic field, and preserved in its regolith. With a smaller Earth-Moon distance and the stronger solar wind in the Archean and Hadean, this transfer would have been enhanced. By studying the volatiles trapped in the lunar regolith, we might have opportunities to better understand the composition of the early Earth’s atmosphere and the conditions that influenced the evolution of life.

Tinghong Zhou is a postdoctoral researcher at the University of Rochester mentored by Professor John A. Tarduno. Her research focuses on the long-term evolution of the geomagnetic field and its correlation with the Earth’s deep interior, and the origin of the lunar magnetism. Email address: tzhou16@ur.rochester.edu

Not all who wander are lost – updating the International Geomagnetic Reference Field

The Earth’s magnetic field is not static and varies on many different time scales. The main source of the long-term field variation comes from the outer core where the magnetic field is generated by the motion of liquid nickel-iron which in turn ‘drags’ the field lines with it. Thus, the field changes strength and shape slowly over decades. Every five years, IAGA issues an updated version of the magnetic field to capture these slow changes known as secular variation. This series of ‘maps’ or models are known as the International Geomagnetic Reference Field (IGRF) and goes back 125 years. In November 2024, the 14th generation of the model was released, valid from January 1900 to December 2030.

Figure 1: Strength of the magnetic field in microTelsa on the Earth’s surface at 2025.0. Note the low strength region known as the South Atlantic Anomaly.

The magnetic field is represented by a series of numbers known as Gauss coefficients. Using the mathematical technique of spherical harmonic analysis, the magnetic field can be represented continuously in time and space rather than as a 3D grid of cells. This means we can provide a snapshot of the magnetic field above, at or below the Earth’s surface using a very compact set of just 195 numbers, which gives an approximate resolution of 3000 km. This captures the vast majority of the core field and allows us to calculate of Declination angle, Magnetic Dip and Total Field Intensity (see Figure 1) anywhere in the world. We can also track the location of the magnetic poles (see Figure 2).

The first IGRF for 1965 was issued in 1968 when it was difficult to get timely datasets of magnetic measurements from observatories – the data usually took several years to produce and was distributed by post! When the modern internet era began in the 1990s, data could be circulated more rapidly and the IGRF began to be produced in a more timely fashion. Today, we live in a golden era of magnetic field measurement: from hundreds of high-quality geomagnetic observatories to dedicated magnetic missions such as ESA Swarm and Macau Science Satellite. Together these freely available, near-real-time datasets and cheap powerful computer make core field modelling widely accessible.

Figure 2: Estimated location of the geomagnetic and magnetic dip poles from 1900 to 2030. While the north dip pole has accelerated over the past 20 years moving from Canada toward Siberia, the south dip pole has moved much more slowly.

For the 14th generation, 19 teams of geomagnetic scientists from four continents submitted candidate models for the magnetic field in 2020 and 2025 and a forecast of secular variation between 2025 and 2030. In total, there were 47 candidate models to evaluate. The method for combination was determined by a panel of experts using a variety of different technical analyses. The final models were agreed by majority vote and the new coefficients were issued to the official IAGA website on 20th November 2024.

The IGRF is used for research for deep Earth and space weather forecasting, part of many industry applications for correcting surveys (archaeology, oil/gas, mineral exploration) as well as standard pointing and navigation uses. The IGRF truly is an international effort involving thousands of observers, scientists and engineers from around the world. Without their contributions, this would not be possible. We thank them all for their work.

Authors: Ciaran Beggan is senior researcher at British Geological Survey in Edinburgh. This is his fourth venture with the IGRF, starting as a PhD student back in 2009. Clemens Kloss is a postdoctoral researcher at DTU Space in Copenhagen who specialises in improving our view of the Earth’s core magnetic field minus the annoying effects of the aurora.

A paleomagnetist on board the JOIDES Resolution ocean drilling vessel

I was having a shower at the beginning of our last day on the ship- warm and comfort shower- and suddenly I smell something different, something like mold. I come out to the deck to realise that we have docked in Reykjavik, that it was the smell of land, the smell of the end of our two months expedition spent in the middle of the North Atlantic Ocean. A turmoil of feelings where silence would prevail. The entire scientist staff, the technical staff, some of the crew, the Capitan, we were all standing still, under a Nordic cold sun, watching the docking operations. The ever-changing colour of the Ocean turned to dark green port-like waters, full of birds, docks and ducks, land all around! It has been two months without seeing (and smelling) land.

We were coming back from our two months sailing in the legendary ship, the JOIDES Resolution (JR), for the International Ocean Discovery Program (IODP), Expedition 395 “Reykjanes Mantle Convection and Climate: Mantle Dynamics, Paleoceanography and Climate Evolution in the North Atlantic Ocean” with Ross Parnell-Turner from Scripps, California, Anne Briais from Toulouse in France as Chiefs, and Leah LeVay from IODP Texas A&M University as Project Manager/ Staff Scientists. We should have sailed in 2020 but because of the global Covid-19 pandemic, the JR Expedition sailed with only a few technical staff and became Expedition 384; one year later the expedition was postponed, becoming Expedition 395C, with only Leah as a Science Staff. Finally, this year, the entire Science Staff could sail. After a good amount of last-minute shopping, including chocolate, tea, biscuits and a hard disk, we set sail from the Ponta Delgata port in Sao Miguel (Azores, Portugal) on the 12th of June to drill a transect of 4 out of the 6 sites originally planned (2 were completed during Expedition 395C) from the East to West in the North Atlantic Ocean, south of Iceland. 

My job as a shipboard paleomagnetist was to measure all the sediment and hard rock cores in the Superconducting Rock Magnetometer to reconstruct the Earth's magnetic field changes in polarity to provide an age of the sediments. We compare these polarity changes recorded in the oceanic sediments (normal polarity is like the present day setting while the reverse polarity is when the North pole flips to the South Pole) with a global reference scale (called Geomagnetic Polarity Time Scale; Ogg 2020) to estimate the age of the sediments, and then we combine the paleomagnetic observations with the encounters of microfossils which also provide an independent age. We had 12 hours shifts and at the end of each shift (at noon and at midnight), we had a crossover meeting with all the other groups of scientists- the sedimentologists, the physical properties scientists, the geochemists, the stratigraphic correlators, the palaeontologists, us the paleomagnetists, the outreach officer, the staff scientists and the two chiefs. 

My typical day started with a one hour gym session in the morning, breakfast/lunch (which was always delicious), crossover meeting with my counterpart in the same role, Sarah, and then here we go, measuring all day meters and meters of ‘boring’ muds. I say ‘boring’ as their properties, colour, granulometry did not change much, but what I really mean was ‘amazing’ and ‘ideal’ for paleomagnetic studies as they require homogenous lithology and continuous sedimentation to capture with a clean signal all the polarity changes! Do not think that Sarah and I did all by ourselves... An amazing team of technicians was always there, ready to help answer questions and fix some mistakes (yes, we make mistakes and it’s ok). For every week there was a weekly report, for every completed site there was a site report and site summary and a meeting with everyone else to share the fresh off the measurements and interpretation results. Yes, hard work! In a hectic around the clock pace of laboratory work, data interpretation and report-writing, science was unveiling under our amazed eyes. Fuelled by coffee, music and peer comfort with frequent and short breaks (and a longer one for lunch/dinner), we made it! We drilled all the sites, exceeding the expectations, drilling more than 4 km of sediment cores, and 120m of basalts, nearly breaking the record of the deepest site ever drilled in one expedition. The Ocean was clement, calm for most of the time, blue, grey, silver, black, flock of birds were around us and some cetaceous visited us. It was simply an amazing experience, for the amazing group of scientists bringing their different expertise to the table, to achieve the expedition's goals and advance science…but I am sad that this program will end in just nine months.

That’s it. The JR needed repairs, but the main funding body cut the expenses out and none of the other international contributors stepped up to challenge. Nobody else will be able to sail on the legendary JR, breaking the boundaries of science by deep ocean drilling. We take comfort that the legacy remains for future scientists, of many kilometers of rock in the Core repositories of College Station in Texas and in Bremen but the specialised expertise to conduct a state of art floating laboratory are sadly lost, forever.

by Dr. Anita Di Chiara (she/her)

Researcher

INGV - Rome

One of the crew on the JR Expedition 395

 

 

 

Photo Credits-

 

1 and 2: Jen Field, Outreach Officer

3: Dr. Sevi Modestou, shipboard sedimentologist

Instabilities of the Earth’s magnetic field over multi-millennial timescales

Earth’s magnetic field varies over a wide range of time scales. The very long multi-millennial scales are associated with variations of the internal core field. The short timescales, annual and shorter, are related to field contributions generated by current systems in the ionosphere and magnetosphere. The most dramatic changes occur at the longest periods, namely geomagnetic reversals and excursions. They represent a significant decrease in the field intensity, with reversals having a complete polarity flip and longer duration in contrast to excursions. While the reversals are global events, the excursions can be of global and regional nature. For the latter, only part of the Earth is affected with very low intensity and transitional directions. The last geomagnetic reversal, Matuyama-Brunhes, happened 780,000 years ago, and the last global excursion, the Laschamps excursion, occurred 41,000 years ago. These events are studied with indirect measurements, paleomagnetic data from sedimentary cores and volcanic rocks. Global data compilations enable us to reconstruct the field’s spatial and temporal variations and to better understand the core processes responsible for these variations.

Magnetic field lines during a period of high dipole moment, for example 1900 AD (top) and the Laschamps geomagnetic excursion, 41,000 years ago (bottom). The figures show the dipole-dominated structure at stable periods and very complex, multipolar structure of the magnetic field in transitional epochs. The color maps represent the radial field component at the Earth’s surface. The blue lines are pointing inward and red lines in an outward direction.

Credit: Maximilian Schanner, GFZ Potsdam, 2022. https://sec23.git-pages.gfz-potsdam.de/korte/pymagglobal-3d-fieldlines/

doi: 10.5880/GFZ.2.3.2020.005

While the current configuration of the geomagnetic field can be approximated with a dipole (having two poles, magnetic north and south pole), during geomagnetic excursions, the dipole is not a dominant component. The figures present the magnetic field lines of a dipole vs. non-dipole-dominated field, where the former is characteristic of the present-day and the latter of the Laschamps excursion. Over very long time scales, tens of thousands to millions of years, the field closely approximates a geocentric axial dipole. This hypothesis allows using the paleomagnetic data for reconstructing plate tectonics.

Direct observations indicate that the magnitude of the Earth's magnetic axial dipole has decreased since 1840, and predictions of geomagnetic secular variation show that it will likely continue to decrease over the following decades. A weakened geomagnetic field may lead to numerous hazards like satellite outages, disturbances in communications and navigation, and induced currents in pipelines and transmission lines. The following questions come naturally:

Will the geomagnetic field go through an excursion or a reversal in the future? Most probably yes, considering the dynamic nature of the field and the frequency variations of these transitional events.

Is the geomagnetic field heading towards an excursion or reversal now? Probably not, based on analyses of available paleomagnetic field models covering the past 10,000 and 100,000 years and the fact that the field at present seems to be clearly stronger than the long-term average.

Can we predict such events? No, this remains an open question because it is hard to foresee the long-term changes. The geomagnetic field intensity needs to decay further for several centuries to recognize and distinguish a future transitional event from normal secular variation.

Sanja Panovska is a postdoctoral researcher at the Helmholtz Centre Potsdam – GFZ German Research Centre for Geosciences. Her primary research focus is reconstructing the long-term evolution of the geomagnetic field based on paleomagnetic data over different timescales from the Holocene, to 100,000 years and over the Earth’s geological history. She is also involved in studies on productions of cosmogenic isotopes, paleomagnetosphere and paleoaurora.