Where is the magnetized material located on the lunar surface?

Magnetometers onboard spacecraft have detected magnetic field signals originating from the lunar crust. These signals are known as magnetic anomalies and are generated by rocks that are permanently magnetized. Lunar magnetic anomalies are distributed heterogeneously over the lunar surface and the geological processes that gave rise to them is under debate. Thus, the Moon's geological history can be further assessed by inferring the shape of the underlying magnetized material. Up to now, these sources were not fully described for such geological assessment studies.

Joana Oliveira and her colleagues, in a recently published work, evaluated the ability of a methodology up to now used to infer the direction of the magnetization, called the method of Parker, to recover the location and shape of the magnetized material by using orbital magnetic field data only.

Lunar magnetic field map at 30 km altitude using Tsunakawa et al. 2015 model.

Through a series of tests, the authors of this study have shown that the Parker’s method can constrain the shape of the source of a magnetic anomaly, provided that the respective part of the crust is magnetized along a common direction.

"We tried to take it a step further to crack the unidirectional assumption by testing complex bodies with different directions, and we were surprised by how this method was still able to recover most of the magnetized structure”, Joana said.

The authors also applied the method to two lunar magnetic anomalies related to two visible geological features an impact crater and an albedo anomaly, also know by swirls. Results show that the inferred shape and location of the magnetized material are in good agreement with the associated geological features and suggest that one originated by an impact event and the other by volcanic activity.

Parker inversion results for the Mendal-Rydberg basin. The magnetized material (related to the dipole moments) is correlated with the inner depression in blue color of the topography map, despite the magnetic field signal being shifted to the southwest from the center of the basin. Figure adapted from Oliveira et al. 2024.

Future applications can focus on constraining the origin of the many lunar magnetic anomalies that are not associated with visible geological features.

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Joana S. Oliveira is an archive scientist working for the European Space Agency (ESA) JUICE and Heliophysics missions, with a background in planetary sciences. She is interested in learning about the history of rocky planets through their magnetic field signals.

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Unraveling Earth's Ancient Geography: Advancements in Paleomagnetic Analysis and Plate Reconstructions

Plate tectonics plays a crucial role in shaping Earth's geography, impacting the evolution of life and climate. To really understand the long-term evolution of Earth’s systems, we need to quantify the past motions of the tectonic plates. Plate reconstructions have been crucial in figuring out an enormous array of Earth processes.

Conventionally, past plate motions are inferred from physical characteristics of the sea floor, specifically its magnetic anomalies and fracture zones. Together, those features have allowed the construction of global plate reconstructions back to the Cretaceous (~130 Ma). But the inherent nature of plate tectonics masks its own origin story: oceanic lithosphere records are progressively destroyed by subduction, so they cannot be used in deeper time. Before 130 Ma, plate motions can only be quantified through the study of paleomagnetism.

Over long time scales (~105-106 years), the geomagnetic field can be approximated by a geocentric axial dipole (GAD), where the vertical field component is specifically linked to latitude and the horizontal component consistently points north. This means that if a rock can record the direction of the paleomagnetic field during its formation and the GAD hypothesis remains valid (at least back to ~540 Ma), we can establish its original paleolatitude and azimuthal orientation! However, analytical limitations have so-far prevented us from using this tool to its full potential. For example, owing to the axial symmetry of the Earth’s magnetic field, the determination of paleolongitude from paleomagnetic data –although theoretically possible– cannot be constrained. Paleolongitude has thus remained the greatest uncertainty in pre-Cretaceous plate reconstructions (top panel figure).

Paleomagnetic records are collected from individual rock samples and subsequently grouped to develop global-scale paths called apparent polar wander (APW) paths. These APWPs represent the time-dependent position of Earth's spin axis relative to a given block of lithosphere or continent.

Hypothetical APWP track (filled dots).
Top panel: Conventional paleomagnetic reconstruction where paleolongitude remains unconstrained.
Bottom Panel: the APWP segment traces a small circle, the centre of which represents the Euler vector. Rotations about Euler poles can completely define plate motion so PEP analysis yields east-west motion.

Euler’s theorem states that any displacement across the surface of a sphere can be represented by a rotation about an axis (i.e. Euler vector). Consequently, if a continent rotates about a fixed axis, the corresponding paleomagnetic poles (i.e. APWP segment) will trace an arc which can be defined by a small circle, the centre of which represents the Euler vector. The inversion of paleomagnetic data to retrieve Euler vectors – or paleomagnetic Euler pole (PEP) analysis – is of particular interest because it offers the possibility to recover full kinematic descriptions of past plate motion. Because an Euler vector can fully express the kinematics of a continent, if paleomagnetic data can be used to compute Euler vectors describing a given plate’s history of motion, its paleolongitude is determinable!

This exciting method could overcome the paleolongitudinal indetermination of paleomagnetism, but despite being first conceptually introduced more than half a century ago has seen limited application. This appears to be due, at least in part, to the fact that paleomagnetic data is inherently noisy, with noise coming from both intrinsic (geomagnetic secular variation) and extrinsic (e.g. measurement errors, erroneous age assignments in rocks, inclination shallowing, etc.) uncertainties. For instance, current APWPs describe plate motion in 10 Ma steps, yielding a crude description of plate latitudinal and azimuthal motion.

Recent advances in APWPs construction methods have demonstrated that through new methodologies and computational methods, it is possible to generate APW paths with unprecedented spatial and temporal resolution (~1Ma). These new methods may offer new insights into Earth's deep time evolution. Great things are on the horizon!

Leandro Gallo is a Maria Skłodowska-Curie postdoctoral fellow at the Center for Planetary Habitability (PHAB), a Center of Excellence funded by the Norwegian Research Council and hosted at the University of Oslo (UiO), Norway. His research focus is reconstructing the long-term changes in the ancient spatial configuration of continents (paleogeography). A major focus of this research is on paleomagnetic data synthesis tools, combining data-analysis, data-science and statistics to constrain polar wander through deep time.

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.

Opportunities and challenges with palaeomagnetic data sources

In our quest to understand the geomagnetic field of past millennia, we rely on two primary sources of data. The first source is archaeomagnetic data, which provides us with valuable information about the geomagnetic field based on ancient artefacts. However, one major challenge we encounter is the uneven distribution of this data in both space and time. It's like trying to put together a historical jigsaw puzzle without all the pieces.

But don't worry, there is another data source that helps overcome these limitations and provides a more comprehensive understanding of the past geomagnetic field. Sedimentary records, a treasure trove of information spanning vast time periods and offering improved spatial coverage. Imagine these records as growing bars, each representing a time series of the geomagnetic field at a specific location. In contrast, archaeomagnetic data appears as dots, providing snapshots of the geomagnetic field at specific locations. As we venture further back in time, sediment data becomes increasingly essential since archaeomagnetic data becomes sparser.

The spatial and temporal data coverage.

Now let's unravel the different magnetization mysteries within these sediments. Archaeomagnetic data captures the geomagnetic field through a fascinating process known as thermoremanent magnetization (TRM). Picture this: ancient artifacts, like pottery and kiln structures but also lava flows are heated and then left to cool down. But here's the magical part - during this process, they become magnetized, preserving a snapshot of the geomagnetic field at that exact moment. It's like capturing a piece of geomagnetic field's history in a magnetic time capsule.

The magnetization process in sediments is known as detrital remanent magnetization (DRM). During the sedimentation process, magnetic particles settle in such a way that their magnetic moments tend to point in the direction of the geomagnetic field. It's like they have an ancient compass within them, pointing in the direction of the geomagnetic forces. With the accumulation of additional sediment material, the magnetic particles become mechanically fixed within the sediment structure, preserving their magnetic orientations. It's as if they have been frozen in time, capturing the magnetic field's influence at the lock-in moment.

The magnetization in sediments is influenced by several factors, such as the interaction between the magnetic particles and the substrate at the sediment-water interface (depositional DRM), as well as the consolidation and dewatering process of the sediment (post-depositional DRM). Within depositional DRM, various effects come into play. For instance, there is the inclination error, which arises when non-spherical particles settle flat on the sediment-water interface. This leads to a distortion of the inclination, resulting in smaller inclination values than expected.

In our investigation, we're particularly interested in unraveling the secrets of post-depositional DRM. Initially, only the larger sediment particles become mechanically fixed shortly after deposition. Smaller particles, on the other hand, enjoy a freer journey, moving within water-filled voids and pore spaces for a longer duration. However, as the sediment consolidates and dries out, these smaller particles slowly become locked in too. It's a mesmerizing process, like witnessing magnetic particles tell stories of the ever-changing geomagnetic field.

Check out the figure below that illustrates the journey of magnetic particles during the lock-in or pDRM process.

The sedimentation process.

A The lock-in adventure begins when the particles settle on the sediment-water interface. Sediments are composed of a mix of magnetic and non-magnetic particles, creating a vibrant playground. During the early stages of the lock-in process, particles rotate freely and align with the geomagnetic field. It's like a magnetic ballroom dance conducted by the geomagnetic field forces.

B As time passes and sedimentation continues, the surrounding material consolidates. Larger magnetic particles begin to lose their mobility and get locked in. They find their forever spots, holding onto the memories of the geomagnetic field at that time. But what about the smaller particles? They're still lively and free, closely following the twists and turns of the geomagnetic field.

C After ample sedimentation and consolidation, the lock-in process reaches its grand finale. Each particle becomes a storyteller, carrying a piece of the geomagnetic field's history within it. The sediment layer becomes a mosaic of magnetic moments, depicting diverse states of the geomagnetic field throughout the entire lock-in period. It's like a magnetic symphony composed of the melodies of the geomagnetic field.

The magnetic moment of a whole sediment layer represents a weighted average of the geomagnetic field over the lock-in period. This is where the concept of a lock-in function comes into play. The lock-in function assigns weights to the different geomagnetic field values, reflecting their significance during the lock-in process.

The investigation of the lock-in process and the development of a modeling concept to estimate the lock-in functions for individual core samples is the primary goal of our studies. Our research outcomes provide in-depth details, methodologies, and findings that shed light on the fascinating world of sediment records and post-depositional DRM. Visit https://sec23.git-pages.gfz-potsdam.de/korte/pdrm/ for more information. With our results we make sediment data a more reliable data source for modeling the geomagnetic field. It's time to unlock the secrets of the geomagnetic past!

Lukas Bohsung is a second year PhD student at the University of Potsdam and the Helmholtz Centre Potsdam — GFZ German Research Centre for Geosciences in Germany. His main focus is on investigating and modeling magnetization processes in sediments to make sediment records a more reliable data source for geomagnetic field reconstructions. He can be contacted via email here.