Published on 16 July 2026
A new study combining cutting-edge laboratory experiments with geomagnetic data from ESA's Swarm satellites has mapped the distribution of iron in Earth's lower mantle for the first time – shedding new light on the deep processes that have shaped our planet over billions of years.
The research, published in Science Advances, was led by Kui Han at the University of Bayreuth, Germany, and brings together two fields that have been developing in parallel: mineral physics and geomagnetism.
A window into the deep Earth
The lower mantle – a vast region extending from around 660 to 2,900 kilometres below Earth's surface – is dominated by a mineral called bridgmanite, a dense magnesium iron silicate that makes up roughly 80% of the lower mantle by volume. The concentration of iron incorporated into bridgmanite's crystal structure varies from place to place, and understanding where iron is concentrated, and why, is important. Iron influences mineral density, drives mantle convection, and has played a role in the long-term habitability of the planet.
The interior of Earth cannot be directly sampled at these depths, and so scientists have traditionally relied on seismic tomography – mapping how earthquake waves travel through earth – to infer the composition of the lower mantle. However, iron content and temperature produce overlapping signals, leaving a fundamental ambiguity that has limited researchers for decades.
A more sensitive probe
The new study resolves this ambiguity by exploiting a different physical property altogether: electrical conductivity. The team synthesised bridgmanite samples in the laboratory with varying iron contents and subjected them to the extreme conditions of the lower mantle – pressures up to 27 gigapascals and temperatures up to 2,000 K. Their measurements revealed that bridgmanite's electrical conductivity is extraordinarily sensitive to iron content, following a steep power-law relationship, whilst showing comparatively little sensitivity to temperature.
This is a crucial finding. Unlike seismic velocities, electrical conductivity cuts cleanly through the temperature–composition ambiguity. Where temperature and iron produce similar seismic signals, they produce very different electrical signals. Dr Alexander Grayver of the Institute of Geophysics and Meteorology at the University of Cologne contributed his geomagnetic modelling expertise to the study. He says: "It turns out that the conductivity is just so sensitive to iron that offers an almost perfect proxy for studying iron content in the mantle, which plays a big role in Earth's long-term evolution."
From magnetic field to mantle composition
ESA's Swarm mission – a constellation of three satellites measuring Earth's magnetic field with high precision – is one of the tools used to map the electrical conductivity of the mantle from space. Variations in the external magnetic field, driven by solar activity and other sources, induce secondary magnetic signals within the planet. These induced signals carry information about the electrical conductivity of the rocks through which they pass, and Swarm's sensitive magnetometers can detect them.
Alexander describes how the two strands of research came together: "The geophysical models we've been creating were interesting, but now we can add meaning to them. We have this missing piece of the puzzle coming from the lab results, which tell us how to extract something totally new from these models."
The Swarm-derived conductivity data were combined with measurements from the INTERMAGNET network of ground-based geomagnetic observatories, which are essential for calibrating and validating the satellite data. "Ground observations play an essential role," says Alexander. "I don't think this science is possible without ground observations." Together, the space and ground data underpin the global electrical conductivity model of the mantle that the team used to infer iron distributions.
Relics of ancient ocean floor
Applying the laboratory conductivity–iron relationship to the global conductivity model, the team produced maps of iron distribution in the lower mantle at depths of around 825 and 1,225 kilometres. Two features stand out.
The first is the identification of iron-enriched regions beneath the western Pacific and South America, close to major subduction zones – regions where tectonic plates plunge beneath one another and descend into the mantle. The team interprets these as vast reservoirs of ancient oceanic crust: iron-rich basaltic rock that once formed the floor of prehistoric oceans, was carried down into the mantle by subducting plates, and has remained there ever since. These iron-rich remnants of subducted crust have now been traced to depths exceeding 1,000 kilometres.
The second notable feature concerns the large low-shear velocity provinces (LLSVPs) – two enormous anomalous structures at the base of the mantle, one beneath Africa and one beneath the Pacific, which were discovered by seismic tomography decades ago and have puzzled scientists ever since. The study finds evidence of iron enrichment coinciding with the African LLSVP, supporting the hypothesis that these structures are thermochemical in nature – that is, they are compositionally distinct from the surrounding mantle, not merely hotter. Whether they represent ancient subducted material, primordial undifferentiated rock, or something else entirely remains an open question. "If the LLSVPs are, indeed, enriched with iron, this will have implications for the very long-term evolution of the planet,” Alexander says.
The road ahead
Alexander is among those working on a new generation of conductivity models combining data from multiple satellite missions and ground observatories, which he expects to offer greater resolution and insight.
"We are really only at the very beginning," he says. "This study gives us a bit of purpose now, a bit of motivation to continue. We can improve the resolution of the model, we can see where there are additional regions perhaps that are rich in iron."
For Alexander, the study marks the beginning of something larger. Swarm was designed in part to probe the electrical properties of the mantle, which remain far less well understood than its seismic properties. Studies like this one show what becomes possible when satellite observations and laboratory science work in concert, and point towards a new era of geomagnetic exploration of Earth's deep interior.
Source:
European Space Agency. (2026, July 16). Swarm helps reveal the iron architecture of Earth’s deep interior. Earth Online. https://earth.esa.int/eogateway/success-story/swarm-helps-reveal-the-ir…
Reference:
Han, K., Özaydın, S., Fei, H., Man, L., Wang, F., Chanyshev, A., Withers, A.C., Grayver, A. and Katsura, T. (2026). Lower-mantle iron heterogeneity constrained by the electrical conductivity of Al-bearing bridgmanite. *Science Advances*, 12, eaec7875.