The scientific controversy stems from decades of contradictory measurements on the rock samples that the Apollo astronauts carried home. On Earth, the churning of liquid iron in the outer core generates a global magnetic field through a process analogous to a bicycle dynamo, which converts mechanical motion into electrical energy. Some researchers who have studied lunar rocks contend that a strong magnetic field operated over a long stretch of time between about 4.25 and 3.5 billion years ago. Others examining similar samples find no evidence of any such field at all. Compounding the confusion, magnetised lunar rock does not automatically prove a dynamo. Massive meteorite or asteroid impacts could, in principle, have triggered magnetisation processes on the Moon, imprinting magnetic signatures in the crust without any internally generated field.

To sidestep the ambiguities of the sample record, geophysicist Anna Mittelholz and PhD student Xi Yang of ETH Zurich’s Department of Earth and Planetary Sciences, together with colleagues at the Institute of Space Research, the German Aerospace Center (DLR) and the Technical University of Berlin, turned to orbital data instead. Their analysis drew on gravity measurements from NASA’s twin GRAIL probes and on magnetic field models constructed from orbital measurements gathered by the Lunar Prospector and Kaguya missions. The target of their scrutiny was a region called Dewar, located on the far side of the Moon, the hemisphere that never faces Earth and one that remains poorly characterised compared with the near-side terrain studied since the Apollo era.

Dewar turned out to be a remarkable stroke of fortune. It hosts one of the strongest magnetic field anomalies on the lunar far side, and that magnetic signal coincides spatially with a distinct gravity anomaly, meaning the region contains rock that is simultaneously more strongly magnetised and denser than its surroundings. In most cases, the origin of magnetic anomalies measured from lunar orbit remains unknown, because a magnetic signal alone cannot reveal what lies beneath the surface. Gravity data, however, provide a window into subsurface density and therefore into the material hidden below. Where the two signals overlap, they can be combined and attributed to a specific geological structure, and Dewar offered precisely that opportunity. For the first time, the team created an accurate model of the subsurface by jointly processing the gravity and magnetic field data.

The model revealed something extraordinary hidden beneath the Dewar region: a buried rock body approximately 60 kilometres wide that extends to a depth of around 9 kilometres. It is much denser than the surrounding lunar crust and, at the same time, strongly magnetised. When the researchers combined this subsurface picture with surface geochemistry and a distinctive arched topography, they concluded that the body is solidified magma that once rose from the Moon’s interior, in other words, a buried volcanic complex. The age of the structure could be pinned down from the various deposits of impact material scattered across the lunar surface around it, yielding an estimate of about 4.2 billion years.

That age and composition allowed the team to perform a calculation with far-reaching implications. Because the iron content of such a rock body is known, the researchers could estimate the minimum strength the magnetic field must have had while the magma cooled slowly and locked in its magnetisation. Their conclusion is striking: the lunar magnetic field at that time was very likely stronger than 10 microtesla. For comparison, Earth’s magnetic field today stands at around 50 microtesla. A young Moon, barely a few hundred million years after its formation, may therefore have been generating a field approaching a fifth of the strength that shields our planet today.

The researchers also ruled out the leading alternative explanation. Impact-generated magnetisation requires specific conditions, and the Dewar region lies outside the areas considered plausible candidates for such processes. This exclusion matters enormously for the dynamo debate, because a magnetised volcanic complex of impact-independent origin, dated to 4.2 billion years ago, is exactly the kind of evidence that has been missing from the contradictory Apollo sample record. As Yang put it, the team can therefore be almost certain that the magnetic field must originate from a longer-lasting dynamo generated in the Moon’s core. The question, however, is not entirely settled. It remains unclear how the Moon’s small core could have generated such a strong magnetic field, and the ETH researchers do not yet consider the existence of an early lunar dynamo to be fully resolved. What has changed is the framing of the problem. The debate has shifted from asking whether a dynamo existed to asking how it worked, and the researchers are now examining the question from an entirely new perspective.

The study also sheds light on one of the Moon’s most photogenic mysteries: the lunar swirls. These bright, curved or striped patterns stand out sharply against the darker surface around them, and wherever a swirl appears, researchers invariably find a magnetic anomaly. A swirl also decorates the surface in the Dewar region, directly above the buried anomaly. One leading explanation holds that swirls form only where the magnetic field runs horizontally at the surface, as it does at the Dewar Swirl. Such a horizontal field deflects the solar wind, the constant stream of charged particles streaming from the Sun, thereby protecting the surface beneath from space weathering. That protected patch remains brighter than its surroundings, painting the swirl onto the landscape over billions of years.

Beyond their scientific elegance, the findings carry practical weight for the coming era of crewed lunar exploration. Because magnetic field lines could offer protection from solar wind radiation, swirls mark locations where such shielding constellations exist, information that Mittelholz describes as important for future astronauts planning extended stays on the surface. The team’s findings are also intended to help upcoming lunar missions select priority targets for on-site measurements, and they may guide the analysis of newly collected lunar samples in reconstructing the Moon’s magnetic evolutionary history. Perhaps most intriguingly, the method itself is portable. Yang notes that the same technique, combining gravity and magnetic data from orbit to infer the presence of a planetary dynamo, could be applied to other celestial bodies. Mars is an especially tempting case, although data of sufficiently high quality is currently lacking. For now, the buried magma of Dewar stands as a silent witness to a time when the young Moon, like Earth today, generated its own magnetic heart.

Subject of Research: Evidence for an early core-generated magnetic field on the Moon from the Dewar magnetic anomaly

Article Title: The far side of the Moon provides clues to a previous magnetic field

Article References: The far side of the Moon provides clues to a previous magnetic field. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Moon, lunar dynamo, magnetic field, Dewar region, far side of the Moon, GRAIL, Lunar Prospector, Kaguya, volcanic complex, lunar swirls, paleomagnetism, solar wind

The Hall effect, discovered by Edwin Hall in 1879, is one of the foundational measurement tools of condensed matter physics. When a current flows through a conductor in the presence of a perpendicular magnetic field, the Lorentz force pushes charge carriers to one side, producing a voltage across the material at right angles to the current. That transverse voltage encodes the density and sign of the mobile charges and, in its quantized variants, has underpinned decades of research into exotic quantum states. Extending the Hall effect to composite quasiparticles such as trions is therefore more than a technical curiosity: it provides a direct probe of the transport behavior of particles that carry both charge and internal structure.

Trions occupy a fascinating middle ground between ordinary excitons and free electrons. An exciton is electrically neutral, a bound electron–hole pair that can move through a semiconductor much like an atom of light, while a trion—often written X− or X+ depending on whether the extra particle is an electron or a hole—carries net charge equal to that of a single electron. Because trions combine a finite charge with a substantial optical dipole moment, they can be created and detected optically yet still respond to electrical and magnetic forces. This dual character makes them attractive candidates for optoelectronic applications and for fundamental studies of many-body physics, but it also makes their transport properties notoriously difficult to isolate from those of the surrounding charge sea.

The experimental platform used in the new study was an electron–hole double layer, a structure in which two sheets of charge carriers of opposite polarity are separated by an insulating or semiconducting barrier. Electrons reside in one layer and holes in the other, each able to move freely within its own plane, while the Coulomb attraction between the layers binds them into excitons and, in the presence of excess carriers, into trions. Double layers of this kind, typically realized in atomically thin van der Waals heterostructures built from transition metal dichalcogenides or in carefully engineered quantum well systems, allow researchers to tune the balance between free carriers and bound complexes simply by adjusting gate voltages.

When the researchers applied a perpendicular magnetic field to this double layer system, they observed that the trion fluid—collectively, the gas of charged three-particle complexes drifting through the device—developed a transverse voltage characteristic of the Hall effect. Crucially, the measured Hall response could not be explained by the motion of free electrons or free holes alone. Its sign and magnitude tracked the optical signatures of trion formation, indicating that the composite particles themselves were participating in the transport. In effect, the experiment showed that an electrically charged but internally complex quasiparticle can behave, at the level of Hall physics, as a well-defined carrier with its own effective Hall coefficient.

Technically, the observation is a considerable feat of disentanglement. In an electron–hole double layer, the Hall voltage measured in either layer receives contributions from several populations: the free majority carriers, the minority carriers of opposite sign, the neutral excitons, and the charged trions. The theory developed alongside the experiments accounts for this mixture using a multi-component transport formalism, in which each species contributes its own conductivity tensor, modified by inter-particle scattering and by the mutual attraction between the layers. The Hall response of the trion component depends on its charge, its effective mass—which is dominated by the heavy bound complex rather than the light bare electron—and the statistics of the excess carriers that stabilize it. The agreement between the measured Hall coefficients and these predictions provides quantitative evidence that trions transport as coherent entities rather than as transient associations of separately drifting particles.

The result settles a question that has lingered since trions were first identified in semiconductor optical spectra decades ago. Because a trion is only weakly bound compared with an atom, and because its lifetime in a photoexcited sample is short, many researchers doubted whether trions could sustain the kind of steady-state drift required for a Hall measurement. The new work demonstrates that, in the right regime, a trion fluid behaves hydrodynamically and electrically much like an ordinary charged gas. The effective temperature and density of the trion population can be controlled through optical pumping and electrostatic gating, and under suitable conditions the composite particles establish their own drift velocity in response to an applied electric field, exactly as required for a Hall signal to develop.

Beyond its fundamental significance, the trion Hall effect opens practical avenues. Trions are central to proposals for excitonic and optoelectronic devices in which information is carried by light-matter bound states rather than bare electrons, promising lower-energy operation and new ways to couple optical and electrical signals. A measurable Hall response provides a purely electrical handle on trion populations, complementing optical spectroscopy and enabling faster readout of composite-particle dynamics. It could also serve as a diagnostic in quantum emitter and single-photon technologies, where trion states often mediate the optical response of defect centers and quantum dots. In van der Moiré heterostructures, where flat electronic bands amplify correlation effects, a trion Hall measurement might even reveal interaction-driven phases that are invisible to conventional transport.

The demonstration also connects to broader themes in modern condensed matter research. Hall effects have become a versatile language for characterizing quasiparticles of every description, from Weyl fermions in topological semimetals to strange metals and superconductors. Each new carrier class that exhibits a Hall response adds a fingerprint that can be used to identify and study it. Trions, being charged composites with internal degrees of freedom, sit in an unusual position in this landscape: their Hall coefficient reflects not only their charge-to-mass ratio but also the correlated motion of the constituent electron and hole, potentially encoding information about binding energies, scattering channels and the inter-layer coupling strength. The theoretical framework validated here offers a template for extracting such details in other layered systems where excitonic complexes proliferate.

Looking ahead, the researchers and their colleagues anticipate several extensions. Applying stronger magnetic fields may drive the trion fluid toward quantum Hall regimes, where quantized Hall signatures of composite particles could emerge. Studying how the trion Hall effect evolves as the layer separation shrinks toward the atomic limit could illuminate the crossover from loosely bound three-body states to the strongly correlated exciton fluids seen in recent experiments on monolayer semiconductors. And because the double layer geometry is compatible with contactless detection methods, the approach may generalize to systems where fabricating electrodes is impractical. For now, the observation stands as a clean confirmation of a decade-old theoretical expectation and a reminder that even the most familiar effects in physics can acquire new life when the carriers doing the drifting are as unconventional as charged pairs of light and matter.

Subject of Research: The Hall effect in a trion fluid formed within coupled electron–hole layers of a semiconductor device

Article Title: Trion Hall effect in electron–hole double layers

Article References: Trion Hall effect in electron–hole double layers. (n.d.). https://doi.org/10.1038/s41567-026-03444-8

Image Credits: AI Generated

DOI: 10.1038/s41567-026-03444-8

Keywords: trion, Hall effect, exciton, electron–hole double layer, condensed matter physics, quasiparticles, semiconductor, van der Waals heterostructures, quantum Hall physics, optoelectronics, magnetic field, transport phenomena