The research team simulated hundreds of possible ancient landscapes, using models of how rivers shape mountain ranges, to test which ones could plausibly have been buried beneath the volcanic deposit. They found that only relatively low-relief landscapes – akin to gentle mountain foothills – could have done so. ¹

From this, the team inferred that this part of the Andes was being built slowly, with rocks pushed upward by no more than a quarter (0.26) of a kilometre every million years – equivalent to 2.5 cm, or about an inch, every 100 years.

If rocks had been pushed upward faster in this region, the landscape would probably have developed steeper slopes that could not have fitted beneath the immense volcanic deposits.

The finding lends support to the idea that the Andes grew slowly and steadily over tens of millions of years rather than popping up more recently, in the last few million years.

This rate is slow compared with many rapidly uplifting parts of active mountain belts, including parts of the Himalaya, where uplift and erosion can occur at rates of several millimetres to centimetres per year.

A hidden ‘Pompeii’ of Andean landscapes

Lead author Dr Byron Adams, based at UCL Earth Sciences, said: “This landscape was buried by a giant volcanic eruption – a little like Pompeii, but on a vastly larger scale. Instead of covering a town, hot mixtures of volcanic ash, rock fragments, and gas swept across an entire landscape, engulfing the terrain beneath them.

“Pompeii shows how volcanic eruptions can freeze a moment in human history. This study shows that much larger eruptions can also freeze moments in Earth history, burying whole landscapes beneath volcanic deposits and preserving clues to how mountains were being built before the eruption.

“We cannot dig down to see the buried landscape, but we can use the shape of the volcanic blanket and what we know about how rivers shape mountains to infer what is hidden beneath it.”

A new way of piecing together geological history

The paper’s authors noted their estimate of rock uplift was consistent with earlier research looking at minerals in the rock that recorded specific changes in temperature over the last 50 million years. These changes of temperature represent the rock cooling as it travels upward through the Earth’s crust.

Dr Adams added: “This approach gives us a new way to look back at Earth’s history. Current methods to estimate the speed at which mountains build look at chemical ‘clocks’ in the rock but these are limited to specific moments in time. Our method can estimate rock uplift over a much longer period.

“In this case, we estimate it would have taken millions of years to produce the subdued landscape we infer prior to the eruption.

“There is debate over whether the Andes grew slowly and steadily over 40 or 50 million years or whether they rose extremely slowly and then popped up more recently, in the last six to 10 million years. Our findings, which cover a large part of the middle of that history, support the slow but steady hypothesis.”

Co-author Dr Frances Cooper, also based at UCL Earth Sciences, said: “We’ve been working in this part of the Andes for many years to understand how the mountains developed. The Andes have a major influence on regional and global climate, so reconstructing their history is important for understanding long-term climate change.

“What’s particularly exciting about this study is that it gives us a new way of piecing together that history. The same approach could be applied to volcanic deposits elsewhere in the world, helping us reconstruct landscapes buried for millions of years.”

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Mott insulator

NiS₂ shares its crystal structure with pyrite, the mineral sometimes called fool’s gold. At ordinary conditions, its electrons do not move freely through the lattice: the strong electrical repulsion between them opens an energy gap that keeps the bulk of the material insulating. This behaviour defines what physicists call a Mott, or charge-transfer, insulator, a category of materials that simple band theory cannot explain. Band theory predicts that any material with a partially filled outer electron shell should conduct electricity, yet compounds like nickel oxide obstinately insulate instead. In 1949, the physicist Nevill Mott showed why: if two electrons crowding the same atomic site cost more energy than is available, the electrons stay locked in place and no current flows, regardless of what band theory predicts. NiS₂ behaves in much the same way.

Yet electrical-transport measurements had repeatedly found that crystals of NiS₂ conduct through their surfaces. Earlier scanning-tunnelling experiments helped explain part of the puzzle: the broad, flat surface remains insulating, but the electronic gap narrows sharply near atomic-scale steps, tiny cliffs just one or two atoms high where the surface abruptly changes level. A new study ¹ confirms this picture and examines the steps with much higher spatial resolution than before.

A not that smooth surface

A scanning tunnelling microscope (STM) works by bringing an extremely sharp needle close enough to a surface that electrons can leak across the gap between tip and sample. Measuring that leakage at different energies reveals both the surface’s shape and how easily electrons can enter or leave it. In NiS₂, these measurements uncovered electronic states concentrated tightly along the step edges: the states extend for a long distance parallel to a step but stay confined to roughly one to two nanometres across it. In other words, the electrons are effectively free to move in one direction while remaining trapped in the other two, forming genuine one-dimensional conduction channels.

Two distinct kinds of surface step were observed, depending on whether the outermost atomic layer exposes nickel or sulfur atoms. Both carry electronic states associated with their steps, but they are not equally important. The nickel-terminated steps host states that sit right at the energy where electrical conduction happens, making them the most likely explanation for the surface conductivity that transport experiments had measured for years. The sulfur-terminated steps also host states inside the insulating gap, but their energy is positioned further away, making them less directly responsible for carrying current.

The study also tested what happens when a magnetic field is applied perpendicular to the surface. The step-edge states survived fields as strong as 10 tesla, a field far stronger than any produced by a laboratory magnet used for everyday purposes. Their intensity weakened somewhat, particularly on the sulfur-terminated steps, but the states were never destroyed. This robustness matters because NiS₂ already displays complicated magnetic behaviour of its own, which might easily have disrupted a more fragile conducting state.

The deeper explanation lies in how electrons are distributed inside the crystal. In an ordinary insulator, the occupied electronic states can usually be pictured as clouds centred on the atoms themselves. NiS₂ is different: calculations show that part of its electron density is naturally centred at special positions between atoms, specifically near the midpoints of pairs of sulfur atoms, where no atom actually sits. Physicists call these obstructed charge centres, and materials built around them belong to a class known as obstructed atomic insulators, a concept that grew out of a broader effort over the past decade to classify materials by the geometry, and not just the energy, of their electron states.

Not ordinary metallic defects

This arrangement is not a chemical accident; it is protected by the symmetry of the crystal. When the crystal is cut to form a surface, though, the special positions that hosted this electron density can no longer fit together the way they did in the bulk. At a step edge, this mismatch creates what is known as a filling anomaly: an imbalance in how the available electronic states can be occupied, which leaves behind a state inside the insulating gap, localized right at the edge. Calculations of artificial step structures reproduce the main features seen under the microscope, including this localization, supporting the idea that the conducting channels are not ordinary metallic defects caused by broken chemical bonds, but a direct consequence of the crystal’s underlying electronic geometry.

The study also settles a long-standing question about NiS₂’s magnetism at low temperature. Below about 39 kelvin, the nickel magnetic moments arrange themselves into a non-collinear antiferromagnetic pattern, with neighbouring moments pointing in different directions that largely cancel out. Below about 30 kelvin, they rearrange again into a state that produces a small but measurable net magnetization. New neutron-diffraction measurements support a revised model of this low-temperature arrangement, resolving a debate about the material’s magnetic ground state that had lasted for roughly fifty years.

Taken together, the results give NiS₂ a significance well beyond its peculiar conducting surface. The material shows that strong repulsion between electrons, magnetism, and the geometry of electronic states can coexist within a single crystal. Such geometric protection is normally associated with topological materials, in which certain electronic states are shielded from disorder or defects by the crystal’s symmetry. The new results suggest that this protection can survive even when strong interactions push a material into being an insulator, making NiS₂ a rare setting for studying how the quantum-mechanical arrangement of electrons decides where electricity is allowed to flow within an otherwise insulating crystal.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.

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By 1800, water had already changed once. It was no longer one of nature’s simple terms, but a compound of hydrogen and oxygen (see Is Water H₂O? (1) From Element to Compound). The new chemistry had a powerful account of that change. What it still wanted was a more direct experiment: water itself, giving up its constituents.

Then came Volta’s pile.

It was not a spectacular object. Discs of metal stacked in pairs, pieces of moist material between them. Yet it altered the experimental life of electricity. Static machines produced sparks, shocks and theatrical moments. The pile offered something steadier. Electricity could now be made to act for long enough to enter chemistry.

Nicholson and Carlisle tried it on water almost at once. They placed wires from the pile in the liquid. Gas appeared, or rather the two sides of the reaction began to declare themselves. Hydrogen was released at one wire; oxygen, in the earliest arrangement, fixed itself to the other, nearly two inches away. Later, with gold or platinum electrodes, oxygen could be collected as gas. The awkward fact remained. The two supposed products of one decomposition appeared at opposite electrical ends of the apparatus.

At first glance, this looked like the experiment the new chemistry had been waiting for.

The trouble was that it looked a little too good.

A cleaner decomposition

The first article in this series followed water from element to compound. That change had not been a simple passage from error to fact. It involved a wider transformation of chemical practice: new gases, new names, balances, combustion experiments, recomposition, and a growing confidence in quantitative composition.

Electrolysis seemed to bypass much of that history. No flame, no combustion vessel, no burning of inflammable air with oxygen, no droplets collected on glass. The battery appeared to make water surrender its constituents directly. If water was composed of hydrogen and oxygen, here was synthesis in reverse: pass electrical action through the liquid and the two gases would come out.

Strictly speaking, electrical decomposition of water was not new. In 1789, Adriaan Paets van Troostwijk and Jan Rudolph Deiman had used repeated sparks of static electricity to obtain a mixture of hydrogen and oxygen from water, then recombined that mixture into water. Their experiment mattered. It was also awkward. The gases came together, in small quantities, and separating them for testing was difficult.

Nicholson and Carlisle’s version looked sharper. The Voltaic pile produced sustained action. The products could be made to appear separately. Hydrogen here, oxygen there. Each could be collected, examined and identified.

That was the improvement. It was also where the puzzle began.

When neatness becomes suspicious

Suppose water is decomposed. What, exactly, has been decomposed? A particle of water, perhaps. Yet if that particle is split into hydrogen and oxygen, why do the products not appear together? Why does hydrogen emerge at one wire and oxygen at the other? Why should the layout of the battery govern the place where each product appears, while the water between the wires shows no visible sign of being torn apart?

This was not an experiment going wrong. The gases appeared reliably. They appeared at the expected poles. That was precisely what made the situation uncomfortable.

Nicholson noticed the difficulty at once. Hydrogen appeared at one wire; oxygen was found at the other, nearly two inches away. The space between them did not look like a region where water was being visibly pulled into its parts. No trail of bubbles marked the route. The products simply appeared at opposite ends of the electrical arrangement, as if the battery had assigned them places.

Others saw the same embarrassment. Cuvier put it bluntly: if oxygen and hydrogen came from the same particle of water, why did they appear at two distant points, each always at its own wire? Haüy pressed the problem in molecular terms. If one molecule of water was decomposed, why were its products separated? If two molecules were decomposed, why did one give only hydrogen and the other only oxygen?

The question was almost childishly simple. That was why it would not go away.

Today we speak of ions, electrodes and charge transport. Those words make the old difficulty vanish before we have properly felt it. Around 1800, chemists had poles, attractions, affinities, particles, electrical fluids, acids, alkalis, metals, and a growing suspicion that electricity and chemistry belonged together. The experiment was clear enough. The mechanism was not.

Chang calls this the distance problem. The phrase is exact. The gases appeared with too much order for a simple story of local decomposition.

The distance problem

The obvious interpretation could still be defended. Perhaps electricity pulled the constituents of water apart. Oxygen went to one pole, hydrogen to the other. Many experiments already suggested that substances had different electrical tendencies, so the idea was hardly absurd.

It left the hardest part untouched.

What travelled through the water? Hydrogen? Oxygen? Something already separated? Something charged? If the constituents were detached somewhere between the electrodes, why did they not appear on the way? If they moved invisibly, why did they not recombine? And if decomposition happened only at the wires, how could it be the decomposition of the same portion of water?

Changes in apparatus did not remove the difficulty. Davy tried water held in separate gold cups, connected only by a moist bridge of fine asbestos, an arrangement he attributed to William Hyde Wollaston. The products still appeared at distant sites. The more carefully experimenters separated the places of action, the more pressing the question became: where, exactly, was water being decomposed?

That was the oddity. Electrolysis was not messy. If anything, it was too orderly. Hydrogen went one way, oxygen another. The battery gave a geography to the products of decomposition.

A flame did nothing like this. When inflammable air burned with oxygen, water appeared as the product of combination. There were difficulties, of course, but not difficulties of distance. Electrolysis changed the spatial form of chemical evidence. Water did not answer in one place. It answered at two.

Under electrical action, water no longer seemed merely something to be composed or decomposed. It seemed to contain routes.

Ritter changes the verb

Johann Wilhelm Ritter changed the verb.

For Lavoisierian chemists, electrolysis was analysis. Electricity decomposed water into hydrogen and oxygen. Ritter proposed the reverse. Electrolysis was synthesis.

At the negative pole, water combined with negative electricity and yielded hydrogen. At the positive pole, water combined with positive electricity and yielded oxygen. No product needed to travel from a decomposed particle somewhere in the middle of the liquid. The gases were made where they appeared.

It was a radical move. It was not foolish. Ritter took the most striking feature of the experiment seriously: the gases emerged at different poles. He did not treat that separation as an inconvenience to be patched up later. He built his interpretation around it. If hydrogen appears here and oxygen there, perhaps the two gases are being formed locally by two different electrical processes.

The price was high. Water became elementary again. Hydrogen and oxygen became compounds of water and electricity.

To modern chemistry, this looks perverse. Seen from the apparatus, it had one plain advantage: the gases were produced where they were found. Electricity was still widely treated as something with material agency. If a pile drove something into a liquid, why could that something not combine chemically with the liquid? Why must electricity be only the knife, and never part of what is made?

The phlogistonist echo was clear enough. If inflammable air could be read as water combined with phlogiston, hydrogen could be read as water combined with negative electricity. Identify phlogiston with negative electricity, and Ritter’s scheme comes surprisingly close to older anti-Lavoisierian accounts of water.

Priestley, from America, also saw that the new experiment did not force the Lavoisierian reading. He followed the debate through Nicholson’s journal and argued that electrolysis need not be understood as the decomposition of compound water. His intervention matters because it shows how differently the same electrical process could be read. For Lavoisierians, the gases were constituents released from water. For Priestley, they could still be products of another chemical-electrical process.

Ritter’s view did not last. Chemistry was becoming more firmly Lavoisierian, and elementary water had little room left in the emerging discipline. Still, electrolysis itself had not supplied a neat refutation. Ritter had solved the distance problem rather well. His view lost ground as chemists built an electrochemical practice around compound water.

Keeping water a compound

The Lavoisierian problem was delicate. Chemists had to hold three claims together. Water was composed of hydrogen and oxygen. Electricity decomposed it. Yet the products appeared at different electrodes, with no visible decomposition in between.

So the theory needed help.

One option was invisible transfer: one constituent moved through the liquid to its proper pole. Another was a chain of decompositions and recompositions, in which neighbouring particles handed hydrogen and oxygen along until the products emerged at the ends. In related versions, the whole liquid was imagined as electrically ordered, so that the final gases appeared only where that order met the metal.

These were rescue hypotheses. The phrase can sound dismissive, although it need not be. They were attempts to make a powerful practice intelligible. Electrolysis worked. Its effects could be repeated, varied and extended. What was missing was a mechanism that made the spatial separation of the gases sit comfortably with compound water.

The chain idea had a certain elegance. No single particle of hydrogen or oxygen needed to travel the whole distance. A constituent could be passed along through a succession of exchanges, like a message moving through a crowd. The gas would appear only at the end of the chain.

But what exactly was being passed? How were the exchanges coordinated? Why did the process end at just the right pole? The explanation helped, though it left plenty in the dark.

The transfer idea had its own attraction. If chemical particles had electrical characters, perhaps they could be drawn through the liquid. Again, however, the path remained hidden. The water between the wires did not display the process one wanted to see. Without a developed theory of ions and charge transport, the mechanism remained speculative.

None of this made the hypotheses useless. They kept research going. They suggested new apparatuses, new distances, new electrodes, new liquids and new comparisons. An experiment need not be fully understood before it becomes useful.

That is more or less what happened.

When water stopped being negotiable

By the early nineteenth century, compound water was becoming part of the working furniture of chemistry. Not every detail had been explained. The distance problem remained. Yet fewer and fewer chemists were prepared to reopen the older question of whether water might be elementary.

Water as a compound was no longer simply a hypothesis under trial. Increasingly, it was a condition for further work.

That shift mattered. Once water was taken as compound, electrochemistry could ask other questions. What moved towards each pole? What kinds of substances were decomposed by the pile? How did acids, alkalis, salts and metals behave under electrical action? What changed when the electrode, distance or solution changed? How much chemical change corresponded to how much electrical action?

The material practice hardened. Trough batteries became easier to handle. Gold and platinum electrodes helped prevent the products from disappearing into reactions with the metal. Separate cups connected by wet asbestos became a standard arrangement. The theory remained unsettled, but the apparatus was becoming disciplined.

The unresolved puzzle did not stop electrochemistry. Experimenters varied the arrangement. They separated vessels, introduced bridges, changed electrodes, added acids and salts, and studied what appeared where. The field learnt to handle the phenomena before it possessed a settled account of what moved inside the liquid.

Later developments brought more discipline. Faraday gave electrolysis a quantitative discipline in the 1830s. By the end of the nineteenth century, Arrhenius’s theory of ionic dissociation would help make sense of processes that had long remained obscure. That later clarity, though, should not be read backwards into 1800. For decades, electrochemistry advanced with a problem at its centre.

What electricity did to water

Electrolysis changed what water could be made to do in the laboratory. It did not simply reveal a truth waiting inside the liquid. It altered the route by which that truth could be pursued.

The first article in this series followed water into the balance sheet of chemistry. Water became a substance that could be produced, analysed, recomposed, weighed and named through its constituents. Electrolysis added another sort of depth. Water could now be interrogated electrically. Its constitution became a problem of poles, currents, attractions, transport and hidden motion.

Lavoisierian chemistry had given water constituents. Electrochemistry gave it pathways.

The familiar classroom demonstration hides that difficulty. The bubbles are easy to see. Their meaning was not. Hydrogen here, oxygen there: that simple separation forced chemists to imagine invisible processes inside a liquid that otherwise looked continuous and unchanged.

Electrolysis made compound water harder to abandon, without making it transparent. It turned water into an electrochemical object: something that could conduct, polarise, transport, release and exchange. The question was no longer only whether hydrogen and oxygen belonged to water. It was how electrical action could make them appear at all.

Still, electrolysis could not settle the formula. It could make water yield hydrogen and oxygen. It could not tell chemists how those elements should be counted.

That was the next difficulty. Once water had constituents, chemistry still had to learn how to count them.

References

Chang, H. (2012). Is Water H2O? Evidence, Realism and Pluralism. Dordrecht: Springer. doi: 10.1007/978-94-007-3932-1.

Davy, H. (1807). ‘The Bakerian Lecture, on Some Chemical Agencies of Electricity’. Philosophical Transactions of the Royal Society of London, 97, pp. 1–56. doi: 10.1098/rstl.1807.0001.

Faraday, M. (1834). ‘Experimental Researches in Electricity.—Seventh Series’. Philosophical Transactions of the Royal Society of London, 124, pp. 77–122. doi: 10.1098/rstl.1834.0008.

Nicholson, W. (1800). ‘Account of the New Electrical or Galvanic Apparatus of Sig. Alex. Volta, and Experiments Performed with the Same’. Journal of Natural Philosophy, Chemistry, and the Arts, 4, pp. 179–187.

Singer, G. J. (1814). Elements of Electricity and Electro-Chemistry. London: Printed for Longman, Hurst, Rees, Orme, A. Brown, and R. Triphook.

Volta, A. (1800). ‘On the Electricity Excited by the Mere Contact of Conducting Substances of Different Kinds’. Philosophical Transactions of the Royal Society of London, 90, pp. 403–431. doi: 10.1098/rstl.1800.0018.

Wilkinson, C. H. (1804). Elements of Galvanism, in Theory and Practice: With a Comprehensive View of Its History, from the First Experiments of Galvani to the Present Time. 2 vols. London: John Murray.

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Tomoyuki Ikai, lead author and professor at Nagoya University’s Graduate School of Engineering, is a polymer chemist inspired by natural helical structures such as DNA and proteins. Polymers are large molecules built from many small, repeating units linked together in a chain. Ikai had been working on ladder polymers, where each repeating unit is locked into place and cannot freely rotate around the bond connecting to its neighbor (unlike a typical flexible linear polymer).

In 2019, Ikai and his collaborators built the first rigid, helix-shaped polymer. In 2021, they perfected a chemical method to fuse these building blocks together without flaws. This method is what let them build the new graphene nanoribbon.

“We used the same fusion approach already used to make predictable helical ladder polymers. But instead of a stable building block, such as [6]helicene, we used [4]helicene, a 4-ring unit written off as too unstable to be useful,” Ikai said.

Soak, twist, repeat

These units are difficult to control on their own because they switch their chirality too fast to hold one handedness. The team’s method fuses these small, restless units into one long chain. Once joined, they found that a neighbor effect took hold: each unit’s twist began to match its neighbors, and long stretches of the ribbon settled into a single, shared spiral.

To lock it in one overall direction, the researchers dissolved the ribbon in a natural chiral liquid, a liquid made of molecules that come in two mirror-image versions. They tested six candidates before they found the right one: beta-pinene, a compound found in pine trees and citrus peel.

Because beta-pinene molecules can be arranged in left- and right-hand mirror-images, these two versions each push the ribbon toward the opposite spiral. This also changed the twist direction of light the ribbon gave off, a property called circularly polarized luminescence, and is the first graphene nanoribbon shown to switch its light this way.

The switching induced a remarkably high degree of helical bias, with almost all of the helicene units aligning in the same direction. The switching effect is strongest at –90°C and grows weaker as the temperature rises toward room temperature.

“For a long time, people saw [4]helicene as unsuitable for chiral materials, because it could not be controlled,” Ikai said. “In this paper, we show that once you link enough of these units together, the instability itself becomes the useful part.”

A chiral solvent’s nudge toward one handedness is usually very weak on its own. Here, the ribbon’s own structure, with each unit mechanically linked to its neighbors, amplifies that weak nudge into a strong, nearly one-handed result.

An adaptive material

The researchers point to several possible future uses: The switchable light could serve optical devices. The ribbon’s chemical sensitivity to beta-pinene suggests it may work as a chiral sensor, and a similar design could one day help detect hazardous or unstable chemicals. Because graphene conducts electricity well, the ribbon may also suit electronic sensors.

The molecule’s twisted shape connects to spintronics, a field that uses the spin of electrons rather than just their charge. Ikai noted that a right-handed or left-handed structure can, in principle, let electrons of one spin pass through more easily than the other, a property helical structures are known for in physics literature.

The current design only holds its twist while it stays in the chiral liquid. If the solvent is removed, the ribbon drifts back to a mixed, unstable state. Ikai’s next goal is a form of “chiral memory,” a way for the ribbon to lock in its twist even after the solvent is gone. This next step builds on memory techniques the researchers have explored in other helical systems and is now applied to graphene for the first time.

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Arsenic contamination in drinking water poses a critical global public health hazard. Prolonged exposure to inorganic arsenic—predominantly present in groundwater as arsenate, where arsenic exists in its +5 oxidation state—can lead to severe dermatological lesions, cardiovascular diseases, and cancers. To mitigate these health risks, the World Health Organization has established a strict provisional safety guideline of 10 micrograms of total arsenic per liter of drinking water. Because arsenic occurs naturally through the weathering of rocks and minerals in aquifers worldwide, engineering low-cost, scalable, and highly efficient materials for water purification remains a central goal in modern materials chemistry.

Iron-based compounds, particularly iron oxyhydroxides, are among the most effective materials for trapping arsenate. At a fundamental chemical level, arsenate ions readily substitute for surface hydroxyl groups on iron oxyhydroxides, forming strong chemical bonds that permanently anchor the contaminant to the solid surface. However, translating this surface chemistry into practical filters presents a persistent engineering hurdle: maximizing the amount of active iron exposed to water. Simply increasing the iron concentration during material synthesis causes iron nanoparticles to aggregate into large, dense clusters. As these clusters grow, much of the iron becomes buried within the interior bulk, leaving only a tiny fraction accessible at the surface to bind arsenate.

A promising design strategy ¹ overcomes this limitation by decoupling the nucleation of iron particles from their subsequent growth on a flexible, high-surface-area polymer matrix. The foundation of this system is a composite fibrous membrane fabricated from poly(vinyl alcohol) and pectin via electrospinning. In electrospinning, a high-voltage electric field draws a liquid polymer mixture into ultra-fine threads, which solidify into an interconnected, highly porous mesh of nanofibers. Poly(vinyl alcohol) provides structural integrity and water stability upon heat treatment, while pectin—a natural polysaccharide rich in carboxyl and hydroxyl functional groups—serves as a molecular anchor capable of capturing iron ions from solution.

To grow high-density nanostructures without triggering particle agglomeration, a sequential, two-step chemical treatment is applied. First, the electrospun membrane is immersed in a solution containing iron(II) ions (Fe²⁺). The carboxylate groups on the pectin molecules coordinate with these cations, forming uniformly dispersed, nanometer-scale iron seed clusters (~2 nanometers in size) across the nanofiber surfaces as the material dries and oxidizes. In the second step, the seeded membrane is placed in a solution containing iron(III) ions (Fe³⁺). Instead of forming isolated new particles in solution, the newly added iron deposits onto the pre-existing seeds, driving controlled anisotropic growth along preferred crystallographic axes.

This seed-mediated approach yields an extraordinarily high density of surface-exposed iron nanostructures, achieving a 12-fold increase in surface atomic iron content compared to single-step doping methods. High-resolution electron microscopy reveals that these nanostructures adopt an elongated, rice-grain morphology measuring approximately 29 nanometers in width and 110 nanometers in length. Structural characterization using X-ray scattering, X-ray diffraction, and Mössbauer spectroscopy identifies the predominant active iron phase as akaganeite (β-FeOOH), a tunnel-structured, poorly crystalline iron oxyhydroxide known for its exceptional binding affinity toward arsenate.

When evaluated in water purification tests, the nanostructured membrane demonstrates exceptional superadsorbent performance. Using a minimal material dosage of 1 gram per liter, the membrane reduces initial arsenate concentrations from 1000 micrograms per liter down to below 10 micrograms per liter within 24 hours at pH 6, achieving roughly 99% arsenic removal and satisfying WHO safety limits. The material retains high efficiency across a broad operational pH range of 5 to 8, and ICP analysis confirms that iron leaching into the treated water remains safely below 5 micrograms per liter.

Kinetic and isotherm analyses reveal that arsenate removal occurs via surface chemisorption governed by cooperative binding behavior. As initial arsenate molecules bind to active sites on the akaganeite surface, local chemical and electrostatic interactions make adjacent sites even more receptive to further adsorption, resulting in accelerated uptake kinetics. Furthermore, the material performs reliably in complex water environments. Naturally occurring co-contaminants such as fluoride do not interfere with arsenate capture, and the presence of heavy metal cations—such as copper and cadmium—actually enhances arsenate removal by forming secondary surface complexes that create additional binding sites. Although chemical regeneration using sodium hydroxide solution results in a slight decrease in capacity over repeated cycles due to incomplete desorption, the membrane maintains an 81% to 87% removal efficiency, demonstrating strong potential for practical, reusable water remediation systems.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.

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Take a walk in London’s Trafalgar Square this summer and wilting under the heat will be all sorts of body sizes, some tall, some short: a real mix of sizes.

Picking just two from the crowd to represent the whole human species would be a tall order. But that is exactly what we do when we find fossils of ancient hominins (human relatives).

Anthropologists can estimate a person’s height with reasonable accuracy from the long bones – your femur (thigh bone) works well. On average, the size of this bone
scales well with overall height, given a large samples of bones.

This is exactly what a group of scientists have done using fossils dredged from the seafloor off Taiwan. To add to the mystery, these bones are those of Denisovans, currently one of the most enigmatic species of hominins.

The Denisovans were a widespread species of Homo (the genus which encompasses our own species, as well as close relatives such as Neanderthals). They were initially known only from genetic sequences that had been extracted from isolated fragments of material, such as a small finger bone, at Denisova Cave in the Altai Mountains of Siberia.

The apparent absence of substantial fossil material from this species made them mysterious, although in recent years, fresh discoveries have added to our understanding. These include the Dragon Man (or Harbin) skull, recovered from a river in northeast China. DNA and protein analysis has now linked this large skull to the Denisovans.

The remarkable feature of the fossils analysed by Yousuke Kaifu, from the University of Tokyo, and colleagues, is that they were dredged from the Penghu Channel in the Taiwan Strait.

Most rare hominin fossils are carefully excavated, allowing researchers to record their exact position within layers of sediment and their relationship to associated animals, artefacts and environmental evidence.

These bones were instead recovered from the seabed among the fossilised remains of terrestrial ice age mammals, probably deposited when sea levels exposed the channel as dry land around the time when the ice age was at its height. Their precise discovery locations and geological contexts were not recorded.

This makes the fossils scientifically unusual: their anatomy and preserved proteins contain valuable evidence. However, much of the contextual information normally supplied by an archaeological excavation has been lost.

Recovered long bones

The bones – an incomplete femur (thigh bone) and tibia (the largest bone in the shin) – have been assigned to two Denisovans on the basis of analyses of their recovered ancient proteomes. A proteome is the collection of proteins preserved within a fossil. These proteins can retain evidence of evolutionary relationships, even when DNA has been lost.

These bones produced slightly different proteome signals, which is why the authors suggest that these represent two individuals. The newly described femur and tibia are likewise not directly dated.

Because they were recovered by dredging and lack secure geological provenance, they can only be placed broadly within the Middle to Late Pleistocene, using the animal remains and preservation evidence. The Pleistocene epoch lasted from around 2.58 million years ago until 11,700 years ago.

Aside from assigning the bones to Denisovans, the researchers have used them to infer the stature of the two individuals. They are incomplete, which complicates matters somewhat, but skilled anatomists can be reasonably confident on the complete bone sizes.

The lack of context for the fossils – normally provided by the sediments in which they are recovered from – means we don’t know if the two individuals lived at the same time or at least a similar interval.

But the bones are long and when the other body elements are scaled based on the leg bones, it suggests that these individuals were tall. In the new paper, Yousuke Kaifu and colleagues say that the estimated height of Penghu 2 (one of the newly described specimens) was approximately 1.8 metres (5’11”), with a weight of around 83kg.

Their estimates for the other fossil, Penghu 3, give a height of about 1.9 metres (6’3″) and a weight of about 91kg. These estimates, the team writes in their study, “are comparable to the largest known Pleistocene Homo from Africa and Europe”.

Inferring tallness as a species-level trait based on two individuals is challenging, although it has not prevented some headlines from claiming that Denisovans were tall. Think back to our walk in Trafalgar Square.

Proceed with caution

The dangers of reconstructing stature are illustrated by the famous Turkana Boy or Nariokotome Boy, a remarkably complete Homo erectus skeleton discovered in Kenya in 1984. He was unusual because his skull, teeth and long bones were all complete and available for study.

He was already around 1.6 metres tall when he died, but estimates of his age have differed. Dentally, he appears younger than his skeleton suggests. His skeletal development appeared comparable to that of a modern human aged around 11-13, while microscopic growth increments in his teeth suggested that he was only about eight or nine.

The discrepancy reflects uncertainty over whether Homo erectus grew slowly like us, including an adolescent growth spurt, or matured more rapidly.

Different assumptions consequently produced different projections of his adult height, although more recent modelling suggests he would probably have reached approximately 1.76–1.80 metres.

He is the only example of such a complete Homo erectus skeleton, but does this mean that all members of his species would have been as tall? We should be sceptical – just as we should be about the height of Denisovans.

This article is republished from The Conversation under a Creative Commons license. Original article.

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Light and sound behave differently. Light from a flashlight or laser travels in a straight line, whereas sound readily spreads in all directions—that is why voices can be heard over a wall. The question, “Could sound, like light, be sent only where we want it?” led to the team’s highly directional speaker, MiPAL. By modulating ultrasound with audible content, the system can deliver sound only within a targeted area. A representative technology is the parametric array loudspeaker (PAL), which uses the nonlinear interaction of two ultrasonic waves in air to generate and concentrate audible sound in a specific direction.

Compared with conventional speakers, PALs can confine sound to a much narrower region. Traditional systems, however, require tens to hundreds of ultrasonic emitters to be precisely arranged and individually controlled, making them expensive and structurally complex. More recent PAL designs have simplified the hardware, but the vibrating diaphragm can also excite unwanted vibration modes, allowing sound to leak sideways and undermining directivity.

The team addressed this problem by integrating two types of metamaterials into a single ultrasonic transducer. An acoustic metasurface was mounted at the front. Acting like a lens for sound, it reshapes the irregular ultrasonic wavefronts radiated by the diaphragm into a straight, narrow beam, much like a laser. Elastic meta-units were placed at the rear. Acting more like a sound barrier, they suppress unwanted vibrations at selected frequencies and keep sound from leaking to the sides. In effect, the design uses a double safeguard: it focuses the sound at the front and blocks parasitic noise at the back.

As a result, the single-element device directed sound over a frequency range of 500 Hz to 10 kHz—more than four octaves, covering a large portion of the audible spectrum. The researchers also conducted an experiment simulating aircraft seating, where different announcements or music might need to reach individual passengers without disturbing their neighbors. After playing real broadcast and music signals and measuring sound intensity at different angles, they observed a clear improvement after the elastic meta-units were added.

Another key advantage is ease of fabrication and adaptation. The metamaterial structures are modular and can be produced using 3D printing, allowing the device to be readily redesigned for different seat spacing, angles and other operating conditions.

Once commercialized, the technology could let passengers on planes or trains listen to personalized announcements or content without disturbing the person next to them. In museums and exhibition halls, explanations could be heard only by visitors standing in front of a particular display. The “sound only I can hear” once seen only in films may soon become part of everyday life.

“By co-designing the metamaterials and the ultrasonic device as a single system, we overcame the limitations of cost, complexity and design flexibility at the same time,” said Professor Junsuk Rho. “Research on acoustic and elastic metamaterials has largely remained at the stage of validating principles and demonstrating concepts. This study is the first to integrate metamaterials into functional device and demonstrate their potential for practical use.”

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Take a glass of water. Nothing about it seems to ask for a theory. It has no colour worth naming, no dramatic smell, no visible architecture. It runs, freezes, boils, evaporates, dissolves, erodes, circulates and nourishes. It belongs to weather, geology, physiology, cooking and plumbing before it belongs to chemistry.

Then science gives us the famous abbreviation: H₂O.

There is a curious finality in those three symbols. They seem to do what ordinary experience cannot. Rain, steam, ice, rivers, blood and laboratory wash bottles are all brought under a clean chemical statement. Two atoms of hydrogen, one atom of oxygen. For many people, H₂O is more than a formula. It is almost an emblem of scientific knowledge, the moment when science gets behind appearances and tells us what something really is.

Yet the formula stands at the end of a long story. Before water could be written as H₂O, it had to acquire a chemical interior. Chemists first had to learn to treat the most familiar liquid in the world as something that could be taken apart, recomposed and assigned constituents. The first transformation was not from one formula to another. It was from element to compound.

This is the starting point of Hasok Chang’s Is Water H₂O?, a book that does for water something close to what Inventing Temperature did for thermometry. Temperature now looks like a number waiting to be read from a scale. Chang showed that the scale had to earn that appearance of simplicity. Fixed points had to be stabilised, instruments compared, extrapolations justified. Measurement became routine only after it had first been constructed as a reliable practice.

Water presented another kind of difficulty. The problem was not how to make a phenomenon answer numerically, but how to make a familiar substance answer analytically. Chemists had to learn to look at water as more than something that flowed, froze, boiled, dissolved, condensed or could be purified. They had to ask whether it had an inside: whether it could be taken apart, made again, and understood through its constituents.

To us, that question feels almost unavoidable. In the eighteenth century, it was anything but.

When water had no parts

For much of European natural philosophy, water was an element. That claim can sound childish to us, though only because we hear it through the categories that later displaced it.

The idea was older than Aristotle. Thales had imagined water as the origin of things; Empedocles later placed it among the four basic “roots” of the world, alongside earth, air and fire. Aristotle gave that inherited scheme a more systematic form. But these were not elements in the modern chemical sense. No one was classifying them by atomic number, nuclear charge or molecular structure. They belonged to a physics of qualities and transformations, of hot and cold, wet and dry. Water was simple because, in that world, it occupied a basic place in the order of natural change.

By the eighteenth century, that older grammar had been heavily rewritten. Chemistry had become crowded with acids, alkalis, precipitates, metallic calxes, distillations and newly isolated “airs”. Water was everywhere in this world. It served as solvent, medium, reagent, product and experimental environment. It could be purified, frozen, boiled, condensed and recovered.

Still, none of that made it automatically compound. A substance can be experimentally familiar and still have no recognised chemical interior.

The difficulty is easy to miss because we already speak the language that solved it. We say that hydrogen burns in oxygen to form water. But eighteenth-century chemists did not begin with hydrogen and oxygen. They had inflammable air, dephlogisticated air, fixed air, nitrous air and other gaseous bodies whose identities were still unsettled. The experiment did not come with modern subtitles. The vocabulary in which its meaning now appears self-evident was itself one of the products of the revolution.

Water became a compound when certain operations began to count as analysis and synthesis: taking water apart, producing it again, and treating the substances involved as its constituents. This was more than a taxonomic adjustment. It changed the standard of chemical knowledge. To know a substance now meant, increasingly, to know what it was made of.

The wrong theory that worked

The defeated rival was phlogiston theory. Posterity has enjoyed defeating it again and again.

In the usual caricature, phlogiston was a ghostly fire-stuff. Combustible bodies contained it and released it when they burned. Metals contained it too. When heated in air, they lost phlogiston and became calxes. Heat the calx with charcoal, and the metal returned because phlogiston had been restored. Then Lavoisier introduced oxygen, and the ghost vanished.

What makes this story so satisfying? Precisely what makes it misleading. It tells the Chemical Revolution from the winner’s seat, where oxygen looks inevitable and phlogiston appears only to make its exit.

From inside the eighteenth-century laboratory, the story looks less like a comedy of error. Phlogiston was not a fantasy floating above the experiments. It belonged to things chemists and metallurgists handled every day. Metals dulled and lost their malleability when calcined. Calxes could be brought back to metal with charcoal. Ores became usable metals in furnaces. Fire, air, metals, respiration and reduction seemed to be tied together by a common traffic of activity. Phlogiston named that traffic. It was a mistaken name for a real pattern.

No one shows this better than Joseph Priestley. He was not a dim survivor of pre-modern chemistry, nor an experimental genius trapped in mere confusion, but one of the great investigators of the eighteenth century, a dissenting minister, political radical and virtuoso of pneumatic chemistry. In Leeds, living beside a brewery, he studied the “fixed air” rising from fermentation, now carbon dioxide. Later, by heating the red calx of mercury, he obtained a far more remarkable gas. Flames burned in it with unusual brilliance; animals could breathe it for longer than ordinary air; Priestley even tried it himself, noticing what he described as a peculiar lightness in his chest.

We call that gas oxygen. Priestley did not. To him, it was dephlogisticated air.

The name sounds absurd only when detached from the operations that made it meaningful. In phlogiston chemistry, ordinary air could become loaded with phlogiston. Air deprived of phlogiston would therefore be especially ready to absorb it again. It should support combustion powerfully and sustain respiration unusually well. That is exactly what Priestley saw. His interpretation was wrong, but it was not arbitrary. It made sense of real experimental behaviour.

Nor was the winning term free of theory. Lavoisier called the gas oxygen, meaning “acid-generator”, because he believed oxygen was the universal principle of acidity. That too was false. The defeated word preserved one mistake; the victorious word preserved another. One became a fossil of discarded chemistry. The other became familiar enough to hide its own history.

A few drops on the glass

The next turn in the story came from a gas with a wonderfully plain eighteenth-century name: inflammable air. We now call it hydrogen.

Henry Cavendish had studied this light, combustible gas in the 1760s. It could be produced by reacting metals with acids, and it burned readily. When inflammable air was burned together with dephlogisticated air, something extraordinary happened. Water appeared. Two invisible airs had produced the most familiar liquid in the world.

It is hard, now, not to let modern chemistry finish the sentence for us. Hydrogen combines with oxygen to form water. Of course. But the droplets on the glass did not settle the matter by themselves. They had to be interpreted.

A phlogistonist could tell the story differently. Perhaps inflammable air was water combined with phlogiston. Perhaps dephlogisticated air was water deprived of it, or at least air peculiarly ready to receive it. On that reading, the experiment did not reveal that water was composed of two gases. It showed that ordinary water could be restored through the right exchange of phlogiston.

That may sound evasive from our side of the Chemical Revolution. At the time, it was not an absurd reply. A flame, a gas, a misted vessel, a change in weight. None of these comes with its meaning attached. Evidence becomes evidence inside a working system of concepts, operations and standards. What counts as a substance? What counts as a component? What counts as analysis rather than transformation? Which variable should be trusted most, appearance, activity, weight, or reproducibility?

The new chemistry changed the setting in which the experiment was read. The appearance of water was no longer merely a striking event at the end of combustion. It became part of a quantitative account. How much inflammable air had disappeared? How much oxygen had been involved? How much water had been produced? Could water be decomposed and then recomposed, with the numbers matching closely enough to make the same substance appear on both sides of the operation?

At that point, water was no longer simply appearing on the glass. It was entering the balance sheet of chemistry.

The balance makes composition visible

A metal is heated in air. It dulls, loses its shine, gives up its malleability and turns into a calx. In phlogiston chemistry, this looked like a loss. The metal had given up the principle that made it metallic.

Then came the awkward fact on the bench. The calx was heavier.

Heavier, after a supposed loss.

That did not destroy phlogiston theory in a single blow. Good chemists could still defend it, and did. Yet the weight gain gave oxygen chemistry an opening. Perhaps calcination was better understood as combination with something from the air. Follow the weights carefully and chemical change begins to look different. Appearances still matter. Activity still matters. So do colour, heat, flame, residue and reversibility. But weight gives the whole operation a new discipline. It turns transformation into material accounting.

The same habit of attention changed the status of water. A substance that can be broken down into measured quantities of other substances, and then produced again from them, starts to lose its old simplicity. It has become analysable. It has constituents.

This was the strongest hand of the new chemistry. Combustion, calcination, gases and water could now be drawn into the same quantitative order. Chemical names changed too. A name was expected to do more than preserve origin, appearance, use or inherited convention. Ideally, it should tell chemists what a substance contained.

The point needs care. This was never a struggle between careful measurers and careless speculators. Cavendish was one of the most exact experimentalists of the century. Richard Kirwan, among the serious defenders of phlogiston, treated weight as a genuine difficulty. The disagreement concerned the authority of weight. Should it count as one experimental constraint among others, or should it become the privileged route to chemical identity?

That question slowly shifted the terrain. Phlogiston chemistry was most at home in a language of principles, powers and transformations. What gives a body combustibility? Why does a metal lose its metallic character? How can a calx be revived? What passes from one substance to another when reaction occurs?

The new chemistry pressed elsewhere. What does this substance contain? In what proportions? Can those constituents be separated and brought together again while the material account remains intact?

Once that style of reasoning became dominant, water’s old simplicity became harder to defend. Water could still be boiled, condensed, purified and recovered, just as before. Now it could also be weighed into a different story. It could be decomposed, recomposed, renamed and treated as a body with an interior.

No single knockout blow

The usual story of the Chemical Revolution likes a clean scene. Priestley isolates oxygen and fails to recognise it. Lavoisier understands what Priestley could not. Phlogiston collapses. Modern chemistry begins.

History was less obliging.

Priestley knew the new chemistry and resisted it. Cavendish remained cautious. Other competent chemists hesitated, adopted parts of the oxygen system, or kept some version of phlogiston alive. Their reluctance was not simple blindness. Phlogiston theory still connected combustion, respiration, calcination, reduction and the behaviour of airs. It had also helped produce many of the experimental facts that later counted in favour of oxygen chemistry.

Lavoisier’s system, for all its power, carried its own burdens. His theory of acidity was wrong. The very word oxygen preserved that mistake, since it named the gas as the supposed generator of acids. His chemistry also relied on caloric, an imponderable fluid of heat. Seen from the present, caloric does not look obviously more respectable than phlogiston. Lavoisierian chemistry was elegant, disciplined and transformative. It was not modern chemistry arriving whole and immaculate.

So the real historical question is more interesting than the textbook one. Why did one way of doing chemistry become more convincing, more portable, more teachable and more productive than another?

Much of the answer lies in the rise of compositionism. Oxygen chemistry made substances legible as combinations of measurable constituents. Reactions became material accounts. Chemical names began to function as compressed statements of composition. Laboratory operations, weights, gases and nomenclature could now be made to speak the same language.

Phlogiston still made sense of many transformations, but it sat uneasily in that emerging world. It named a principle of activity, a way of thinking about combustibility, metallic character, reduction and chemical power. The new chemistry increasingly wanted components. It asked what substances were made of, how much of each constituent they contained, and whether those constituents could be separated and brought together again.

As compositionism gained ground, phlogiston began to lose its natural habitat. It could still speak about combustion, reduction and chemical activity, but chemistry was moving towards another language: constituents, proportions, balances, material accounts. The revolution changed the answer, certainly. More importantly, it changed the kind of answer chemistry wanted.

The first metamorphosis of water

When water became a compound, it did more than shift from one scientific category to another. Its identity for chemistry changed.

Before this transition, water could pass through laboratory practice while keeping its apparent simplicity. It could be boiled, frozen, distilled, condensed, purified and recovered. It appeared everywhere, as liquid, vapour, solvent, medium, residue or product. Familiarity gave it stability. It did not yet give it an interior.

After the Chemical Revolution, the same substance began to occupy a different role. Water could be produced from other bodies, analysed through them, and assigned constituents. The liquid that returned from vapour or appeared after combustion now carried a hidden depth. It was no longer merely recovered; it could be accounted for.

Nobody saw that hidden constitution. There were no water molecules under a microscope, no direct image of hydrogen and oxygen joined together. The evidence came through laboratory work: gases prepared and collected, vessels sealed, mixtures burned, droplets condensed, weights compared, names revised. Compound water became credible through practice before it became imaginable as a molecular structure.

This is why the story should not run too quickly towards H₂O. Lavoisierian chemistry helped establish water as a compound of hydrogen and oxygen. It did not settle the formula. It was one thing to say that water was made from hydrogen and oxygen; it was another to decide how many atoms of each entered the compound. That question would take nineteenth-century chemistry into a different set of difficulties.

First came the more basic metamorphosis. Water ceased to be one of nature’s simple terms. It became a substance that could be decomposed, recomposed and written in the language of other substances.

The cost of clarity

The new chemistry clarified immensely. It disciplined measurement, stabilised nomenclature and helped make possible the modern science of composition. But clarity selects. It illuminates some things by pushing others into shadow.

Phlogiston theory had been concerned with chemical activity. Why do substances burn, reduce, revive, calcine, combine and transform? Early compositional chemistry was much better at saying what entered and left a reaction than at explaining why the reaction happened at all. It provided an excellent account book, though not yet a full theory of chemical agency.

Some questions once handled by phlogiston later returned in different forms. Chemical potential energy, oxidation-reduction and electrons all came to occupy parts of the conceptual space that phlogiston had once held. That does not vindicate phlogiston as a literal substance. It suggests something subtler and more useful. A false theory can preserve real questions. A victorious theory can be right and still leave work unfinished.

This is a more realistic picture of scientific progress. Oxygen chemistry won for good reasons. But those reasons were not contained in one devastating experiment before which only fools could hesitate. They belonged to a wider reorganisation of chemical practice.

Water’s first great chemical transformation was therefore not the discovery of a formula. It was the making of a new kind of object. The familiar liquid became something that could be analysed, recomposed, weighed and named through its constituents. Only after that could the later question become pressing: not whether water was compound, but what exactly its composition was.

Electricity seemed about to settle the matter. It made water yield hydrogen and oxygen directly, as if nature had finally supplied the decisive experiment.

Instead, the proof produced a new puzzle.

References

Chang, H. (2012). Is Water H₂O? Evidence, Realism and Pluralism. Springer. doi 10.1007/978-94-007-3932-1.

Chang, H. (2004). Inventing Temperature. Measurement and Scientific Progress. Oxford University Press.

Cavendish, H. (1784). ‘Experiments on Air’. Philosophical Transactions of the Royal Society of London, 74, 119–153.

Kirwan, R. (1789). An Essay on Phlogiston and the Constitution of Acids. London, J. Johnson.

Lavoisier, A.-L. ([1789] 1965). Elements of Chemistry. Dover.

Priestley, J. ([1796] 1969). Considerations on the Doctrine of Phlogiston, and the Decomposition of Water. Kraus Reprint.

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How do you explore a place where the Sun never shines? China’s space programme will soon embark on one of the most innovative lunar exploration missions in recent years.

We’ve known for nearly two decades that some form of water exists on the Moon. What we still don’t know is exactly how much accessible water ice lies inside permanently shadowed craters, how deeply it is buried or what form it takes.

What we do know is that accessing that water ice will be a real challenge: it is found at the bottom of deep, hard-to-traverse craters where the Sun never shines, remaining untouched for billions of years at temperatures well below –200°C. These are among the most valuable places on the Moon, but the hardest to reach and explore.

Lunar exploration has come a long way since the Soviet Union’s Luna-1 probe reached the Moon’s vicinity in January 1959. The Apollo missions and their successors showed the challenge is no longer simply to reach the Moon. It is to explore places human and conventional rovers cannot easily go, and ultimately determine whether the Moon itself can provide some of the resources that future explorers will need.

China has spent two decades building an increasingly ambitious lunar exploration programme, named after Chang’e, the ancient Chinese goddess of the Moon. The Chang’e-7 mission is expected to set course for our celestial neighbour later this month, although a recent failure involving a different Chinese rocket has introduced some uncertainty over the launch date. Here’s what makes this latest mission so interesting.

What is Chang’e-7 set to achieve?

The mission will test several novel approaches to lunar exploration. It will consist of an orbiter, a lander, a rover and a hopper.

In addition, the Queqiao-2 lunar relay satellite is already in place. Launched separately in March 2024, it subsequently supported the Chang’e-6 mission and will facilitate communication between Earth and the lunar surface during the Chang’e-7 mission.

We don’t know precisely where the mission will land, but a site on or near an illuminated ridge close to the Shackleton crater in the Lunar South Pole’s Aitken region is a prime candidate.

The hopper will be the mission’s real star. Unlike a wheeled rover, it’s designed to leap over broken terrain and descend into craters where sunlight never reaches, places conventional vehicles may simply be unable to enter.

Adding it to the mission makes sense, given the steep crater walls, broken terrain, permanent shadow and the need for autonomous navigation and communications.

Chinese scientists won’t be looking for signs of water; we already have compelling evidence for lunar polar water. The question at the mission’s core is where exactly that water is located, in what form and concentration, and whether it could realistically be accessed.

This marks a transition from simply exploring the Moon to determining whether its resources can actually be used.

How NASA is taking a different tack

Other nations are similarly interested in addressing that same question. The United States is pursuing the same longer-term objective of making sustained lunar operations possible. Although NASA will be confronted by the same physical problems, the American space agency has opted for a different solution.

Chang’e-7 is a coordinated state mission whose elements have been designed around a single, specific objective and which can use the already operational Queqiao-2 infrastructure.

NASA, by contrast, is deliberately trying something different: building a lunar marketplace through its Commercial Lunar Payload Services (CLPS) programme. The agency buys lunar delivery, mobility and technology services from companies that own and operate their own spacecraft.

NASA currently has 17 contracted deliveries carrying more than 60 instruments.

And the American effort has already yielded useful experience: Intuitive Machines’ IM-2 mission, Athena, attempted strikingly similar undertakings to Chang’e-7. It carried a drill and mass spectrometer looking for volatiles, a small hopping vehicle intended to explore difficult terrain and an experimental mobile communications network. However, the lander fell over at the landing site and was powered down just one day after arriving.

In essence, the destination dictates the problems; different space programmes are experimenting with different ways to resolve them.

Learning how to live on the Moon

Finding lunar resources is only the first step. Chang’e-7 will attempt to determine where useful lunar resources are found near the Moon’s South Pole and characterise their nature.

Chang’e-8, slated for potential launch in 2028, will move towards demonstrating how they might actually be used.

Crewed exploration will likely follow, culminating in the construction of the International Lunar Research Station, jointly led by the China National Space Administration and Russia’s Roscosmos space agency.

On the other side of the Pacific, the CLPS will feed technology and science into the broader NASA-led Artemis programme, which the agency explicitly describes as laying the foundation for sustained lunar exploration.

Ultimately, the lunar South Pole doesn’t care which flag is on a spacecraft. Any explorer entering its permanently shadowed craters faces the same darkness, severe cold, broken terrain and communications problems.

Getting to the Moon was one of the great technological challenges of the 20th century. Learning how to live and work there may be one of the great challenges of the 21st.

 

This article is republished from The Conversation under a Creative Commons license. Original article.

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At the center of most galaxies lies a supermassive black hole, with a mass millions to billions of times that of our sun and some of the most extreme gravity in the universe.

Some stars that venture too close to such black holes live to tell the tale. Rather than being completely destroyed, they survive to make repeated close passes, producing a new flare of light each time.

These repeating partial tidal disruption events (rpTDEs) give astronomers the opportunity to watch the same star and black hole interaction unfold again and again, thanks to wide-field time-domain surveys that repeatedly scan large areas of the sky for objects that change in brightness.

But in several cases, researchers have noticed a puzzling pattern: The successive flares grow progressively dimmer. For years, theoretical models couldn’t explain why.

Now, a team of astrophysicists at Syracuse University has shown that the answer may lie in a previously overlooked factor—the star’s spin.¹

When stars meet black holes

In a typical tidal disruption event (TDE), a black hole’s tidal force—the difference in gravitational pull across a nearby star—tears the star completely apart.

As the disrupted stellar debris falls toward, or “accretes” onto, the black hole, it loses energy that is emitted in the form of light over the course of days to months.

While black holes themselves emit no light, TDEs provide a short-lived supply of fuel that lights up the surrounding region, allowing astronomers to study these otherwise invisible objects indirectly.

If a star orbiting a black hole does not come close enough to be completely ripped apart, it can, however, lose a fraction of its mass, resulting in a partial TDE. In a repeating partial TDE, the surviving core continues orbiting the black hole, losing more material with each new close pass, a few months to several years apart.

The dimming mystery

How much material a star loses during repeated encounters depends partly on its internal structure. Bandopadhyay compares a low-mass star to a fluffy meringue, which can become increasingly vulnerable to the black hole’s tidal forces. By contrast, a higher-mass star has a more centrally concentrated, onion-like star and can shed its outer layers while its dense core remains relatively unaffected, losing decreasing amounts of mass over time.

Those differences can help explain why rpTDEs don’t all behave the same way. But one pattern in particular has mystified researchers. Of the roughly 10 repeating systems identified to date, four have produced flares that grow progressively dimmer.

Decreasing mass loss might seem like an obvious explanation. But previous hydrodynamical simulations showed that, surprisingly, even as the material lost decreased with each encounter, the predicted flares retained roughly the same brightness.

“We were puzzled by this for two years,” Bandopadhyay says.

Their previous work had revealed another effect of the black hole’s tidal forces.In addition to stripping material from the star, they exert a torque that causes the star to spin faster with each close encounter. As a result, although less material falls back toward the black hole, it returns over a shorter period of time, helping to keep the predicted flare at roughly the same brightness.

To reproduce the dimming astronomers were actually observing, the researchers needed what Bandopadhyay called “a new ingredient”—a star that was already spinning rapidly before its first encounter with the black hole.

The new study found that this initial rotation prevents the star from being significantly spun up during each passage. Without that additional spin-up, the timescale over which the stripped material falls back remains relatively constant. As the star loses less material with each encounter, the peak fallback rate—and the predicted brightness of the flare—can finally decline.

Tracing the star’s past

But why would the star already be spinning so rapidly?

“It is also extremely difficult to ‘bind’ a star to a supermassive black hole so tightly that it orbits the black hole in a matter of months, and yet they seem to do so in rpTDEs,” Coughlin says.

The so-called Hills mechanism may explain both. Under this scenario, a pair of closely orbiting stars passes near a supermassive black hole, which tears the binary apart, ejecting one star and capturing the other.

In a close binary, the stars can become tidally locked, causing each to rotate on its axis at the same rate that the pair orbits each other. The tighter the binary, the shorter that orbital period and the faster a tidally locked star spins. The binaries capable of leaving a captured star on the short orbit observed in rpTDEs would have to be extremely tight—also leaving a tidally locked star spinning rapidly before its capture.

“Ananya’s work demonstrates that each of these peculiarities can be explained by the same underlying phenomenon: the tidal destruction of a binary system and the capture of one of the stars,” Coughlin says. “From a theoretical standpoint, this is a major step forward in our understanding of the physics at play in these systems.”

Zooming out, Coughlin notes that Hills capture may also have produced some of the stars orbiting Sagittarius A*, the supermassive black hole at the center of the Milky Way. The new findings could therefore help explain some of the properties of stars in what he calls “our own cosmological backyard.”

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A 2025 study by South Korean researchers caught the attention of the astronomy community when it suggested that the evidence behind dark energy could be wrong. Dark energy makes up about 70% of the universe but nobody knows what it is. This makes it a frequent target for scepticism.

Scientists had long known that the universe was expanding. But in the late 1990s, researchers discovered that this expansion was accelerating. Astrophysicists concluded that something must be driving the acceleration. They named this unknown quantity “dark energy”.

The 2025 re-analysis, by a team at Yonsei University in Seoul, suggested that the expansion of the universe was not in fact accelerating.

Our team at the University of Southampton has now gone through the Yonsei team’s findings. Our results, published in Monthly Notices of the Royal Astronomical Society (MNRAS), suggest that the results were a false alarm.

However, the Yonsei University team have said they stand by their results and that a follow up study supports their finding that the universe may not be accelerating.

What is dark energy?

In the late 1990s, cosmology was upended by a startling discovery. Scientists knew the universe was expanding, but they had assumed that gravity was gradually slowing down this expansion.

However, by observing incredibly bright, thermonuclear explosions of white dwarf stars, two independent teams of astronomers found the opposite. Because these explosions, known as Type Ia supernovae, have almost the same intrinsic brightness, comparing how bright they appear gives a precise distance.

In 1998, data revealed that distant supernovae were fainter than they should have been in a slowing universe. Because faintness implies distance, the observations placed these supernovae farther away than expected, and implied that the cosmic expansion was not slowing.

In fact, when the calculations were run, it was clear that the expansion of the universe was actually speeding up.

There was no convincing physical explanation for this “cosmic acceleration”. Physicists labelled the mysterious, repulsive force that causes the acceleration “dark energy”.

Cosmic calibration

To understand the Yonsei University group’s claims, we must understand how astronomers use supernovae as “standard candles” to measure cosmic distances. Type Ia supernovae are remarkably alike, but they are not identical.

Cosmologists correct for these differences. They use subtle calibrations to account for relationships between the brightness of the supernovae and measurements like their colour, the duration of their light curves, and the type of galaxies they explode in.

One of those corrections is well known but subtle. It links the brightness of a supernova to the size of the galaxy it exploded in. Big galaxies hold more stars, and so more mass. After the standard corrections, supernovae in those big (massive) galaxies come out a few per cent brighter than ones in small galaxies. Nobody knows why.

But bigger galaxies typically have older stars that are made up of heavier elements, which could in turn influence the properties of the supernovae that some of those stars become. The authors of the 2025 study proposed a far bigger evolutionary effect. They argued that the brightness of Type Ia supernovae changes significantly as the universe ages.

Specifically, they claimed that older white dwarf stars, which are more common in the nearby, present-day universe, produce brighter explosions. That would mean today’s supernovae are inherently much brighter than their distant, early-universe counterparts.

If true, the faintness we observe in distant supernovae would not be a result of their being further away, a relationship that is in turn driven by dark energy, but rather an evolutionary trait of the stars themselves. This claim threatened to dismantle almost three decades of progress in astronomy.

Addressing the claims

Extraordinary claims need careful testing. At the University of Southampton we embarked on an audit of the data, using observations from the Dark Energy Survey (DES), an astronomy project designed to constrain dark energy’s properties, alongside the same dataset used by the authors of the 2025 work. In doing so, we sought to replicate and test their conclusions.

Our re-analysis revealed what we saw as two problems in the Yonsei study: a technical omission and a flawed assumption about stellar populations. Once corrected, the data fell back in line with standard cosmological results, including those of DES.

Crucially, the 2025 study did not account for how supernovae in big galaxies come out a few per cent brighter than ones in small galaxies. When our team reapplied this correction to the same dataset, the correlation between a supernova’s brightness and galaxy age – presented by the Yonsei team in their paper – became far weaker.

This doesn’t mean that the age of a white dwarf star has no effect on the brightness of a resulting supernova. Indeed, most researchers in the field would likely accept that the brightnesses of supernovae are more affected by the age of the stars that explode than by the mass of the galaxy that it is in.

However, measuring a galaxy’s mass is far less expensive in terms of telescope time and requires fewer technical assumptions than measurements of the ages of galaxies, which are challenging. What we showed is that the galaxy mass calibration is adequate to the accuracy required to constrain dark energy.

Secondly, the 2025 analysis relied on a flawed proxy, incorrectly assuming that the overall age of a host galaxy is identical to the specific age of the star that exploded. But galaxies are not uniform like this: even large, ancient galaxies contain localised pockets of young stars.

By overestimating the difference in age between the stars that explode nearby and those that explode in the distant universe, by a factor of three to five, the Yonsei University study overestimated the amount by which the age-brightness calibration could bias supernova distance measurements.

Once these omissions were corrected, the data behaved as expected under the standard model of cosmology. As such, evidence for an accelerating universe remains secure.

Science friction

While the 2025 study turned out to be incorrect, the challenge it posed was valuable. Challenging accepted ideas and rigorously testing our observations is fundamental to scientific progress.

Their claims provided an opportunity for cosmology teams to return to their baseline data, interrogate their core assumptions, and test different ways of calibrating the measurements. When measuring distances, however, the age effect and the mass effect are highly correlated, and therefore we (the Southampton team) believe it is incorrect to make a full age-based adjustment to distances that have already been corrected for the mass effect.

The Southampton team was invited, among others, to a workshop at Yonsei in June 2026, where we discussed our methods and talked about ways to include both stellar mass and stellar ages into the brightness corrections.

In a follow up study, recently accepted by MNRAS, the Yonsei University team maintain that the ages of stars have an important effect on the supernovae they produce. They say that two methods used in earlier studies have suppressed the importance of this phenomenon.

When these effects are accounted for consistently, they explain, it leads to results similar to those in the 2025 study. “We therefore continue to find evidence that stellar-population age is an important factor in Type Ia supernova standardisation,” authors Junhyuk Son and Hyejeon Cho, told The Conversation.

“Because the typical ages of supernova host populations change with cosmic time, understanding this effect is important for precision measurements of the Universe’s expansion history.”

Future measurements will depend on understanding supernova explosions better. It is a job we hope the community can take on together. But none of this removes the need for dark energy: it is still there, and still unexplained.

Ultimately, we have shown that existing measurements of dark energy are robust. Rather than debating whether cosmic acceleration exists, the astrophysical community can return to the exciting task of figuring out what dark energy actually is and how we best measure it.

Over the next 10 years, the Vera C. Rubin Observatory in Chile will carry out a project known as the Legacy Survey of Space and Time, which will produce a sample of supernovae 20 times larger than the DES sample used in our analysis. There is no better time to be investigating the mysteries of dark energy.

 

This article is republished from The Conversation under a Creative Commons license. Original article.

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