The research, led by geologists Bernard Charlier of the University of Liège and Olivier Namur of KU Leuven, rests on a specialised discipline known as experimental petrology. Because no spacecraft has ever returned samples from Mercury and no meteorite has ever been convincingly linked to the planet, scientists cannot simply analyse rock in the way they do for the Moon or Mars. Instead, they recreate the planet’s interior in miniature. Using high-pressure, high-temperature apparatus, the teams subjected synthetic mixtures approximating Mercury’s interior to temperatures between roughly 1,250 and 2,170 degrees Celsius, at pressures equivalent to those found deep within a rocky planet, and watched how carbon behaved as molten metal separated from molten silicate.

The logic of the approach follows from how all the terrestrial planets formed. Mercury, Venus, Earth and Mars each began as accumulating material from the disc of gas and dust surrounding the young Sun. The heat released during accretion, together with contributions from radioactive decay and giant impacts, was sufficient to melt the growing planets, producing global oceans of magma. This molten stage was decisive: as a magma ocean crystallised, it sorted elements between a dense metallic core sinking to the centre and a silicate mantle left behind above. The mantle later partially melted to generate the magmas that built the crust, and the gases exsolved during this early evolution may have formed the planet’s first atmosphere. Whatever the magma ocean stage decided, the planet inherited for the next four and a half billion years.

The key variable governing carbon’s fate turned out to be oxygen. Geochemists quantify this with a parameter called oxygen fugacity, an effective measure of how oxidising or reducing the environment is. Under relatively oxidising conditions, the experiments showed, carbon is strongly siderophile, meaning it prefers dissolving in metallic iron and follows the metal down into the core. Earth, which formed under comparatively oxidising conditions, exemplifies this behaviour. Mercury, however, is the most reduced of the terrestrial planets, its rocks remarkably poor in oxidised iron. Under those highly reducing conditions, the experiments revealed that carbon becomes far less attracted to metal and instead stays dissolved in the silicate magma.

That single shift in chemical affinity has dramatic consequences. As the reduced magma ocean cooled and crystallised, the carbon it hosted could no longer remain dissolved and precipitated as graphite, the soft, low-density crystalline form of pure carbon. Because graphite is too light to sink through the molten silicate, it floated. Accumulating at the top of the magma ocean, it formed a primitive crust of graphite, a crust made not of silicate rock like every other planetary crust in the inner solar system, but of carbon. The team’s model indicates this primordial graphite crust was on the order of 40 to 120 metres thick, a figure that matches the carbon-rich surface layer of roughly 1 to 3 per cent by mass inferred from observations by NASA’s MESSENGER spacecraft, which orbited Mercury from 2011.

The primordial graphite crust did not survive intact. Meteorite bombardment and the vigorous volcanism that built Mercury’s younger volcanic plains disrupted and redistributed the carbon-rich layer, mixing it into the surface deposits we see today. This history helps explain why MESSENGER detected abundant carbon darkening Mercury’s terrain, a property that had previously been difficult to reconcile with any simple crustal model. The new experiments provide the missing physical mechanism: a flotation crust of graphite, generated by the planet’s own extreme chemistry rather than by any exotic delivery of carbon from outside.

The same experiments resolved a second, long-standing puzzle about Mercury’s interior. Geodetic data from MESSENGER show that the planet’s core is extraordinarily large, accounting for roughly 70 per cent of Mercury’s total mass, and yet it is less dense than pure iron would be. Something light must be dissolved in the metal. Carbon had been proposed as a candidate light element, and the idea was appealing: if carbon stayed in the core, it would simultaneously explain the density deficit and the surface graphite. The new measurements close that door. Under precisely the reducing conditions required to float a graphite crust, the core remains strikingly poor in carbon, containing less than 5,000 micrograms per gram, or less than 0.5 per cent. That concentration is far too low to account for the observed density shortfall.

The culprit, the researchers conclude, is primarily silicon, with sulphur playing a secondary role. These elements dissolve in metallic iron under reducing conditions and are light enough to depress the core’s density to the value MESSENGER measured. They carry an additional consequence with far-reaching implications: both silicon and sulphur substantially lower the melting point of iron alloy. That depression helps explain how Mercury’s core has remained at least partially liquid for 4.5 billion years, despite the planet’s small size and the steady loss of its primordial heat. A liquid, convecting layer in the core is the dynamo that sustains Mercury’s magnetic field, one of the surprising discoveries of the Mariner 10 and MESSENGER eras. The same light elements that lighten the core, in other words, may also be what keeps the planet’s generator running.

By tying Mercury’s extreme chemical reduction to both its graphite crust and its core composition, the work provides a unified framework that extends well beyond one planet. It speaks to the nature of the original, more massive proto-Mercury that some models propose existed before a giant impact stripped away much of its mantle. It informs the study of hypothetical super-Mercuries, dense rocky exoplanets detected around other stars whose interiors may follow similar rules. It bears on carbon-rich exoplanets generally, where reduced conditions might likewise produce graphite flotation crusts rather than carbon sequestered in cores. And it even reaches back to the early Earth, which accreted largely from reduced materials, meaning the partitioning behaviour measured in these experiments constrains how our own planet distributed its carbon between core, mantle, atmosphere and the biosphere that eventually depended on it.

The timing of the work is no accident. BepiColombo, a joint mission of the European Space Agency and the Japan Aerospace Exploration Agency, is entering the final phase of its long journey and will place two spacecraft in orbit around Mercury, carrying instruments far more sensitive than those of MESSENGER. As Charlier notes, future observations could confirm and quantify graphite as a major phase on Mercury’s surface, turning a laboratory inference into a measured planetary fact. If the orbiters detect the spectral signature and thickness of the carbon-rich layer the experiments predict, scientists will have a rare example of a planet whose earliest crust, core chemistry and magnetic history can all be traced back to a single parameter: how little oxygen was present when its magma ocean cooled, more than four billion years ago.

Subject of Research: Carbon distribution during Mercury's magma ocean evolution and the formation of its graphite crust and core composition

Article Title: How Mercury formed its graphite crust and core

Article References: How Mercury formed its graphite crust and core. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Mercury, graphite crust, magma ocean, experimental petrology, oxygen fugacity, planetary cores, MESSENGER, BepiColombo, silicon, sulphur, magnetic field, reduced planets

The research team, led by Hemanthkumar Manne, Nisha Kumari and Sonia Nain of Chaudhary Charan Singh Haryana Agricultural University, together with colleagues at Uttar Banga Krishi Vishwavidyalaya and Government Degree College Ramban, grew the mustard cultivar RH 749 under three escalating levels of lead stress—100, 200 and 300 parts per million—and then sprayed subsets of the plants with either 100 or 200 ppm of sulphur. Crucially, rather than taking a single snapshot, the investigators sampled the crop at 30, 60 and 90 days after sowing, capturing how the battle between lead toxicity and sulphur-mediated defense unfolds across the plant’s life cycle.

The damage inflicted by lead was severe and, in many respects, worsened as the season progressed. At the highest lead dose of 300 ppm, levels of malondialdehyde—a classic molecular signature of lipid peroxidation and cellular membrane damage—climbed by 30.93 percent at 30 days, 32.73 percent at 60 days and 34.82 percent at 90 days after sowing. Electrolyte leakage, which measures how badly lead has punched holes in cellular membranes, surged even more dramatically, rising by 204 percent at the earliest measurement and remaining elevated by 116 percent and 94.3 percent at the later time points. These figures confirm that lead progressively dismantles the structural integrity of plant cells, setting off a cascade of oxidative destruction.

Yet the plants were not passive victims. Lead exposure triggered a powerful internal counterattack: by 90 days after sowing, the worst-stressed plants had boosted their levels of ascorbic acid by 53.1 percent, glutathione by 27.96 percent and—most strikingly—phytochelatins by a remarkable 362.73 percent. Phytochelatins are small, sulphur-rich peptides that bind heavy metals and lock them away in cellular vacuoles, effectively quarantining the toxin. The enormous surge in phytochelatin production suggests that the mustard plants were pouring their sulphur resources into metal detoxification, a strategy that previous work in rice, pakchoi and poplar has also implicated in heavy metal tolerance.

The cost of this defense, however, was paid at the expense of growth and productivity. Under 300 ppm lead stress, the plants’ transpiration rate fell by 33.6 percent, photosynthetic rate by 20.76 percent and stomatal conductance by 33.36 percent at 90 days after sowing. With stomata closing and the photosynthetic apparatus compromised, yield and oil content—the very traits that make mustard a valuable oilseed—declined sharply. The findings underscore a familiar dilemma in stress physiology: plants can either spend their energy fighting toxins or building harvestable biomass, and lead forces them toward the former.

This is where sulphur changed the story. When lead-stressed plants received the higher sulphur spray of 200 ppm, malondialdehyde accumulation dropped by 13.49 percent and electrolyte leakage by 18.62 percent at 90 days after sowing. The sulphur treatment steadied cellular homeostasis, restoring transpiration, photosynthesis, oil content and yield toward healthier levels. In effect, the foliar spray supplied the raw material the plants needed to mount their chemical defense without cannibalizing their own growth machinery—a distinction that could matter enormously to farmers cultivating mustard on contaminated land.

At the enzymatic level, the researchers found that lead stress activated the plant’s sulphur assimilation pathway, increasing the activity of ATP sulfurylase (ATPS), the gateway enzyme that converts sulphate into a biologically usable form, and glutathione S-transferase (GST), which conjugates toxins to glutathione for safe removal. With sulphur assimilation ramped up, the downstream products of the pathway—ascorbic acid, glutathione and phytochelatins—accumulated in greater quantities, giving the plants a larger arsenal for neutralizing lead ions and the reactive oxygen species they generate. The ascorbate-glutathione cycle, long recognized as the central hub of plant redox regulation, was thus supercharged by the extra sulphur supply.

Perhaps the most compelling evidence came from the molecular level. Transcriptomic analysis revealed that expression of the BjATPS gene—the gene encoding ATP sulfurylase—rose under lead stress, but when plants were sprayed with sulphur, its expression nearly doubled. This finding provides a mechanistic explanation for the whole-plant results: exogenous sulphur does not merely act as a passive nutrient but actively amplifies the transcriptional program of sulphur assimilation, feeding the glutathione and phytochelatin production lines that detoxify lead. It also builds on earlier work by the same group, which had shown that sulphur mitigates lead toxicity in mustard through biochemical and transcriptomic strategies, and on studies demonstrating that ATP sulfurylase activity correlates with stress tolerance in mustard cultivars.

The temporal dimension of the study adds practical weight to its conclusions. By showing that oxidative damage accumulates steadily across the season while the protective response peaks late, the researchers highlight that sulphur supplementation must be timed to support the plant through its most vulnerable phases. Their conclusion is straightforward: sulphur application alleviates lead stress in Brassica seedlings by regulating oxidative biomarkers and antioxidants, thereby easing yield constraints. Because sulphur is already a familiar, affordable fertilizer ingredient, the authors suggest it could be a suitable choice for farmers seeking to mitigate lead toxicity in mustard—serving simultaneously as a defense activator against metal stress and a regulator of normal plant growth and development.

Beyond the immediate agronomic implications, the study contributes to a growing body of science on heavy metal stress in crops, a field increasingly urgent as industrial pollution, mining runoff and wastewater irrigation contaminate farmland worldwide. Brassica species are already prized for their phytoremediation potential, and understanding how sulphur metabolism underpins their metal tolerance could inform breeding programs aimed at developing cultivars that thrive on marginal, contaminated soils. With lead exposure posing risks to both crop productivity and human health, a cheap foliar spray that boosts the plant’s own detoxification machinery offers a rare win-win: cleaner fields, healthier plants and better harvests from soil that would otherwise surrender to toxicity.

Subject of Research: Sulphur-mediated alleviation of lead toxicity in Indian mustard (Brassica juncea)

Article Title: Temporal dynamics of sulphur mediated alleviation of lead toxicity in Brassica juncea

Article References: Manne, H., Kumari, N., Nain, S., Vaishnavi, K., & Zaid, A. (2026). Temporal dynamics of sulphur mediated alleviation of lead toxicity in Brassica juncea. Plant Cell Reports, 45(10), Article 302. https://doi.org/10.1007/s00299-026-03990-8

Image Credits: AI Generated

DOI: 10.1007/s00299-026-03990-8

Keywords: Brassica juncea, lead toxicity, sulphur, phytochelatins, glutathione, ascorbic acid, oxidative stress, ATP sulfurylase, photosynthesis, antioxidant defense, heavy metal stress, Plant Cell Reports