A magma that would freeze solid on Earth can keep flowing on Mercury, and sulfur is the reason. New laboratory experiments published in Geochimica et Cosmochimica Acta in early 2026 show that sulfur-rich, oxygen-poor melts mimicking Mercury’s interior chemistry remain liquid at temperatures well below the crystallization point of comparable terrestrial basalts. The results suggest that volcanism on the solar system’s innermost planet persisted far longer than conventional cooling models have assumed, potentially reshaping the timeline of Mercury’s geological evolution.
What the experiments found
Researchers synthesized basaltic melts under the extremely low-oxygen conditions that characterize Mercury’s interior, then dissolved sulfur into those melts at concentrations consistent with spacecraft observations. As sulfur content increased, the liquidus temperature, the point at which the first crystals begin forming in a cooling magma, dropped significantly. The identity and order of the minerals that crystallized also shifted, producing assemblages rarely seen in Earth’s volcanic systems.
In plain terms: a Mercurian magma enriched in sulfur would still be flowing freely at temperatures where an equivalent lava on Earth had already locked into solid rock. That wider thermal window between fully liquid and fully solid means Mercury’s magmas could have traveled farther through the crust, erupted more easily at the surface, and stayed mobile for longer stretches of geological time.
The chemistry behind this traces to data from NASA’s MESSENGER spacecraft, which orbited Mercury from 2011 to 2015. MESSENGER’s X-ray spectrometer revealed unexpectedly high sulfur concentrations on the planet’s surface, a finding that forced scientists to reconsider how magmas behave when oxygen is nearly absent. Mercury’s oxygen fugacity, a measure of how much oxygen is available to participate in chemical reactions, falls far below the iron-wüstite buffer, a standard reference point in geochemistry. Under those conditions, sulfur dissolves into silicate melt through pathways that have no direct parallel in terrestrial volcanism.
Separate high-temperature experiments on sulfur and iron partitioning under Mercury-like conditions have shown that dissolved sulfur also prevents early formation of iron sulfide minerals. That keeps iron dispersed throughout the silicate mantle rather than locked into dense phases that would sink toward the core. The mechanism helps explain a long-standing puzzle: why Mercury retains iron in its mantle at all, given that the planet formed under conditions that should have driven nearly all of it into its oversized metallic core.
Taken together, these lines of evidence point to sulfur as a structural player in Mercurian magmas, not a trace impurity. By suppressing crystallization temperatures and altering mineral stability, sulfur fundamentally changes how Mercury’s interior melts, moves, and solidifies.
What scientists still don’t know
The new experiments capture behavior near the liquidus, the narrow temperature range where magma transitions from fully liquid to partially crystalline. They do not yet replicate the full range of pressures found deep in Mercury’s mantle, where higher pressures could alter both mineral stability and how much sulfur the melt can hold. Extending these results to mantle depths will require a new generation of high-pressure experiments.
Thermodynamic models for sulfide speciation in Mercurian magmas can extrapolate from the experimental data across a wider range of temperatures, pressures, and compositions. But the parameter space is vast, and the number of direct experiments constraining it remains small. The exact magnitude of the temperature depression across different magma compositions has not been consolidated into a single published figure; instead, it must be pieced together from multiple experimental series and calculations, each carrying its own uncertainties.
The absence of physical rock samples from Mercury compounds the challenge. MESSENGER measured elemental abundances remotely, and those readings carry calibration uncertainties that are difficult to fully resolve. The starting mixtures used in laboratory experiments are carefully designed analogs, but they may not capture the true diversity of Mercury’s magma chemistry. Without rocks in hand, every experimental result carries an asterisk.
There is also the question of whether all of Mercury’s eruptions tapped similarly sulfur-rich sources. The planet’s surface preserves evidence of both ancient, widespread lava plains and younger, more localized volcanic vents. If some magmas were less sulfur-rich or experienced different redox conditions during their ascent, their crystallization behavior could have diverged from what the current experiments describe. Mercury’s volcanic history may turn out to be more varied than any single set of lab conditions can represent.
Weighing the evidence
The strongest support for the headline finding comes from the peer-reviewed experimental data itself: controlled laboratory measurements showing how sulfur shifts liquidus temperatures and crystallization sequences in reduced basaltic melts. That work is reproducible and directly targets Mercury’s unusual chemical environment.
A second layer of support comes from thermodynamic models that extend the experimental results to conditions not yet tested in the lab. These models are peer-reviewed and internally consistent, but they depend on assumptions about Mercury’s bulk composition and mantle structure that remain debated. They are best understood as frameworks for generating predictions that future experiments and missions can test.
The foundational layer is MESSENGER’s detection of abundant surface sulfur and low iron, which established the boundary conditions making the experimental work planetarily relevant. Without those spacecraft measurements, the lab results would be an interesting exercise in high-temperature geochemistry but would lack a clear target world.
What comes next at Mercury
The practical implication is that Mercury’s volcanic past likely lasted longer and produced more varied eruptions than older thermal models predicted. If sulfur kept magmas liquid at lower temperatures, volcanic activity could have continued well after the planet’s interior cooled below the threshold that would shut down eruptions on an Earth-like body. That extended activity window helps account for the extensive lava plains and younger volcanic features mapped from orbit, features that sit uneasily with the picture of a rapidly frozen world.
The European-Japanese BepiColombo mission, which entered Mercury orbit in late 2025, carries instruments designed to map surface sulfur at higher spatial resolution than MESSENGER achieved. Those measurements will offer a direct check on whether the sulfur concentrations assumed in the new experiments match what actually exists across Mercury’s surface. Strong regional variations in sulfur abundance would point to multiple magma reservoirs with distinct histories; uniformly high sulfur would strengthen the case that the experimental compositions are broadly representative.
Beyond Mercury, the findings sharpen models of how rocky planets differentiated early in solar system history, separating into core, mantle, and crust. Many planet-forming environments, particularly those closer to young stars, may have experienced similarly reducing, sulfur-rich conditions. Working out how that chemistry affects melting, crystallization, and the separation of metal from rock on Mercury gives researchers a template for interpreting the interiors of rocky exoplanets and for reconstructing the earliest stages of planetary assembly closer to home.
Mercury’s unusual combination of a massive metal core, a thin silicate shell, and sulfur-saturated, highly reduced magmas makes it a natural laboratory for probing volcanic behavior that Earth simply does not sample. The new liquidus data represent one step toward converting that uniqueness into quantitative understanding, but they also underscore how much remains to be discovered as BepiColombo’s science campaign ramps up through 2026 and beyond.
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*This article was researched with the help of AI, with human editors creating the final content.
