How Mercury’s Sulfur-Rich Magmas Differ From Earth’s

New laboratory experiments simulating Mercury’s volcanic past reveal that sulfur-rich magmas on the innermost planet behave fundamentally differently from those on Earth, offering fresh insight into the planet’s geological evolution and the role of volatile elements in shaping rocky worlds.

Researchers at NASA’s Johnson Space Center and collaborating institutions recreated conditions similar to those found in Mercury’s mantle by heating synthetic rock mixtures rich in sulfur and silicates to temperatures exceeding 1,400 degrees Celsius. Their findings, published in Geochimica et Cosmochimica Acta, show that these sulfur-bearing melts do not crystallize in the same way as terrestrial magmas, instead forming unusual mineral assemblages dominated by iron sulfides and silica-poor phases.

Unlike Earth’s magmas, which typically evolve through well-understood processes involving minerals like olivine, pyroxene, and plagioclase, Mercury’s sulfur-rich melts suppressed the formation of these common silicates. Instead, the experiments produced significant quantities of troilite (iron sulfide) and altered the sequence of mineral precipitation, suggesting that volcanic activity on Mercury may have produced surface compositions quite unlike anything seen on our planet.

“We’re seeing that sulfur doesn’t just act as a minor impurity in Mercury’s interior — it changes the very behavior of molten rock,” said Dr. Rebecca M. E. Williams, planetary scientist at the Lunar and Planetary Institute and lead author of the study. “This means that models of Mercury’s thermal and volcanic history based on Earth-like processes are likely incomplete.”

The study builds on data from NASA’s MESSENGER mission, which orbited Mercury from 2011 to 2015 and detected unusually high levels of sulfur on the planet’s surface — up to 4 percent by weight in some regions, far exceeding Earth’s average of less than 0.1 percent. Scientists have long debated whether this sulfur originated from the planet’s core, was delivered by impacts, or was concentrated through volcanic outgassing.

By demonstrating that sulfur-rich magmas resist forming typical silicate minerals and instead favor sulfide-bearing phases, the experiments support the idea that much of Mercury’s surface sulfur could have been released during ancient volcanic eruptions. The altered crystallization behavior also affects how heat moves through the mantle and how long volcanic activity could have persisted.

These findings have implications beyond Mercury. Understanding how volatiles like sulfur influence magma behavior helps scientists interpret data from other rocky bodies, including the Moon, Venus, and exoplanets orbiting close to their stars, where high temperatures and different compositions may produce similarly exotic melts.

Future work will focus on refining the experimental conditions to match more precise estimates of Mercury’s mantle composition and pressure profiles. Researchers also plan to analyze how sulfur affects the viscosity and gas content of magmas, which could explain the distinctive smooth plains and volcanic features observed by MESSENGER.

As planetary scientists continue to piece together the geologic story of the inner solar system, studies like this underscore that planetary composition doesn’t just affect what a world looks like — it changes how it works from the inside out.