Sulfur Plays the Role of Oxygen in Mercury Magmas Reshaping How the Planet Formed

by Clarence Oxford
Los Angeles CA (SPX) Apr 16, 2026
Mercury is a small, rocky planet about which researchers know relatively little. Two missions passing over the planet have revealed that Mercury is covered by an iron-poor and sulfur-rich crust. It is also reduced - a chemical state in which substances have gained electrons - and is in fact the most reduced planet in the solar system.

Researchers at Rice University have now used a meteorite analog to recreate Mercury rocks in the laboratory and found that sulfur plays a structural role on Mercury that on Earth belongs to oxygen. The work fundamentally changes how scientists think about the planet's interior evolution and crust formation.

"Mercury's surface looks completely different than Earth's," said Rajdeep Dasgupta, the Maurice Ewing Professor in Earth Systems Science and director of the Rice Space Institute Center for Planetary Origins to Habitability. "We couldn't study its magmatic evolution using assumptions built off our understanding of Earth, and missions data are difficult to interpret. We had to find ways to bring the planet closer to our lab - specifically, through the meteorite Indarch."

Indarch, a meteorite that landed in Azerbaijan in 1891, closely matches the chemical makeup of Mercury. "Indarch chemically is as reduced as rocks on Mercury," said Yishen Zhang, a postdoctoral researcher in Dasgupta's lab and first author on the paper. "It is believed to be a possible building block of the planet."

Zhang used a model melt composition based on Indarch to cook Mercury rocks in a high-pressure, high-temperature facility, mixing Indarch's chemical ingredients together and adjusting conditions to match those on Mercury. "This process of cooking a rock can show us what happened chemically inside of Mercury," Zhang said. "By using the temperature, pressure and chemical constraints derived from spacecraft observations and models, we recreate Mercury-like conditions to understand how magmas form and evolve there - even without direct samples from the planet."

What Zhang found is that sulfur lowers the temperature at which these reduced melted rocks begin to crystallize. That means sulfur-rich magmas on Mercury may stay molten at lower temperatures than similar magmas on Earth. The reason lies in Mercury's unique chemical composition: low iron, high sulfur and a strongly reduced chemical state.

Sulfur is a promiscuous element that likes to bind to other elements, usually iron. Iron-rich planets like Mars and Earth have most of their sulfur bound to iron. Mercury's low iron content, however, meant that its sulfur was available to bind to major rock-forming elements like magnesium and calcium instead. On Earth, these rock-forming elements would typically bind to oxygen, resulting in a stable structure called a silicate network. When sulfur replaces oxygen in that network, the structure becomes weaker and crystallizes at a lower temperature.

"As Indarch may represent Mercury's proto-planet state," Zhang said, "these experiments show that Mercury likely formed with sulfur occupying a structural position that on Earth belongs to oxygen. This fundamentally changes how the planet's mantle solidified."

"This is a fascinating glimpse of how Mercury may have evolved as a planet to its unique current-day surface chemistry," Dasgupta said. "More importantly, it provides a way for us to think about planets not based on how Earth was formed, but based on their own unique chemistry and magmatic processes under vastly different conditions. What water or carbon does to magmatic evolution of Earth, sulfur does on Mercury."

Research Report:The effects of sulfur on near-liquidus phase relations of highly reduced basaltic melts with implications for magmatism in Mercury