Mercury’s Sulfur-Rich Magma May Rewrite How Solar System’s Innermost Planet Formed

New research from Rice University suggests that sulfur keeps Mercury’s interior molten at lower temperatures, offering new clues about the evolution of the planet’s strange crust and mantle.

Yishen Zhang and Rajdeep Dasgupta provide new insights into the role of sulfur in the thermochemical evolution of Mercury and other rocky planetary systems of similar reduction. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington.

“Mercury’s surface is completely different from Earth’s,” said Professor Rajdeep Dasgupta, director of the Rice Space Institute Center for Planetary Origins to Habitability.

“We could not study its magmatic evolution using assumptions based on our understanding of Earth, and the mission data are difficult to interpret.”

“We had to find ways to bring the planet closer to our laboratory, in particular thanks to the Indarch meteorite.”

Indarch, a meteorite that landed in Azerbaijan in 1891, closely resembles Mercury in chemical composition.

Researchers realized they could use Indarch to study how Mercury’s unique chemical composition shaped the planet, sharing their results in a recent publication.

“Indarch is as chemically reduced as the rocks on Mercury,” said Yishen Zhang, a postdoctoral researcher at Rice University.

“It is thought to be a possible building block of the planet.”

The scientists used a model fusion composition from Indarch to cook their own mercury rocks in a high-pressure, high-temperature facility.

The process was quite simple: mix Indarch’s chemical ingredients in a small glass vial, change the plant settings to match Mercury’s conditions, add the chemicals, and cook.

“This process of cooking a rock can show us what happened chemically inside Mercury,” Zhang said.

“Using 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.”

The authors found that sulfur lowers the temperature at which these reduced molten rocks begin to crystallize.

This means that sulfur-rich magmas on Mercury can remain molten at lower temperatures than similar magmas on Earth.

The reason for this significantly reduced crystallization temperature is due to Mercury’s unique chemical composition: low iron, high sulfur, and chemically reduced state.

Sulfur is a promiscuous element: it likes to be bound with other elements, usually iron.

Iron-rich planets like Mars and Earth have most of their sulfur bound to iron. However, mercury’s low iron content meant its sulfur sought new binding partners.

Specifically, it could bind to major rock-forming elements like magnesium and calcium.

On Earth, these rock-forming elements typically bond with oxygen, giving rise to a stable structure called a silicate network composed of silicon, oxygen, and rock-forming elements.

However, when sulfur replaces oxygen, this network weakens and crystallizes at a lower temperature.

“As Indarch can represent the protoplanetary state of Mercury, 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,” Zhang said.

“This is a fascinating insight into how Mercury may have evolved as a planet to its current unique surface chemistry,” Professor Dasgupta said.

“More importantly, it allows us to think about planets not in terms of how Earth formed, but in terms of their own unique chemistry and magmatic processes under very different conditions.”

“What water or carbon does to magmatic evolution on Earth, sulfur does on Mercury.”

The results appear in the journal Geochimica and Cosmochimica Acta.

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Yishen Zhang and Rajdeep Dasgupta. The effects of sulfur on the near-liquidus phase relationships of highly reduced basaltic melts with implications for magmatism in Mercury. Geochimica and Cosmochimica Actapublished online February 26, 2026; doi: 10.1016/j.gca.2026.02.034