Sulfur's Role in Mercury's Magma: Insights from a Meteorite

A new study has used the chemical composition of a meteorite to simulate magmatic processes on Mercury. The research indicates that sulfur significantly lowers the temperature at which melted rocks on Mercury begin to crystallize, suggesting its magmas may remain molten at lower temperatures than similar magmas on Earth. The findings offer a framework for understanding planetary formation based on unique chemical conditions.

Research Overview and Methodology

Researchers from Rice University conducted experiments to understand magma formation on Mercury, a planet whose direct study is challenging. The team used the chemical composition of the Indarch meteorite, which landed in Azerbaijan in 1891, as a model for Mercury's potential building blocks.

The Indarch meteorite is considered chemically similar to Mercury, sharing characteristics of being iron-poor, sulfur-rich, and in a chemically reduced state—a condition where substances have gained electrons. Mercury is noted as the most chemically reduced planet in the solar system.

To simulate planetary conditions, postdoctoral researcher Yishen Zhang created a model melt composition based on Indarch. This synthetic material was then subjected to high-pressure and high-temperature conditions in a laboratory facility, with parameters adjusted to match those derived from spacecraft observations and models of Mercury.

Key Findings

The central finding of the experiments is that sulfur lowers the crystallization temperature of reduced, melted rocks under Mercurian conditions.

The researchers attribute this effect to the planet's unique chemical composition. The study explains the mechanism as follows:

• On iron-rich planets like Earth and Mars, sulfur typically binds to iron.
• On Mercury, due to its low iron content, sulfur binds instead to major rock-forming elements like magnesium and calcium.
• On Earth, these elements would bind with oxygen to form a stable silicate network in rocks.
• In the Mercury simulation, sulfur replaces oxygen in this network. This substitution results in a weaker structural network that crystallizes at a lower temperature.

Consequently, sulfur-rich magmas on Mercury may remain molten at lower temperatures than comparable magmas on Earth.

Interpretations and Implications

Based on the experimental results, the researchers propose that Mercury likely formed with sulfur occupying a structural role in its mantle that, on Earth, is filled by oxygen. They state this would fundamentally change how the planet's mantle solidified compared to Earth.

"The work provides a way to think about planet formation based on unique chemistry rather than assumptions derived from Earth," said Rajdeep Dasgupta, the Maurice Ewing Professor in Earth Systems Science.

First author Yishen Zhang stated that the experimental process of "cooking" the synthetic rock model shows what happened chemically inside Mercury, allowing insight into magma evolution without direct samples from the planet.

Research Support and Context

The research was supported by NASA grants (80NSSC18K0828 and 80NSSC24K0988) and the Rice Space Institute Center for Planetary Origins to Habitability. The results were published in a recent scientific paper with Yishen Zhang as first author.

The study addresses a gap in planetary science, as direct analysis of Mercury's magmatic history has been limited to data from a small number of spacecraft flybys, and its surface chemistry is markedly different from that of Earth.