Mercury is a small, rocky planet about which researchers know relatively little. Two missions, taking readings as they passed 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 the substances have gained electrons. In fact, it's the most reduced planet in the solar system.
"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, looks very similar to the chemical makeup of Mercury. The researchers realized they could use Indarch to study how Mercury's unique chemical makeup had shaped the planet, sharing their results in a recent publication.
"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 of Indarch to cook his own Mercury rocks in a high-pressure, high-temperature facility. The process was fairly simple: mix Indarch's chemical ingredients together in a small glass vial, change the settings in the facility to match the conditions on Mercury, add in the chemicals and cook.
"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 Mercurylike 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 for this significantly decreased crystallization temperature, Zhang found, is because of Mercury's unique chemical composition: low iron, high sulfur and the chemically reduced state.
Sulfur is a promiscuous element — it likes to be bound 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 looking for new binding partners. Specifically, it could bind to major rock-forming elements like magnesium and calcium.
On Earth, these rock-forming elements would typically bind to oxygen, resulting in a stable structure called a silicate network made up of silicon, oxygen and rock-forming elements. When sulfur replaces oxygen, however, that network becomes weaker and crystalizes 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."
This work was supported by NASA grants (80NSSC18K0828 and 80NSSC24K0988) and by the Rice Space Institute Center for Planetary Origins to Habitability.
