Mercury is a small, rocky world that scientists still understand only in limited detail. Data from two flyby missions show that its surface is made up of a crust low in iron but rich in sulfur. The planet is also in a reduced chemical state, meaning its materials have gained electrons—making it the most reduced planet in the solar system.
“Mercury’s surface is entirely unlike 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 rely on Earth-based assumptions to understand its magmatic history, and mission data are challenging to interpret. So we needed a way to study the planet in the lab—specifically by using the meteorite Indarch.”
Indarch Meteorite Offers Clues to Mercury’s Chemical Evolution
Indarch, which fell in Azerbaijan in 1891, closely matches Mercury’s chemical composition. This led researchers to use it as a proxy to investigate how the planet’s unusual chemistry influenced its development, with their findings reported in the journal Geochimica et Cosmochimica Acta.
“Chemically, Indarch is just as reduced as Mercury’s rocks,” said Yishen Zhang, a postdoctoral researcher in Dasgupta’s lab and the study’s lead author. “It’s thought to be one of the building blocks of the planet.”
Zhang recreated Mercury-like rocks by using a model melt composition based on Indarch in a high-pressure, high-temperature lab setup. The method was straightforward: combine Indarch’s chemical components in a small glass vial, adjust the facility to simulate Mercury’s conditions, then introduce the mixture and heat it.
“This process of ‘cooking’ rocks helps us understand the chemical processes that took place inside Mercury,” Zhang said. “By applying temperature, pressure, and chemical limits based on spacecraft data and models, we can reproduce Mercury-like conditions in the lab and study how its magmas form and evolve—even without direct samples.”
Sulfur’s Role in Lowering Crystallization Temperatures and Shaping Mercury’s Magmas
Zhang found that sulfur lowers the temperature at which these reduced molten rocks begin to crystallize. As a result, sulfur-rich magmas on Mercury can remain liquid at lower temperatures than comparable magmas on Earth. He attributed this notably lower crystallization point to Mercury’s distinctive chemistry: low iron content, high sulfur levels, and an overall reduced state.
Sulfur is a highly reactive element that readily bonds with others, most commonly iron. On iron-rich planets like Earth and Mars, most sulfur is tied up with iron. But because Mercury contains much less iron, its sulfur instead bonds with other major rock-forming elements, such as magnesium and calcium.
On Earth, these rock-forming elements usually bond with oxygen, creating a stable silicate structure composed of silicon, oxygen, and other key elements. But when sulfur takes oxygen’s place, the network weakens and begins to crystallize at lower temperatures.
“As Indarch may reflect Mercury’s early proto-planet stage,” Zhang said, “our experiments suggest that sulfur occupied structural roles typically held by oxygen on Earth. This would have fundamentally altered how Mercury’s mantle solidified.”
“This offers a compelling look at how Mercury may have developed its distinct surface chemistry,” Dasgupta added. “More importantly, it shows that we should understand planets on their own terms—based on their unique chemistry and magmatic behavior under very different conditions, rather than using Earth as the default model. On Mercury, sulfur plays a role in magmatic evolution similar to what water or carbon does on Earth.”
Read the original article on: Phys.Org
Read more: Volcanic rock–based formula reduces cement emissions by two-thirds