Mercury, the smallest and innermost planet in our solar system, has long been a source of intrigue for planetary scientists. Its unique surface chemistry, as revealed by NASA's MESSENGER mission, has left researchers with a conundrum: how can a planet so different from Earth, Venus, or Mars have such a distinct composition? The answer lies in the planet's sulfur-rich, iron-poor chemistry, which may fundamentally alter how its molten rock crystallizes. This is where the Indarch meteorite, an enstatite chondrite, comes into play as a stand-in for Mercury's building blocks. By recreating the pressures and temperatures of Mercury's interior, researchers have discovered that sulfur, in the absence of iron, binds to magnesium and calcium, weakening the silicate network that holds magma together. This allows the melt to stay liquid at temperatures where an Earth-like magma would have already started crystallizing, reshaping our understanding of Mercury's interior history. The implications are far-reaching, suggesting that Mercury's magmas may have run cool and stayed molten longer than previously thought. This behavior has been compared to the role of volatiles on Earth, with sulfur potentially occupying a structural role similar to oxygen. The research also raises questions about Mercury's outsized iron core and the possibility that it began much larger and lost most of its mantle in a giant impact. The broader lesson is that planetary science must move away from assuming Earth as the default case and instead embrace the diversity of rocky-world chemistries. As exoplanet characterization improves, the need for experimental foundations in magmatic physics will become increasingly important, just as it has for hidden magma oceans on rocky exoplanets. While there are still many unknowns, the useful framing is clear: build models from the unique characteristics of each planet, rather than relying on Earth-centric assumptions. This research not only challenges our understanding of Mercury but also serves as a reminder of the importance of experimental grounding in planetary science.
