Sulfides in Mercury's Mantle: Implications for Mercury's Interior as Interpreted From Moment of Inertia

The partitioning of sulfur between Mercury's core and mantle reflects its formation conditions and early evolution. If Mercury's core and mantle equilibrated under reducing conditions, and if Mercury is not depleted in sulfur relative to chondrites, Mercury's mantle should contain large quantities (7–11 wt.%) of sulfur in the form of Ca or Mg‐rich sulfides. Using petrologic constraints, we estimate the quantity of these sulfides and the implications of a sulfide‐rich mantle for Mercury's radial density structure. We find that based on recent measurements of Mercury's outer shell moment of inertia (MoI), a sulfide‐rich, iron‐poor mantle mineralogy is consistent with a low value of Mercury's polar MoI (0.333 MR2). Alternatively, a higher value for Mercury's MoI (0.343 MR2) would require a sulfide‐poor mantle, indicating bulk sulfur depletion or more oxidizing conditions than implied by surface composition. Plain Language Summary Mercury's mantle is unusually rich in sulfur relative to the other terrestrial planets. That sulfur is not bound into the silicate rocks, but rather it is expected to take the form of calcium and magnesium‐rich sulfides. We estimate how much sulfide should be present in Mercury's mantle and what the presence of that sulfide means for the density of Mercury's mantle and core. We interpret recent measurements of the moment of inertia (MoI) of Mercury's mantle and of the whole planet in light of the possible sulfide content of Mercury's mantle. We find that if Mercury's mantle contains large amounts of sulfide, but not very much iron (which is what we expect), then the value of Mercury's polar MoI should be low. Alternatively, if Mercury's polar MoI is high, Mercury's mantle must not have much sulfide in it. If Mercury's mantle has little sulfide, either Mercury does not have much sulfur overall, or the sulfur is in Mercury's core. The latter implies that Mercury's interior chemistry (especially the amount of oxygen) is different from what is predicted from the abundance of elements and minerals measured at Mercury's surface. Key Points Petrologic constraints imply Mercury's mantle may contain 13–20 wt.% low‐density sulfides; this is relevant for geophysical studies A sulfide‐rich mantle is consistent with a low polar moment of inertia (MoI), and implies a smaller, denser core than does a pure silicate mantle If Mercury's polar MoI is closer to the high end of published values, it rules out a sulfur‐rich mantle