Now, Manthilake and colleagues have proposed an explanation: The liquid part of the core is surrounded by a layer of iron sulfide that lets heat out much more slowly than previously assumed, the team reports August 21 in the Journal of Geophysical Research: Planets. A look inside Mercury’s interior is probably layered: a solid inner core, a molten outer core and perhaps a layer of iron sulfide at the boundary between the core and the mantle. Previous work suggests that Mercury’s core has a solid deep interior, plus a liquid outer layer made mostly of iron, sulfur and silicon (SN: 4/22/19).