Revision 1

Accepted · first revision · record

Title: Which light elements make Earth's core less dense than iron?
Plain: Earth's liquid core is about 10 percent less dense than pure iron-nickel would be at core pressure, so lighter elements must be dissolved in it. Which ones (silicon, oxygen, sulfur, carbon or hydrogen) and how much of each is unknown.
Precise: The outer core is roughly 10 percent less dense than Fe-Ni at the same P and T, the inner core a few percent, and the density jump at the inner-core boundary is about $0.5\ \text{to}\ 0.9\ \mathrm{g}/\mathrm{cm}^{3}$ depending on the normal-mode analysis. Determine the concentrations of Si, O, S, C and H in outer and inner core that reproduce density, compressional velocity and the boundary density jump, with solid-liquid partitioning from experiment or ab initio calculation and consistency with core-formation conditions. An answer is a composition with uncertainty of about 1 weight percent per element.
Settled by: Static and shock measurements of density and sound speed of Fe alloys at core conditions combined with ab initio solid-liquid partitioning and normal-mode constraints on the inner-core boundary.
Kind: well-posed
Answer type: identification
References: (empty)
Literature status: open
Status note: (empty)
Posed since: 1952