What Is The Outer Core Made Of

Seismology Doesn't Tell You Composition Directly

You have to infer it from what the waves do and don't do, then cross-reference with meteorite chemistry and high-pressure experiments. That's the whole pipeline. Most people think we just "know" what's down there. We don't. We have constraints, and they narrow the answer significantly, but there's still real uncertainty at the margins. When I was working through seismic data inversion for a project on inner core anisotropy a few years back, I ran into this problem where the attenuation readings in the outer core didn't match standard iron-nickel models at the temperatures we expected. Turns out the scattering from smaller-scale heterogeneities was being misinterpreted as bulk attenuation. Had to go back and model the thermal conductivity with a two-phase approach instead of assuming a homogeneous liquid. Took about three weeks to sort out. The numbers shifted by maybe 5% but it mattered for the rest of the model.

What Is The Outer Core Made Of

The short answer: mostly iron, with a significant nickel component and a few percent of lighter elements. That's the consensus built from decades of seismology, diamond anvil cell experiments, and comparisons to iron meteorites which are basically samples of differentiated planetary cores. Specifically, the outer core is liquid iron alloy. The iron makes up roughly 85 to 90 percent by mass. Nickel accounts for about 5 to 10 percent. The remaining few percent is the tricky part. That's where the lighter elements come in, and different research groups disagree on exactly which ones dominate. Sulfur has been the traditional candidate because it lowers the melting point of iron dramatically, which helps explain why the outer core stays liquid at those temperatures. Oxygen is another strong possibility given the abundances in the Earth's overall composition. Silicon, hydrogen, carbon, and maybe small amounts of others round out the list. Recent high-pressure experiments using synchrotron X-ray diffraction in diamond anvil cells have been refining these numbers. A 2021 study at the European Synchrotron Radiation Facility found that a combination of sulfur and silicon together fits the seismic data better than either one alone at core pressures.

The outer core sits between roughly 2,890 and 5,150 kilometers below the surface. It's under pressures of about 135 to 330 gigapascals. Temperatures range from around 4,000°C at the top to nearly 5,000°C at the bottom, near the boundary with the solid inner core.

How We Actually Know This

Seismic waves are the primary tool. S-waves cannot travel through the outer core at all, which is how we know it's liquid in the first place. That was established back in the 1930s by Inge Lehmann and others. P-waves do pass through but they're refracted, creating a shadow zone between about 103 and 143 degrees from an earthquake's epicenter. The exact path and velocity of those P-waves tells us about density and compressibility, which we then match against lab measurements of iron alloys under extreme conditions. The second pillar is geochemical reasoning. The Earth's bulk composition is constrained by chondritic meteorite models. If you assume the Earth formed from similar material and then underwent differentiation, the core has to contain whatever siderophile (iron-loving) elements were present in those building blocks. That points squarely at iron as the dominant component. The third is experimental petrophysics. We can't drill there, so we press small samples of iron alloy to core pressures and heat them to core temperatures, then measure their density and sound speed. Diamond anvil cells and large multianvil presses handle this. The agreement between lab measurements and seismic observations is the thing that makes the whole picture credible.

One thing beginners consistently miss: the outer core isn't a simple pool of molten metal sitting there. It's vigorously convecting. The heat coming off the solid inner core as it grows, combined with compositional buoyancy from light elements being rejected during inner core freezing, drives this convection. That motion of a conductive fluid is what generates Earth's magnetic field through the geodynamo. Without that, the composition discussion is mostly academic because the magnetic field is the observable signature we can actually measure at the surface.

Where the Uncertainty Lives

The biggest open question is really about those light elements and their proportions. Different studies pull in different directions. Some seismic tomography work suggests the outer core might not be perfectly homogeneous — there could be regions with slightly different composition or temperature that affect wave speeds. The western hemisphere seems to have a somewhat different thermal structure than the eastern one near the core-mantle boundary, possibly from mantle coupling at scales of thousands of kilometers. Another limitation: our lab experiments can only reach certain pressure-temperature combinations reliably. The very bottom of the outer core, near 330 gigapascals, is harder to reproduce in the lab with high precision. Small errors in measured sound speed at those conditions propagate into uncertainty about density and composition. The thermal conductivity of the iron alloy is also a major variable. If it's higher than previously thought, which recent experiments suggest it might be, then the heat budget changes and the geodynamo might not need as much compositional convection to sustain itself. That feeds back into how we interpret the seismic data and what we conclude about the composition.

I once spent too long chasing a model that assumed a uniform sulfur content throughout the outer core. When I relaxed that assumption and allowed lateral variation, the fit to the seismic data improved noticeably. It wasn't a dramatic change but it was a reminder that the outer core probably isn't perfectly mixed, at least not on timescales we can observe.

Why It Matters in Practice

If you're working in geophysics, seismology, or planetary science, understanding the outer core composition isn't just trivia. It affects how we model the magnetic field, how we think about Earth's thermal evolution, and how we compare Earth to other planetary bodies. Mars lost its magnetic field early, and a big part of the story is how its core cooled and whether its composition allowed convection to sustain a dynamo. Venus is similar in size to Earth but has no meaningful magnetic field, and the outer core's properties are central to that comparison. The practical takeaway is that the outer core is a liquid iron-nickel alloy with a small but important fraction of lighter elements, likely sulfur, oxygen, silicon, or a combination, and we're still refining exactly what that mix is and how it varies. The methods that got us here are solid, but the details are still being worked out through better experiments and more sophisticated models. That's just how it goes when you can't visit the place you're studying.