Latent-heat based measurements of the melting curve of iron to the pressure of Earth’s inner core boundary

This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2).

The Earth’s core is a massive ball of metal located 3000 km beneath our feet. Improved knowledge of its basic properties will help Earth scientists better understand the way heat and material move inside the solid Earth’s many layers (inner core, outer core, mantle, and crust). For example, motion of liquid metal in the outer core generates a powerful magnetic field that shields life at the surface from harmful radiation, but the magnitude of forces driving the motion are uncertain.

Another example is that heat from the outer core might melt some minerals in the rocks at the base of the mantle, but the amount of heat flow and the temperature at the base of the mantle are unknown. One notable reason for these uncertainties is that a basic fact about the Earth’s core remains highly uncertain: its temperature. The best way to infer the core temperature relies on a simple idea.

The liquid outer core, which is mostly iron, freezes onto the solid inner core as the Earth cools, meaning the temperature at the liquid-solid interface is the freezing temperature, or equivalently the melting temperature, of the mostly iron liquid material. Hence, the starting point for understanding the core’s temperature, and its effect on the entire Earth, is to accurately determine the melting temperature of iron when subjected to the enormous pressure that exists at the inner core-outer core boundary: 3.3 million atmospheres.

The upcoming project aims to accurately determine this crucial temperature by using a new type of laboratory measurement. In addition to these scientific impacts, the project will engage undergraduate students, and benefit physicists and materials scientists studying high-pressure phase transitions that involve entropy changes - from magnetic transitions, to superconducting transitions, to melting.

The team will also publish the sample preparation details in the video journal JoVE to clearly disseminate their methods to a wide audience.

Experiments will be performed to identify the absorption of latent heat while samples of iron and iron-sulfide are rapidly heated. The samples will be compressed in diamond anvil cells to pressures as high as 3.3 million atmospheres, and heated with short pulses of electricity that flow through the samples. This pulsed Joule heating procedure allows latent heat detection by limiting the amount of heat escaping from samples, and also limits chemical reactions by restricting the total time during which the sample is molten to roughly 1 to 100 microseconds.

The project aims to (a) dramatically improve the accuracy of the melting temperature data for iron up to 3.3 million atmospheres by achieving plus/minus 50 K accuracy up to approximately 1 million atmospheres and plus/minus 200 K accuracy up to 3.3 million atmospheres, (b) to prove that the latent-heat technique can be extended to partial melting of iron alloys by measuring the solidus temperature of Fe(0.9)S(0.1) up to 1 million atmospheres with plus/minus 50 K accuracy, and (c) assimilate the new measurements into models of Earth core adiabats and hydrodynamic models of magnetic field formation and evolution.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.