Carbon-driven iron transition points to a solid lunar inner core
This study resolves the controversy over the lunar inner core's existence by demonstrating that an iron-carbon alloy with a specific high-pressure phase (fcc-II) matches observed seismic velocities, thereby confirming a solid inner core composed of Fe-C with temperatures likely below 1500 K.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Deep beneath the surface of the Moon, hidden from view and untouched by human hands, lies a mystery that has puzzled scientists for decades. While we know the Moon has a core made mostly of iron, much like Earth, the question of whether that core is entirely liquid or contains a solid inner ball has remained a subject of intense debate. For years, data from the Apollo missions, which placed seismometers on the lunar surface, suggested a solid inner core might exist. However, when scientists tried to match those seismic signals to what they knew about iron under extreme pressure, the numbers did not add up. The seismic waves traveling through the suspected inner core moved far slower than pure iron should allow, leading many to doubt the existence of a solid center at all. This discrepancy left a gap in our understanding of the Moon's history, its magnetic past, and how it cools over time. To solve this, researchers needed to find a way to make iron behave in a way that matched the slow speeds recorded by the Apollo instruments.
A team of scientists has now proposed a solution that changes the recipe of the lunar core. By recreating the high-pressure and high-temperature conditions found deep inside the Moon in a laboratory, they discovered that iron mixed with a small amount of carbon undergoes a surprising transformation. When pure iron is squeezed to the pressures found in the Moon's core, it is expected to be a solid metal that transmits seismic waves quickly. However, the researchers found that when iron is alloyed with just a tiny fraction of carbon, it undergoes a structural shift that causes seismic waves to slow down dramatically. This change happens without the iron changing its basic crystal shape, but the internal arrangement of atoms shifts just enough to reduce the speed of sound traveling through it. This specific behavior, which the researchers call an isostructural phase transition, occurs at pressures between 4 and 5 gigapascals, a range that perfectly matches the depth of the Moon's inner core.
The experiments involved synthesizing iron samples with carbon contents ranging from 0.2 to 1.7 percent and subjecting them to pressures up to 7 gigapascals and temperatures as high as 1300 Kelvin. Using advanced X-ray diffraction and acoustic velocity measurements, the team observed that as the pressure increased, the speed of the compressional waves dropped by about 1 kilometer per second. This drop brought the speed of sound in the iron-carbon alloy down to roughly 4 to 4.5 kilometers per second, a figure that aligns almost perfectly with the seismic data collected from the Apollo era. The researchers also found that this transition is reversible and occurs in both the body-centered cubic and face-centered cubic forms of iron, suggesting it is a robust feature of iron-carbon mixtures under these conditions. Crucially, the study ruled out the idea that the slow speeds were caused by the presence of sulfur or nickel, elements that were previously thought to be the main light elements in the lunar core, as their effects did not match the observed seismic data.
This discovery suggests that the Moon's inner core is not made of pure iron, but rather an alloy of iron and carbon. The presence of carbon explains why the seismic waves move so slowly, resolving a decades-old conflict between mineral physics and seismology. The findings also allow scientists to estimate the temperature of the lunar core more accurately. Because the speed of sound in this iron-carbon mixture is sensitive to heat, the data indicates that the temperature of the inner core is likely below 1500 Kelvin. If the core were significantly hotter, the seismic speeds would be even lower than what was observed. This temperature constraint helps refine models of how the Moon has cooled and evolved since its formation. Furthermore, the study supports the idea that carbon is abundant enough in the lunar core to drive this transition, a conclusion backed by calculations of how carbon distributes between the Moon's solid inner core and its liquid outer core.
The implications of this work extend beyond just the structure of the Moon. By confirming the existence of a solid inner core composed of iron-carbon alloy, the study provides a new foundation for understanding the Moon's thermal history and its ancient magnetic field. The presence of a solid inner core is a key driver for the dynamo effect that generates magnetic fields, and knowing its composition helps explain why the Moon's magnetic field faded over time. The researchers emphasize that their results are based on direct measurements of acoustic velocities and X-ray diffraction patterns, offering a concrete physical explanation for the seismic anomalies that have long confused the scientific community. While the exact distribution of carbon in the lunar interior remains a subject for further study, the evidence presented here strongly points to a carbon-rich, solid inner core as the missing piece of the lunar puzzle. This finding does not just fill a gap in a model; it rewrites the physical description of the Moon's heart, turning a theoretical possibility into a measured reality.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.