Earth's core could be much cooler than previously thought

The Earth's liquid outer core may be significantly cooler than previously believed, according to new experimental findings that could resolve a decades-old mystery surrounding the planet’s magnetic field. The results, presented recently at the Goldschmidt Conference, a major international gathering in geochemistry, have sparked debate among scientists. The study, led by Michael Walter of the Carnegie Institution for Science, challenges existing assumptions about the temperature at which iron melts under the extreme pressures found near the boundary between the solid inner core and the liquid outer core. The Earth’s core plays a crucial role in sustaining life on the surface. Composed largely of molten iron, the outer core generates the planet’s magnetic field through convective movements of this liquid metal. This magnetic shield protects the atmosphere from solar winds, making it essential for maintaining conditions suitable for life. However, understanding how this geodynamic engine has functioned over billions of years remains a challenge for geophysicists. The structure of the Earth consists of a solid inner core, roughly the size of the Moon, surrounded by a liquid outer core, comparable in size to Mars. The movement of molten iron within the outer core creates electric currents, which in turn produce the magnetic field. This process, known as the geodynamo, is primarily driven today by the slow crystallization of the inner core. As the inner core solidifies, heat and lighter elements are released, further fueling convection in the outer core. Despite these mechanisms, a long-standing puzzle persists. Many models suggest that the solid inner core began forming less than half a billion years ago. Yet, geological evidence indicates that the magnetic field has existed for at least four billion years. Scientists have referred to this discrepancy as the “new core paradox”, a question of what sustained the geodynamo during the three-and-a-half billion years before the inner core started to form. To address this issue, researchers must determine the exact temperature of the Earth’s core. A key factor is the melting point of iron under the immense pressures present deep within the planet. Since the inner core is solid, its temperature must be below the melting point of iron at those depths. Previous studies estimated temperatures above 6000 kelvin at the boundary between the inner and outer cores, equivalent to more than 5726.85 degrees Celsius. Walter and his team developed a novel method to measure the temperature of iron in the outer core with greater precision. Over five years, they refined their approach using diamond anvils to apply pressures of nearly two million atmospheres, conditions similar to those in the outer core. Unlike earlier experiments, which used lasers to heat the sample, the new technique employed extremely short electrical pulses. This allowed for more accurate temperature control while observing a unique phenomenon: when iron begins to melt, its temperature temporarily stabilizes despite continued energy input. The results revealed a much lower melting point for iron under these conditions. According to the new measurements, pure iron would begin to melt at around 4420 kelvin at the boundary with the inner core. This finding suggests that the outer core may be significantly cooler than previously thought, potentially altering current models of how the geodynamo has operated throughout Earth’s history. These findings could help explain how the magnetic field persisted for so long without the formation of the inner core. If the outer core was indeed cooler, it might have remained active even before the inner core began to crystallize. Further research will be needed to confirm these conclusions and integrate them into broader theories of planetary evolution.