Florida: A comprehensive study conducted by NASA has revealed that the Moon's inner core is a solid mass with a density comparable to iron. This discovery could finally resolve the long-standing debate about the state of the Moon's core—whether it is solid or molten—and enhance our understanding of the Moon's history, and by extension, the history of the Solar System.
Led by astronomer Arthur Briaud of the French National Centre for Scientific Research, the research team stated that their findings challenge previous assumptions about the Moon's magnetic field and support the theory of a global mantle overturn. This theory provides crucial insights into the timeline of lunar impacts during the Solar System's first billion years.
Understanding the interior composition of celestial bodies like the Moon is best achieved through seismic data. The way seismic waves generated by quakes travel through and reflect off materials inside a planet or moon allows scientists to create detailed models of the interior structure. Although lunar seismic data from the Apollo missions exists, its resolution is too low to definitively determine the state of the Moon's inner core. The data supports both models of a solid inner core and a fully fluid core, leaving the matter unresolved.
To address this, Briaud and his colleague’s analysed data from various space missions and lunar laser-ranging experiments. They compiled a profile of the Moon's characteristics, including its deformation due to Earth's gravity, variations in its distance from Earth, and its density.
Their findings are significant. The models that most accurately reflect current knowledge of the Moon suggest active overturn within the lunar mantle, where denser material sinks toward the centre, and less dense material rises. This process may explain the presence of certain elements in volcanic regions on the Moon, adding to the evidence supporting this theory.
Furthermore, the researchers discovered that the Moon's core shares similarities with Earth's core, featuring a fluid outer layer and a solid inner core. According to their models, the Moon's outer core has a radius of about 362 kilometres (225 miles), while the inner core measures approximately 258 kilometres (160 miles) in radius, accounting for about 15% of the Moon's total radius. The inner core's density is around 7,822 kilograms per cubic meter, closely matching that of iron.
Interestingly, a 2011 study led by NASA Marshall planetary scientist Renee Weber found similar results using advanced seismological techniques on Apollo data. Their research also indicated a solid inner core with a radius of about 240 kilometres and a density of approximately 8,000 kilograms per cubic meter.
Briaud and his team's results confirm these earlier findings and strengthen the case for an Earth-like core within the Moon. This has important implications for understanding the Moon's evolution. Shortly after its formation, the Moon had a strong magnetic field, which began to decline around 3.2 billion years ago. This magnetic field is believed to have been generated by motion and convection within the core, making the core's composition critical to understanding why the magnetic field eventually disappeared.