This blog post explores the scientific basis and secrets of its internal structure, based on the latest exploration data, examining why Mercury’s core remains liquid despite intense solar heat and the planet’s small size.
Mercury is the smallest planet in the solar system, with a radius of 2,440 km and a density of 5,430 kg/m³, slightly less than Earth’s. As the closest planet to the Sun, Mercury belongs to the terrestrial planets along with Venus, Earth, and Mars. It is believed to possess a hard, rocky crust and mantle, beneath which lies a dense, iron-rich core. However, precise investigation via spacecraft is essential to more accurately determine its internal structure. Due to intense solar heat and gravitational influence, access itself is difficult; to date, only two spacecraft have been sent to Mercury. The American Mariner 10 spacecraft, during its first close flyby of Mercury in 1974, detected the presence of a magnetic field. Although the magnetic field’s strength was only about 1% of Earth’s, this was a significant achievement, revealing that Mercury is the only terrestrial planet besides Earth to possess its own intrinsic magnetic field. According to the dynamo theory, which states that Earth’s magnetic field is generated by convection in its conductive liquid outer core and the effects of rotation, Mercury’s magnetic field implies that part of its core is in a liquid state. However, Mercury’s small size makes it unlikely that its iron core could remain liquid. Some theories suggest that even if it was liquid in the past, it would have solidified over time. For this reason, geologists have speculated that Mercury’s solid iron core is surrounded by a liquid core composed of iron-sulfur-silicon compounds. However, during the early exploration phase, it was also suggested that the detected magnetic field could be residual magnetism, like a magnet left within the rock after the core solidified.
The second probe, Messenger, launched in 2004, entered Mercury’s orbit in March 2011 and subsequently conducted precise measurements of gravity, magnetic fields, and terrain elevation. The moment of inertia of Mercury, obtainable from gravity data, provides crucial clues for understanding its internal structure. Moment of inertia is a physical quantity indicating an object’s resistance to changes in its rotational state; its value increases as the object’s mass is distributed farther from its rotational axis. This is analogous to the phenomenon where a flat top spins longer than a spindly top when both have the same mass. Assuming Mercury, with mass M, is an object located at a point R radii away from its rotational axis, its moment of inertia is MR². The normalized moment of inertia (C/MR²), obtained by dividing Mercury’s total moment of inertia C by MR², is a key indicator revealing the density distribution within the planet. Generally, the larger the proportion of the planet’s interior occupied by its core, the larger the normalized moment of inertia. Messenger measurements indicate Mercury’s normalized moment of inertia is 0.353, larger than Earth’s 0.331. This implies Mercury’s core radius occupies over 80% of its total radius, a significantly larger proportion than Earth’s approximately 55%.
Planets exhibit slight oscillations due to the eccentricity of their orbital paths, known as longitudinal libration. The magnitude of this libration increases as the moment of inertia decreases. This principle is analogous to how an elongated top wobbles more sensitively to external impacts than a flatter top. While the Earth’s tidal locking phenomenon is commonly known to make only one side of the Moon visible, the actual libration effect allows observation of approximately 59% of the Moon’s surface. If Mercury were completely solid like a hard-boiled egg, the entire planet would vibrate uniformly. However, if it possesses a liquid core, only the outer layer composed of crust and mantle would slide over the core like the shell of a soft-boiled egg, generating longitudinal libration. Therefore, if a liquid core exists, the magnitude of libration is inversely proportional to the outer layer’s moment of inertia Cm, not Mercury’s total moment of inertia C. Measured values of Mercury’s libration match theoretical calculations based on the outer layer’s moment of inertia Cm, providing strong observational support for the liquid core hypothesis.
Scientists have proposed a model of Mercury’s internal structure based on Messenger’s extensive observational data. According to this model, Mercury’s core radius is approximately 2,030 km, and the outer layer’s thickness is about 410 km. The topographic elevation is 9.8 km, relatively small compared to other terrestrial planets. Considering the average crustal thickness is about 50 km, this leads to the interpretation that the mantle thickness is relatively thin at about 360 km, resulting in less active orogenic movement due to mantle convection. The density of the outer layer (ρm) is 3,650 kg/m³, higher than that of Earth’s upper mantle (approximately 3,400 kg/m³). However, analysis by Messenger’s X-ray spectrometer detected almost no heavy iron components in Mercury’s volcanic ejecta, a result considered highly unusual. This result implies a low iron content within the mantle, raising the problem of how to explain the high density of the outer layer. To resolve this contradiction, researchers proposed the possibility of a thick anticrust composed of high-density iron sulfide in the lower mantle, developing a new hypothesis that its thickness could exceed that of the crust. Research since the 2020s continues to regard this model as a valid hypothesis. The BepiColombo mission (scheduled for full-scale observations after 2025) is expected to provide more precise verification of this hypothesis in the future.
