The core radius of Mars: a historical perspective

In August 2021, the first seismological determination of the core radius of Mars was published by the InSight team (Stähler et al., 2021). We take this opportunity to take a mental step backwards and assume a historical perspective on the scientific investigation of planetary cores, and how our knowledge about them, especially in terms of their size, evolved.

The first thoughts about the Earth's interior that we would place into the history of science or rather into that of religion originate in the 17th century. Descartes suggested that the Earth is a former star which produced so many sunspots that it became encrusted in them, and that later processing of sunspot-material resulted in the surface we have today. The innermost part of the Earth, however, is still unaltered solar matter.

Isaac Newton, in the posthumously published "System of the World", suggested that the gravity of a single, isolated mountain could be used to determine the density ratio between the surface and the interior of the Earth. Respective experiments were conducted by Bouguer and, later, by Maskelyne and Hutton - the latter concluded from the result, that the presence of a heavy, metallic core could explain the overall mass of the Earth as well as the density contrast resulting from Newton's experiment. A metallic core was also suggested by Wiechert, by the end of the 19th century. When Oldham demonstrated the S wave core shadow in 1906, he did not make any suggestions about the nature of the central region.

In the early 20th century, it was however doubted that any material could withstand the conditions of the deep interior, or that a segregation of metals could take place. One alternative approach was indeed solar matter, another one a metallic high pressure state of silicate rock: The possibility that the core mantle boundary is a phase boundary like the 410 and 660 km discontinuities was long supported by some.

The consequence of the phase boundary model was that neither the Moon nor Mars could have cores, for the simple reasons that they are too small to provide the necessary internal pressures. This claim fit well with the moment of inertia factors as they were observed back then: Until the mid-1960s it was assumed that the MoIF of the Moon exceeds 0.4, and that of Mars is too close to 0.4 to indicate much differentiation.

In both cases, the space age led to a revision: Having spacecraft near or at the respective bodies turned out to be crucial for a sufficiently precise determination of the MoIF.

In the case of Mars, the Mariner IV mission greatly improved the knowledge of radius, mass, and moment of inertia factor of the planet (because of the atmosphere, even the radius was rather uncertain and observational results depended on the optical wavelength used in photography). After Mariner IV, an iron core suddenly became feasible, if not necessary, again. Mariner VI and VII showed that Mars is neither a small Earth nor a big Moon, but something different - and the global photographic map resulting from the Mariner IX mission showed all the now familiar surface structures for the first time. With Mariner IX it also became possible to map the surface gravity, and the gravity anomaly of Tharsis was discovered - which is so enormous that it biases the J₂ gravity coefficient, and invalidates the previously used hypothesis of hydrostatic equilibrium. Several methods to compensate for this were suggested in the following. A replacement for the hydrostatic assumption became available with precession measurements using Viking radio signals, later augmented by Pathfinder and other missions. It became finally possible to determine the MoIF, which turned out to be significantly below that of the homogeneous sphere. The most significant progress in terms of the estimation of the core radius was however Mariner IX: After this mission, core radii below 1000 km were no longer discussed.

The Viking missions produced important clues for the identification of the SNC meteorites as of martian origin, and thus for improved models of the chemical composition of Mars. This provided better contraints for the densities of core and mantle. A comparison of the core radii discussed in the literature after Viking however shows that none of these models could constrain the core radius with a sufficient precision. Different models were developed, but in the long run, the range of uncertainty of the core radius proved rather stable for more than 40 years.

The results obtained by InSight still build upon the knowledge of geodetic and gravity measurements as well as on geochemistry, but they add seismic data as constraints that are more sensitive to the sought-after structural parameters than to density.

References

Listing all references relevant for the above text would require much more space than is available here. The discussion of the abstract is a condensate from Knapmeyer & Walterová (2022), where all the references can be found.

Knapmeyer, M., Walterová, M. (2022), Planetary Core Radii: From Plato towards PLATO, under review at Advances in Geophysics.

Stähler et al., (2021). Seismic detection of the martian core, Science, vol. 373, 443-448, DOI: 10.1126/science.abi7730