Planetary core radii: from Plato towards PLATO

Abstract

The boundary between rocky mantle and iron core constitutes the most significant discontinuity within the terrestrial planets, the core itself is one of the largest, if not the largest, structural features of these planets with consequences for the entire geodynamical evolution of the planet: It contains a significant amount of the planets iron inventory, and planetary magnetic fields are generated within the core. We take the occasion of the first seismic determination of the core size of Mars to look back into the development of theories about planetary interiors and cores, starting with early mythological narrations. The renaissance produced the first geologically and physically motivated inferences about the Earth's core, which were extended to the Moon, Mars and other planets in the 19th century. Theories based on telescopic observations soon found their limits, and spacecraft missions to the Moon and to Mars provided the necessary precision of radius, mass, and moment of inertia determinations, and finally seismic data, to determine the core radius precisely. Meanwhile, interest extended to beyond the solar system, and we discuss the observational foundations on which models for the core size of exoplanets are based.