The iron-nickel metal cores of terrestrial planets decrease in size relative to the size of the planets, with increasing distance from the Sun. Mercury, the planet closest to the Sun, has a nucleus that represents almost 75% of its mass. Earth and Venus have nuclei representing about 33% of their mass, while Mars has a nucleus representing only about 20% of its mass. This same range in the proportion of metal to silicate material appears in chondritic meteorites, some having high abundances and others having relatively low abundances of metal. The metal concentrations required to form Mercury or the metal-rich chondrites are too high to be explained by the higher temperatures of the solar nebula near the Sun. Instead, another physical or chemical process must have enriched the metallic iron in the inner solar system during its early evolution.
Research by William McDonough of the University of Maryland and Takashi Yoshizaki of the University of Tohoku (Japan) suggests that this composition gradient may have been driven by the early Sun’s magnetic field. Their modeling shows that when particles condense from a nebular gas, the iron-rich particles are affected by the magnetic separation of the silicate particles. This would create a magnetic iron enrichment that would decrease with increasing distance from the Sun as the strength of the solar magnetic field weakened. Planets that formed closer to the Sun would therefore be made up of more metal compared to silicate, resulting in the larger core fractions observed. McDonough and Yoshizaki estimated the strength of the early Sun’s magnetic field relative to the time scale of planetary accretion. They determined that this was consistent with magnetic measurements taken from chondrites that would have formed at a similar distance from the Sun as the terrestrial planets. Since the presence of a protective magnetosphere around a planet, created by its convective metallic core, appears to be a prerequisite for habitability, this research could have important implications for the formation and existence of habitable zones in d ‘other solar systems. READ MORE