Is Pluto an Iron-rich Planet?

Abstract

ANY plausible theory about the origin of the solar system must explain, among other known facts, the chemical fractionation of the planets. The terrestrial planets (of densities 3.9–5.5 g cm⁻³) are principally characterized by Fe, Mg, Si and oxygen, whereas some of the outer planets, of densities 0.7–1.6 g cm⁻³ and rich in hydrogen, helium and other volatiles, may have retained the elemental composition of the original solar nebula. Hoyle and Wickramasinghe^1,2 have suggested that the planets were formed in a hot solar centrifuge³, and that whereas Jupiter and Saturn have the composition of the original solar nebula, the deficiency of hydrogen in Uranus and Neptune arises because the peripheral speed of hydrogen molecules is equal to the escape velocity from the solar system for these orbital distances. Further, the compositions of the more dense Fe-bearing inner planets may be caused essentially by thermal fractionation^1–3, because refractory MgO, Fe and SiO₂ of the principal elements and possible molecules condense first at ∼ 1500 K. The composition of Pluto is crucial to the validity of the nebular centrifuge theory, since from its position in the solar system the theory demands that Pluto not be Fe-rich. Here I show that similarities in the spectra of Pluto and Fe-bearing terrestrial silicates indicate that Pluto may well be Fe-rich.