An alternative view of Earth’s beginnings

Friday, 19 December 2014: 2:55 PM
Anne M Hofmeister and Robert E Criss, Washington University in St Louis, St. Louis, MO, United States
Earth’s current compositional structure and thermal state are linked to the conditions of its formation and differentiation, which can be deduced from physical principles and thermodynamics. The fundamental, shared rotational characteristics of the Solar System, e.g. the circular, co-planar orbits and upright axial spins, clearly record conditions of origin, yet are not explained by disk models. Current planetary spin and orbital rotational energies (R.E.) each nearly equal gravitational self-potential of formation (Ug), revealing mechanical energy conservation. We derive -ΔUg =Δ.R.E. from thermodynamics. Our conservative 3-d model, which allows for different behaviors of gas and dust, explains key Solar System characteristics (spin, orbits, gas giant compositions) and second-order features (dwarf planets, comet mineralogy, satellite system sizes). Gravitational heating during accretion, core formation and differentiation is insignificant because Ug is negative. Most heat production is due to internal radioactivity, and to a lesser extent by surficial impact heating. Core formation produces order and rotational energy associated with differential rotation, not internal heat.

Earth’s beginning involved cooling via explosive outgassing of substantial ice (mainly CO) that was accreted with dust. High carbon content is inconsistent with present models yet is clearly expected from Solar abundances and ice in comets, and this profoundly affected compositional zonation. Reaction of CO with metal provided a carbide-rich core while converting MgSiO3 to olivine via oxidizing reactions. Differentiation and formation of the core and lower mantle greatly cooled the Earth, as outgassing occurred, magmas rose, and radioactive elements were segregated upwards. Reference conductive geotherms, calculated using accurate and new thermal diffusivity data, require that heat-producing elements are sequestered above 670 km; this condition limits convection to the upper mantle. Coupling our estimate for heat producing elements with meteorite data indicates that Earth’s oxide content has been greatly underestimated. Density sorting segregated a Si-rich, peridotitic upper mantle from a refractory, oxide lower mantle with high Ca, Al and Ti contents, consistent with diamond inclusion mineralogy.