Interior Evolution Models as Link between Planetary Composition and Structure

Kurzfassung

Large scale melting by giant collisions is an inevitable consequence of terrestrial planet formation by planetesimal accretion. Hence not only terrestrial planets in our solar system but also the majority of terrestrial exoplanets might have experienced a magma ocean (MO) phase characterized by global and maybe even complete melting of their silicate fraction. The process of MO solidification is linked to the differentiation of the planetary interior into different chemical reservoirs, that can be modified by convection and partial melting processes throughout the history of the planet. Such interior differentiation processes can be modeled to assess a) which chemical reservoirs form and b) how they are distributed in the interior at the present day. The physical properties and radial positions of individual reservoirs are directly linked to potentially observable physical properties of the bulk planet like its density or moment of inertia. Therefore, interior evolution models that simulate magma ocean solidification, solid state convection and ideally decompression melting can act as a link that connects bulk planetary composition and structure. We have applied such a modeling approach to the evolution of the Moon in order to constrain the bulk silicate Moon (BSM) FeO content, which is difficult to establish based solely on petrological arguments. We first used petrological modeling to study the effect of BSM FeO content on the properties of chemical reservoirs in the lunar mantle that were formed during lunar magma ocean solidification. In a second modeling step, we considered the effects of solid state convection on the distribution of these reservoirs in the lunar interior. The results are used to test the consistency of different BSM FeO contents and mantle convection scenarios with the bulk Moon density and BSM moment of inertia. Combining current estimates of the lunar core properties and today’s selenotherm with our lunar interior models, we find that BSM FeO contents of 8 - 13.5 wt% are consistent with the observed bulk Moon properties. Further constraints on the lunar interior structure from seismic and selenodetic data (suggesting e.g. the presence of a dense, partially molten zone at the core mantle boundary [1]) indicate that BSM FeO contents of 9 - 11 wt% are most probable. This estimate could be further limited by tighter constraints on the size and density of the lunar core, e.g. by future seismic investigations. [1] Matsumoto et al. (2015), GRL, 42 (18), 7351-7358