Devolatilization During the Formation of Rocky Planets: Bulk Elemental Composition

Abstract

As the solar nebula condensed, evaporated and fractionated to form the nascent Earth, the bulk elemental composition of the Earth was established. To first order, the Earth is a devolatilized sample of the solar nebula. Similarly, rocky exoplanets are also likely devolatilized samples of the stellar nebulae out of which they and their host stars formed. If this assumption holds, we can estimate the chemical composition of rocky exoplanets by applying a devolatilization algorithm based on the elemental abundances of their host stars. This thesis is an investigation of this potentially universal devolatilization pattern, from which exoplanetary chemistry and habitability are then derived. To quantify (in broad terms) the chemical relationships between the Earth, the Sun and other bodies in the Solar System, the elemental abundances of the bulk Earth are required. The key to comparing Earth’s composition with those of other objects is to have a determination of the bulk composition with an appropriate estimate of un- certainties. We present concordance estimates (with uncertainties) of the elemental abundances of the bulk Earth, by compiling, combining, and renormalizing a large set of heterogeneous literature values of the primitive mantle and of the core. The weighting factor for the concordance estimates comes from our new estimate of the core mass fraction of the Earth: 32.5±0.3 wt% (weight percent). The uncertainties on our elemental abundances usefully calibrate the unresolved discrepancies between standard Earth models made under various geochemical and geophysical assump- tions. We then extend our assessment of terrestrial abundances to the modeling of pro- tosolar abundances based on the latest estimates of solar photospheric abundances and primitive meteoritic abundances. We compare our new protosolar abundances with our estimates of bulk Earth composition, thereby quantifying the devolatiliza- tion of the solar nebula that led to the formation of the Earth. As a function of elemental 50% condensation temperatures (TC), we fit the Earth-to-Sun abundance ratios f to the linear trend log( f ) = a log(TC ) + b. The best fit coefficients are: a = 3.676 ± 0.142 and b = 11.556 ± 0.436. The quantification of the slope a provides an empirical observation upon which modeling of the devolatilization processes can be based. These coefficients determine a critical devolatilization temperature for the Earth TD(E) = 1391 ± 15 K. The resultant devolatilization pattern allows inferences to be made concerning the depletions of elements in the early solar system and is potentially useful for estimating the chemical composition of rocky exoplanets from their known host stellar abundances. We apply the devolatilization pattern to nearby planetary systems to infer the bulk elemental composition of rocky exoplanets – particularly those within the cir- cumstellar habitable zones – from the known host stellar elemental abundances. The estimated bulk planetary composition (rather than the host stellar abundances) is then used as a principal constraint to model the interior composition and structure of such exoplanets. We apply these constraints to four planet host stars: Kepler-10, Kepler-20, Kepler-21 and Kepler-100, to model the interiors including the mantle and core compositions as well as core mass fraction for potential terrestrial exoplanets orbiting these host stars. With respect to the estimates of the interiors, we conclude that a potential terrestrial exoplanet orbiting Kepler-21 would be the most Earth-like while one orbiting Kepler-10 would be the least. Show more