Devolatilization During the Formation of Rocky Planets: Bulk Elemental Composition
Date
2018
Authors
Wang, Haiyang
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Canberra, ACT : The Australian National University
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.
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Keywords
devolatilization, devolatilisation, volatility, volatility trend, planet formation, elemental composition, chemical composition, elemental abundance, Earth, primitive mantle, core, Sun, solar abundance, proto-Sun, protosolar abundance, condensation, evaporation, photoevaporation, meteorite, chondrite, chondrule, exoplanet, extrasolar planet, rocky exoplanet, terrestrial exoplanet, interior, interior structure, mineralogy, constraint, enhanced constraint, habitability, habitable, pale blue dot
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