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With a grain of salt: Variably fractionated volatile lithophile alkali metals and alkali earths in (exo)planetary contexts

by Prof. Stephen J. Mojzsis (Bayerisches Geoinstitut (BGI) Universität Bayreuth)

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Sala Jappelli (Osservatorio astronomico di Padova)

Sala Jappelli

Osservatorio astronomico di Padova
Description

Compared to Earth, moderately volatile lithophile elements (MVLEs) are strongly depleted and fractionated in the Moon. Observed and model bulk compositions of Mars, Venus and Mercury show broad similarities in MVLE to the Earth trend and plot within a ‘planetary’ field of carbonaceous (CC) and non-carbonaceous (NC) chondrites and achondrites. To understand what this means in the solar system context, I compare the ‘planetary’ trends in MVLE and refractory lithophile element (RLE) with those for the Sun (baseline), NC and CC iron meteorites, and the measured comets (Halley, 67P, Wild2). The solar-corrected log10 [K/Na] and log10 [Ca/Na] of Earth is +0.1958 and +0.7416, respectively. The Moon’s values are very different from the ‘planetary’ trend (-0.0312 and +1.6697, respectively), consistent its special history. The angrite meteorites also plot in this ‘lunar’ field. Vestoid meteorites (HEDs) plot between the ‘planetary’ and ‘lunar’ field. Extending this to exoplanets, I document the variations of the major MVLEs K+Na/Ca reported for the extended atmosphere of a hot (>2100K) jovian exoplanet (WASP-76b) orbiting an F7 star. Based on comparison with its host F7 star, jovian planets such as WASP-76B are probably stellar abundance mirrors. Thus in MVLEs and RLEs in hot exo-jupiters helps us to understand our own gas giant compositions. I reassess interpretations of the pollutants on the surfaces of some dense stellar remnants (white dwarfs). Many white dwarf pollutants are often assumed to originate from infall of rocky bodies including asteroids and disrupted planetary crusts. I show from analysis of data from the Planetary Enriched White Dwarf Database that instead many of these compositions comport with Kuiper Belt and Trans-neptunian type objects. This outcome reconciles the dynamical problem of late accretion to WDs being sourced from anywhere except the outer edges of an otherwise extinct planetary system owing to the absence of a mechanism to donate angular momentum.  

 

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