Determining the mantle composition of Venus: a preliminary investigation using thermodynamic modelling

Venus is often referred to as the Earth’s twin due to its similar size and mass. While Venus surface is young (~750 Myr old on average), there is no direct evidence for large tectonic plates as we have on Earth. Most of the surface is thought to be basaltic in composition, although some regions, the so-called tessera, that cover about 8% of Venus’ surface have been suggested to be more felsic, potentially resembling continental crust on the Earth (Gilmore, 2015). The crustal composition carries valuable information about planetary differentiation processes and can be used to constrain the interior composition. Several mantle compositions have been proposed based on the evolution of the Solar System (condensation (e.g., Lewis, 1972) and chondrite models (Morgan and Anders, 1980)) or based on the similarities and differences between Venus and Earth (e.g., difference in mean density and similarity in radius (BVSP, 1981)). However, it is yet to be assessed if these mantle compositions can reproduce the composition of Venus surface rocks.

 

During the 1980’s, three landers (Venera 13 and 14, and Vega 2) successfully completed missions on the surface of Venus, collecting and analysing surface rock material (Surkov et al., 1984, 1986). The surface rock compositions were determined using X-ray fluorescence and then compared to a suite of Earth rocks analysed in the same way. The results show that Venus’ surface is best described as consisting of alkaline basalts (Venera 13) and tholeiitic basalts (Venera 14 and Vega 2) (Fegley, 2014). However, the data compiled from these missions contain large uncertainties and several elements, such as Na, have not been measured (Treiman, 2007). Although these analyses are limited, they remain the most accessible data we have about Venus’ surface, as there have been no further surface-based missions to Venus and there are no known Venus meteorites.

 

A combination of thermodynamic modelling with the Venera and Vega lander data can be used to constrain the composition of Venus’ mantle. Perple_X allows for primary melt compositions to be calculated using a combination of thermodynamic data and Gibbs energy minimisation (Connolly, 2005). Hence, in this study, proposed Venus mantle compositions (BVSP, 1981) and a terrestrial mantle composition (Davis et al., 2009) will be used to generate partial melt compositions in Perple_X (Connolly, 1990, 2005, 2009) in conjunction with the internally consistent database described in Holland, Green, and Powell (2018, 2022). These partial melt compositions will be compared with the Venera 14 and Vega 2 lander data (Treiman, 2007) with a focus on the abundance of major elements SiO₂, Al₂O₃, FeO, MgO, and CaO. Further investigations will evaluate if fractional crystallisation may have played a role in the generation of Venera 14 and Vega 2 rocks.

 

Our models are a necessary step towards understanding the interior of Venus, its magmatic differentiation, and the link to surface composition. Combined with thermal evolution models (Herrera et al., this meeting), our work will provide a useful basis for the interpretation of future measurements of VERITAS and EnVision missions, that aim to characterize the types of rocks and minerals on Venus’ surface with unprecedented detail.

 

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