Capturing the oxidation of silicon carbide in rocky exoplanetary interiors

Abstract

Context. Theoretical models predict the condensation of silicon carbide around host stars with C/O ratios higher than 0.65 (cf. C/O Sun = 0.54), in addition to its observations in meteorites, interstellar medium and protoplanetary disks. Consequently, the interiors of rocky exoplanets born from carbon-enriched refractory material are often assumed to contain large amounts of silicon carbide. Aims. Here we aim to investigate the stability of silicon carbide in the interior of carbon-enriched rocky exoplanets and to derive the reaction leading to its transformation. Methods. We performed a high-pressure high-temperature experiment to investigate the reaction between a silicon carbide layer and a layer representative of the bulk composition of a carbon-enriched rocky exoplanet. Results. We report the reaction leading to oxidation of silicon carbide producing quartz, graphite, and molten iron silicide. Combined with previous studies, we show that in order to stabilize silicon carbide, carbon saturation is not sufficient, and a complete reduction of Fe 2`t o Fe 0 in a planetary mantle is required, suggesting that future spectroscopic detection of Fe 2`o r Fe 3`o n the surface of rocky exoplanets would imply the absence of silicon carbide in their interiors.

Experimental Constraints on Solid Nitride Phases in Rocky Mantles of Reduced Planets and Implications for Observable Atmosphere Compositions

“…While the oxidative behavior of Si 3 N 4 at mantle conditions is important when considering its possible planetary role, the data to do so is not currently available. We expect that Si 3 N 4 will continue to form at mantle conditions as long as SiC is present, and that the presence of silicates would not hinder the reaction as long as the conditions remain reducing enough to form SiC (Hakim et al, 2018), particularly since silicon nitride ceramics are made by heating silica, carbon, and nitrogen (Weiss & Engelhardt, 1910 Ambient pressure phase stability is from Siddiqi and Hendry (1985) with ambient pressure melting/decomposition temperature shown as an open triangle for SiC (Dolloff, 1960), and as an open circle for Si 3 N 4 (Ziegler et al, 1987 (Brown & Shankland, 1981); thick-thin dots (Anderson, 1981)), as well as the temperature profile calculated for a 100-year old Earth magma ocean (Abe, 1993). The colors represent the inferred regions of stability for SiC + N 2 (blue) versus Si 3 N 4 + C (green) based on our experiments as well as on ambient pressure data.…”

“…The presence of mantle nitrides may limit the amount of nitrogen available to be outgassed to the atmosphere, thus rocky planets with oxygen‐poor interiors and surface temperatures below ∼2,000 K would lack large amounts of nitrogen gas or gaseous nitrogen species as a result of this process and would instead have solid silicon nitride as the stable form of nitrogen (Figure 6). The detection of gaseous atmospheric nitrogen species, in addition to the presence of oxidizing Fe 2+ and Fe 3+ (Hakim et al., 2018), may provide a strong indication of an oxidized mantle composition. We note that planetary surface temperatures below 77 K at 1 bar would be too cold to support the presence of gaseous N, however, and so atmospheric N may only be tied to mantle redox for a surface temperature range of ∼77–∼2,000 K at 1 bar.…”

“…While the oxidative behavior of Si 3 N 4 at mantle conditions is important when considering its possible planetary role, the data to do so is not currently available. We expect that Si 3 N 4 will continue to form at mantle conditions as long as SiC is present, and that the presence of silicates would not hinder the reaction as long as the conditions remain reducing enough to form SiC (Hakim et al., 2018), particularly since silicon nitride ceramics are made by heating silica, carbon, and nitrogen (Weiss & Engelhardt, 1910). The products from such a reaction, such as carbon monoxide (Bartnitskaya et al., 1983), however, could help to constrain the atmospheric composition that we would expect to be outgassed on a reducing planet in lieu of nitrogen gas.…”

Experimental Determination of Mantle Solidi and Melt Compositions for Two Likely Rocky Exoplanet Compositions

“…Experimental igneous petrology has been used to study melting on our solar system's planets and moons and has led to insights such as redox conditions, degree of differentiation, melt compositions, location of solidi, magma sources, and planetary cooling rates for these bodies (e.g., Filiberto, 2014;Grove & Krawczynski, 2009;Kiefer et al, 2015Kiefer et al, , 2015Krawczynski & Grove, 2012;Namur et al, 2016;Putirka, 2016;Vander Kaaden et al, 2017;Wadhwa, 2008). Only recently have workers begun to apply concepts from experimental mineralogy and petrology to exoplanets, studies which thus far have focused mostly on Si-C planets and high pressure mineralogy (e.g., Daviau et al, 2019;Duffy & Smith, 2019;Hakim et al, 2018Hakim et al, , 2019Miozzi et al, 2018;Nisr et al, 2017).…”

SiO2‐SiC Mixtures at High Pressures and Temperatures: Implications for Planetary Bodies Containing SiC

“…As SiC has been proposed to be a component in planetary systems, experimentally determining the stability of SiC in the presence of other planetary materials is essential. Recent work considers the oxygen fugacity of planetary mantle assemblages that allow the formation of SiC at pressures of 1 GPa and temperatures less than 2000 K, finding that very reducing conditions are necessary for moissanite formation (Hakim et al, ). This is not surprising as previous SiC literature finds that in the presence of oxygen, oxidation reactions eat away at SiC ceramics at high temperatures and room pressure (Gulbransen & Jansson, ; Jacobson et al, ).…”