Thermal Expansion of SiC in the Deep Interiors of Carbide Exoplanets
Abstract
Recent astrophysical studies have discovered that some planet-hosting stars have very high carbon-to-oxygen ratio (Bond et al., 2010). These stars may host carbon-rich planets rather than silicate planets. Nebular condensation models predicted that some of these carbonrich planets may have the mantles made mainly of silicon carbide (SiC) rather than silicates. These identifications are based on mass-radius relation derived from measurements only up to 43 GPa at room temperature. The study of the equation of state of SiC and its phase transition at high pressure-temperature is therefore important for identifying carbon-rich exoplanets and understanding the structure and dynamics of the mantles in carbon-rich exoplanets.
We have conducted in situ synchrotron X-ray diffraction experiments on SiC in the laser-heated diamond-anvil cell which allow us to achieve the pressure and temperature conditions of the deep planetary interiors. We collected data for the hexagonal (SiC-6H) and the cubic (SiC-3C) phases up to 60 GPa and 2000 K. We fit the unit-cell volume data to the thermal equation of state. The thermal expansion parameter calculated from the equation of state of SiC-6H is shown on the figure below. The thermal expansion parameter of SiC-6H is a factor of 3-4 lower than that of MgSiO3 perovskite (the dominant phase in Earth's lower mantle) at lower pressures. However, we found that pressure does not affect the thermal expansion of SiC much that its thermal expansion becomes comparable to that of MgSiO3 perovskite at 60 GPa. Therefore, while the buoyancy of thermal heterogeneities may be much smaller in shallow depths in the carbide mantle than in the silicate mantle, our new experiments open possibilities for comparable or even stronger thermal convection in the deep carbide mantle.- Publication:
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Search for Life Beyond the Solar System. Exoplanets, Biosignatures & Instruments
- Pub Date:
- March 2014
- Bibcode:
- 2014ebi..confP2.19N