Astrophysical measurements have revealed that certain stars possess sufficiently high carbon-to-oxygen (C/O) ratios, suggesting that the planets orbiting these stars could be primarily composed of carbides rather than silicates. Such carbon-rich planets may exist in solar systems with either stars exhibiting high C/O ratios or proto-planetary disks with locally elevated C/O ratios. Despite their potential prevalence, comprising no more than 12%–17% of planetary systems, there is limited understanding of the surface and interior structures of these planets.
Recent experimental research has shed light on this topic through investigations of silicon carbide (SiC) and water (H₂O) mixtures under extreme conditions using a laser-heated diamond-anvil cell (LHDAC). The study demonstrates that silicon carbide reacts with water to form silica (SiO₂) and diamond at pressures up to 50 GPa and temperatures up to 2500 K:
SiC + 2H2O -> SiO2 + C + 2H2
This implies that if water is incorporated into carbide planets during their formation or delivered later, these planets could be oxidised, resulting in interiors predominantly composed of silicates and diamonds. In the experiment, pure synthetic SiC was mixed with gold powder (10 wt%) to serve as a laser coupler and pressure calibrant. Samples were compressed to pressures between 20 and 40 GPa at 300 K before being subjected to laser heating. A total of 18 LHDAC runs were conducted.
The LHDAC operates on a piston-cylinder mechanism, with each half containing a diamond with a flat tip. Pressure is generated between the two opposing diamonds by pressing on a sample encased within a metallic gasket. Due to the transparency of diamonds, it is possible to access the sample using a wide range of radiation energies, from infrared (IR) to X-rays, for both spectroscopic analysis and heating. This capability allows for an extensive range of pressures and temperatures within the laser-heated diamond anvil cell.
X-ray diffraction (XRD) patterns were collected at high pressures and temperatures, using wavelengths of 0.4133 Å or 0.3344 Å focused on the sample. The XRD patterns consistently showed the conversion of SiC into SiO₂ stishovite across the entire pressure-temperature range. For instance, at 40 GPa before heating, only peaks from SiC-6H and H₂O ice VII (the starting mixture) along with gold were observed. As heating commenced, the 110 diffraction peak at 2.8 Å became immediately visible, followed by other diffraction lines such as the 101, 111, 210, 211, and 220 peaks after approximately 5 minutes of heating. In all LHDAC experiments, the stishovite peaks continued to grow throughout the run, indicating that stishovite remains stable over SiC in the presence of H₂O. Data from samples heated to 1800 K at 42 GPa, and subsequently decompressed to 1 bar, showed that all phases observed at high pressure remained at 1 bar, including the diamond 111 line.
Although not the focus here, Micro-Raman measurements were conducted during DAC runs 14–17 at Arizona State University (ASU). Both XRD and Raman observations support the reaction between SiC and H₂O, demonstrating that water can react with SiC to produce silica and diamond at high pressures and temperatures. This finding suggests that the presence of water during the formation of carbide planets, or its later delivery, can induce such reactions. If water is delivered to carbide planets, the resulting high pressures and temperatures could trigger these reactions locally. In regions of the mantle where water reaches SiC, the reaction would produce diamond and silica. Consequently, a carbide planet would undergo a chemical transformation from the exterior in, potentially resulting in a surface covered with silica and, at greater depths, a combination of diamond and silica.
Additionally, at pressures below the stability of diamond, the reaction likely produces methane. Therefore, at shallower depths and lower temperatures, methane and hydrogen may be produced in hydrated carbon-rich planets. These gases could be released from the planet’s interior, leading to atmospheres rich in reducing gases, which might be crucial for the emergence of life. The conversion of mineralogy to diamond and silicates would also decrease the density of the carbon-rich planet. Overall, the findings provide insights into how carbide planets might readily convert to silicate planets in the presence of water, suggesting that the number of such carbon-rich planets may be even lower than current predictions. The unique mineralogy of these converted planets would render them distinctly un-Earth-like.
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