The physical processes during planet formation span a large range of pressures and temperatures. Giant impacts, such as the one that formed the Moon, achieve peak pressures of 100s of GPa. The peak shock states generate sufficient entropy such that subsequent decompression to low pressures intersects the liquid‐vapor phase boundary. The entire shock‐and‐release thermodynamic path must be calculated accurately in order to predict the post‐impact structures of planetary bodies. Forsterite (Mg₂SiO₄) is a commonly used mineral to represent the mantles of differentiated bodies in hydrocode models of planetary collisions. Here, we performed shock experiments on the Sandia Z Machine to obtain the density and temperature of the liquid branch of the liquid‐vapor phase boundary of forsterite. This work is combined with previous work constraining pressure, density, temperature, and entropy of the forsterite principal Hugoniot. We find that the vapor curves in previous forsterite equation of state models used in giant impacts vary substantially from our experimental results, and we compare our results to a recently updated equation of state. We have also found that due to under‐predicted entropy production on the principal Hugoniot and elevated temperatures of the liquid vapor phase boundary of these past models, past impact studies may have underestimated vapor production. Furthermore, our results provide experimental support to the idea that giant impacts can transform much of the mantles of rocky planets into supercritical fluids.

中文翻译：

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镁橄榄石汽相边界的温度和密度

行星形成过程中的物理过程涉及很大范围的压力和温度。巨大的撞击（例如形成月球的撞击）达到了100s GPa的峰值压力。峰值激波状态会产生足够的熵，以至于随后对低压的减压会与液相汽相边界相交。必须精确计算整个冲击和释放的热力学路径，以预测行星体的撞击后结构。镁橄榄石（Mg ₂ SiO ₄）是一种常见的矿物，在行星碰撞的水力代码模型中代表微分体的地幔。在这里，我们在Sandia Z机上进行了冲击实验，获得了镁橄榄石液相汽相边界的液体分支的密度和温度。这项工作与以前的工作相结合，从而限制了镁橄榄石主Hugoniot的压力，密度，温度和熵。我们发现，在巨大撞击中使用的先前镁橄榄石状态方程中的蒸气曲线与我们的实验结果大不相同，并将我们的结果与最近更新的状态方程进行了比较。我们还发现，由于这些过去模型的主要Hugoniot熵预测不足以及液相汽相边界的温度升高，过去的影响研究可能低估了蒸气的产生。此外，我们的研究结果为巨大的冲击力可以将岩石行星的许多地幔转化为超临界流体的想法提供了实验支持。