Magma ocean formation due to giant impacts
Magma ocean formation due to giant impacts
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DOI：
10.1029/92je02726
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发表时间：
1993-03
影响因子：
--
通讯作者：
W. Tonks;H. Melosh
中科院分区：
文献类型：
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作者：
W. Tonks;H. Melosh
目前对行星吸积最后阶段的理解表明，质量和能量的积累是由几次大的撞击所主导的。这种撞击的一个重要的热后果是融化并形成一个大的熔池。如果地球的均衡调整时间尺度比岩浆池的冷却时间尺度短，熔体可能会挤出到地表，形成深度大致相同的岩浆海洋。虽然在10-15公里处撞击的巨大撞击S−1积累了足够融化整个地球的能量，但这种能量的分布对于决定碰撞的热结果是重要的。我们通过估计初始冲击波产生的熔体体积和相应的岩浆海洋深度来研究巨型撞击的热效应。此外，我们还考察了行星初始温度对产生的熔体体积的影响。在压力-熵空间中绘制的Hugoniot曲线被用来确定完全熔化材料所需的冲击压力。对于室温纯橄榄岩，这一压力约为150 Gpa，部分熔融区较窄。对于初始固相线温度下的纯橄榄石，这一压力约为115 Gpa。部分融化区域延伸到整个星球。一旦熔化压力已知，基于第二Hugoniot方程、冲击波-质点线性速度关系和经验质点速度-距离关系的冲击熔化模型被用来估计发生在撞击现场的径向距离熔化。找到距离后，熔体区域的几何图形将确定关联的熔体体积。还估计了部分熔融体积。估计了火山口形成过程中未被挖掘的熔体比例，并计算了被挖掘和保留的熔体所产生的岩浆海洋深度。该模型还被用来估计被初始冲击波融化的行星的比例。形成月球的巨型撞击的标称条件(射弹/行星质量=0.14，撞击速度=15公里S−1)根据其初始温度产生30-65%的地球熔化。对于15公里的S−一号撞击，如果行星在撞击前接近其固相线，则整个行星的融化需要抛射/行星质量比为>；0.4。均衡松弛可能会产生大量的额外熔化。
Current understanding of the last stages of planetary accretion suggests that mass and energy accumulation are dominated by a few large impacts. An important thermal consequence of such impacts is melting and formation of a large melt pond. If the planet's isostatic adjustment time scale is short compared to the magma pond's cooling time scale, the melt may be extruded onto the surface and form a magma ocean of approximately uniform depth. Although a giant impact striking at 10–15 km s−1 deposits enough energy to melt the entire planet, the distribution of that energy is important in determining the thermal outcome of the collision. We examine the thermal effects of giant impacts by estimating the melt volume generated by the initial shock wave and corresponding magma ocean depths. Additionally, we examine the effects of the planet's initial temperature on the generated melt volume. The Hugoniot curve plotted in pressure-entropy space is used to determine the shock pressure required to completely melt the material. For room temperature dunite, this pressure is about 150 GPa and the partial melt region is narrow. For dunite initially at the solidus temperature, this pressure is about 115 GPa. The partial melt region extends throughout the planet. Once the melting pressure is known, an impact melting model based on the second Hugoniot equation, the linear shock-particle velocity relationship, and the empirical particle velocity-distance relationship is used to estimate the radial distance melting occurred from the impact site. Once the distance is found, the melt region's geometry determines the associated melt volume. Partial melt volume is also estimated. The melt fraction that is not excavated during crater formation is estimated and magma ocean depths resulting from both excavated and retained melt are calculated. The model is also used to estimate the fraction of a planet melted by the initial shock wave. Nominal conditions of the Moon-forming giant impact (projectile/planet mass = 0.14, impact speed = 15 km s−1) generate melting of 30–65% of the planet depending on its initial temperature. Whole planet melting requires projectile/planet mass ratios of > 0.4 for a 15 km s−1 impact if the planet was near its solidus before the impact. Isostatic relaxation may generate a significant volume of additional melting.