在冲击飞溅物中形成球粒- II挥发物保留
Forming chondrules in impact splashes - II Volatile retention
摘要：
解决陨石球粒的起源之谜是陨石学领域最难以实现的目标之一。最近，行星碰撞释放熔岩滴飞溅的想法，长期以来被认为是失宠的，已经被几篇论文重新考虑为球粒的可能起源。这一想法的主要问题之一是缺乏定量和简单的模型，可以用来通过直接与许多已知的陨石球粒观测数据进行比较来测试这一情景。在本系列的第一篇论文中，我们提出了一个简单的热演化模型，该模型假设两个微行星之间的碰撞产生了一个球形对称的熔岩滴云。在本文中，本系列的第二篇，我们使用这个模型来计算挥发性元素，如Na和K是否会在这些液滴中保持丰富，或者它们是否会由于蒸发而耗尽。液滴云的高密度（例如，相邻液滴之间的小距离）导致蒸汽快速达到饱和压力，从而停止进一步蒸发。我们显示在何种程度上，在何种条件下，这保持了这些元素的丰度高，如在陨石球粒。我们发现，对于我们的模型的大多数参数（云质量，膨胀速度，初始温度）的挥发性元素Mg，Si和Fe仍然完全在球粒。由于蒸汽压的饱和，液滴内部的Na和K丰度最初将大部分停留在其初始值，但在某个点处由于云膨胀而开始下降。然而，一旦温度开始下降，大部分或全部蒸汽再次冷凝。最后，Na和K元素保留了它们最初的大部分丰度，尽管偶尔会有所减少，这取决于膨胀云模型的参数。
英文摘要：
Solving the mystery of the origin of chondrules is one of the most elusive goals in the field of meteoritics. Recently the idea of planet(esimal) collisions releasing splashes of lava droplets, long considered out of favor, has been reconsidered as a possible origin of chondrules by several papers. One of the main problems with this idea is the lack of quantitative and simple models that can be used to test this scenario by directly comparing to the many known observables of chondrules. In Paper I of this series we presented a simple thermal evolution model of a spherically symmetric expanding cloud of molten lava droplets that is assumed to emerge from a collision between two planetesimals. In the present paper, number II of this series, we use this model to calculate whether or not volatile elements such as Na and K will remain abundant in these droplets or whether they will get depleted due to evaporation. The high density of the droplet cloud (e.g. small distance between adjacent droplets) causes the vapor to quickly reach saturation pressure and thus shutting down further evaporation. We show to which extent, and under which conditions, this keeps the abundances of these elements high, as is seen in chondrules. We find that for most parameters of our model (cloud mass, expansion velocity, initial temperature) the volatile elements Mg, Si and Fe remain entirely in the chondrules. The Na and K abundances inside the droplets will initially stay mostly at their initial values due to the saturation of the vapor pressure, but at some point start to drop due to the cloud expansion. However, as soon as the temperature starts to decrease, most or all of the vapor recondenses again. At the end the Na and K elements retain most of their initial abundances, albeit occasionally somewhat reduced, depending on the parameters of the expanding cloud model.
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