Experimental Chondrule Formation I - Lightning Discharge Experiments under Microgravity Conditions at the International Space Station

在这个项目中，我们打算研究球粒的形成机制，球粒是太阳系最初100万年中形成的最重要和最常见的固体之一。尽管人们对其形成机制已经了解了近一百年，但科学界仍存在激烈的争论。就在最近，关于球粒形成机制的讨论得到了一些新的冲击，并重新激活了一些最有利的形成机制，如星云闪电放电（例如Johansen和Okuzumi， 2018），星云冲击波（例如Desch等人，2012；Fedkin等人，2012）和小行星碰撞（例如Sanders和Scott， 2012）。此外，关于球粒形成过程本身的几个新方面也被发表（Palme et al., 2015; Friend, 2017）。考虑到所有这些因素，在与太阳星云相关的条件下，例如在微重力下，缺乏实验球粒形成是显而易见的。这里介绍的新实验方法将在Alexander Gerst当前的太空任务期间在国际空间站的微重力条件下进行。样品容器将于2018年12月返回地球，进行详细的分析实验室研究。这个实验被接受为“乌伯弗利格”竞赛框架内仅有的三个实验之一。这里提出的建议将包括样品材料返回地球后的详细分析研究和对实验的全数字视频覆盖的评估。该项目的主要目标是：1)在微重力条件下，闪电是产生类似球粒物体的合理过程吗？2)粉尘团聚体的融化是否会促进颗粒的生长？3)形成的类似球粒的物体的结构细节是什么，它们是否与特定的已知球粒类型有关，这些球粒类型预计会在高冷却速率下形成，如隐晶、径向橄榄石或棒橄榄石球粒。4)闪电对球粒形成的地球化学影响是什么？5)结晶度是多少？6)我们是否看到相分离和/或形成其他相，如辉石，通常在球粒中发现？

With this project we intend to study the formation mechanism of chondrules, one of the most important and most frequent solids formed within the first million years of the Solar System. Although known for almost hundred years the formation mechanism is still a matter of intense scientific debate. Just recently the discussion about the formation mechanism of chondrules get several new impulses and re-activated a few of the most favorable formation mechanism like nebular lightning discharges (e.g. Johansen and Okuzumi, 2018), nebular shock waves (e.g. Desch et al., 2012; Fedkin et al., 2012) and asteroidal collision (e.g. Sanders and Scott, 2012). In addition several new aspects of the chondrule formation process itself were published (Palme et al., 2015; Friend, 2017). Taking all this into account the lack of experimental chondrule formation at conditions relevant to the solar nebula, e.g under microgravity, is obvious.The new experimental approach presented here will be performed under microgravity conditions aboard the International Space Station during Alexander Gerst´s current space mission. The sample container will returned to Earth in December 2018 for detailed analytical laboratory studies. The experiment was accepted as one of only three experiments within the framework of the “Ueberflieger”- competition. The proposal presented here will cover the detailed analytical study of the sample material after returned to Earth and the evaluation of the full digital video coverage of the experiment.The main goals of the project are:1) Is lightning are reasonable process to produce chondrule like objects under microgravity conditions?2) Do melting of dust aggregates enhance particle growth?3) What are the structural details of the chondrule like objects formed and can they be related to specific known chondrule types expected to form at high cooling rates, like cryptocrystalline, radial olivine or barred olivine chondrules.4) What are the geochemical consequences on chondrules formed by lightning discharges?5) What is the degree of crystallinity obtained?6) Do we see phase separation and/or the formation of other phases like pyroxene, commonly found in chondrules?