Abstract
AbstractChondrules are thought to play a crucial role in planet formation, but the mechanisms leading to their formation are still a matter of unresolved discussion. So far, experiments designed to understand chondrule formation conditions have been carried out only under the influence of terrestrial gravity. In order to introduce more realistic conditions, we developed a chondrule formation experiment, which was carried out at long‐term microgravity aboard the International Space Station. In this experiment, freely levitating forsterite (Mg2SiO4) dust particles were exposed to electric arc discharges, thus simulating chondrule formation via nebular lightning. The arc discharges were able to melt single dust particles completely, which then crystallized with very high cooling rates of >105 K h−1. The crystals in the spherules show a crystallographic preferred orientation of the [010] axes perpendicular to the spherule surface, similar to the preferred orientation observed in some natural chondrules. This microstructure is probably the result of crystallization under microgravity conditions. Furthermore, the spherules interacted with the surrounding gas during crystallization. We show that this type of experiment is able to form spherules, which show some similarities with the morphology of chondrules despite very short heating pulses and high cooling rates.
Highlights
The formation of chondrules is considered to be a fundamental step in the evolution of our solar system and chondrules are often termed as the building blocks of the terrestrial planets (Johansen et al 2015; Bollard et al 2017)
The proof-of-concept experiment presented here shows that scientifically useful results regarding chondrule formation, such as the response of dust particles to arc discharges, crystallization under microgravity conditions, and the subsequent thermal evolution of chondrules, can be obtained under very tight technical constraints and we present the analysis of melt spherules formed in microgravity and discuss their relevance as chondrule analogs
The space between the electrodes was empty in 70% of the arc discharges
Summary
The formation of chondrules is considered to be a fundamental step in the evolution of our solar system and chondrules are often termed as the building blocks of the terrestrial planets (Johansen et al 2015; Bollard et al 2017). Several chondrule formation scenarios have been proposed, such as shock waves (e.g., Ciesla and Hood 2002; Morris and Boley 2018), collisions of planetesimals (Krot et al 2005; Sanders and Scott 2012; Lichtenberg et al 2018), dissipation of magneto hydrodynamic turbulence (McNally et al 2013), or nebular lightning (Horanyi et al 1995; Desch and Cuzzi 2000), whereby some are considered more plausible than others (Desch et al 2012) None of these theories is able to explain all features typical for a specific chondrule type by a single formation mechanism
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