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We performed an experiment under long-term microgravity conditions aboard the International Space Station (ISS) to obtain information on the energetics and experimental constraints required for the formation of chondrules in the solar nebula by 'nebular lightning'. As a simplified model system, we exposed porous forsterite (Mg 2 SiO 4 ) dust particles to high-energetic arc discharges. The characterization of the samples after their return by synchrotron microtomography and scanning electron microscopy revealed that aggregates had formed, consisting of several fused Mg 2 SiO 4 particles. The partial melting and fusing of Mg 2 SiO 4 dust particles under microgravity conditions leads to a strong reduction of their porosity. The experimental outcomes vary strongly in their appearance from small spherical melt-droplets ( ∅ ≈ 90 µm) to bigger and irregularly shaped aggregates ( ∅ ≈ 350 µm). Our results provided new constraints with respect to energetic aspects of chondrule formation and a roadmap for future and more complex experiments on Earth and in microgravity conditions. [ABSTRACT FROM AUTHOR]

Abstract: With the hydrodynamic experiment ‘GeoFlow’ (Geophysical Flow Simulation) instability and transition of convection between two spherical shells are traced. The flow is driven by a central-symmetry buoyancy force field in microgravity conditions. We performed experiments for a wide range of rotation regimes, within the limits between non- and rapid-rotation. Here we focus on the non-rotational convection in an isoviscous experimental fluid as in ‘GeoFlow I’ and the preparation of ‘GeoFlow II’, that uses a temperature-dependent viscous fluid. Theoretical predictions on thermal, dielectric and optical performance of the fluid suggest the use of an alkanole, i.e. 1-Nonanol as working fluid for ‘GeoFlow II’. Initial ground based experiments demonstrate the influence of the viscosity contrast on fluid flow patterns. Specific results from the ‘GeoFlow I’ experiment, i.e. steady-state convection above a threshold and transition to chaos, are used as a reference. [Copyright &y& Elsevier]

The formation of chondrules in the solar nebula is still an enigmatic process. In order to reconstruct this process, we performed a chondrule formation experiment aboard the International Space Station (ISS), called EXCISS. The purpose of the EXCISS experiment is to acquire new insights in chondrule formation by "nebular lightnings". During the experiment, forsterite (Mg 2 SiO 4) particles were repeatedly exposed to arc discharges under long-term micro-gravity conditions. The purpose of this paper is to describe the experimental set-up and establish and benchmark approaches for the analysis of the samples once they are returned from the ISS. We produced aggregates of fused Mg 2 SiO 4 particles in our Earth-based experiment which we studied by SEM and X-ray microtomography. The results of our preliminary Earth-based experiments indicate that the chosen experimental parameters will likely lead to melting of Mg 2 SiO 4 particles and the formation of aggregates. The results from the first experiments aboard the ISS revealed that the EXCISS experiment is fully operational. • We built an experimental set-up for a chondrule formation experiment aboard the ISS. • The experimental parameters to fuse forsterite dust particles were adjusted on Earth. • The sample material was characterized by SEM, μ -CT, powder XRD and Raman spectroscopy. • Exemplary natural chondrules were studied by synchrotron microtomography. [ABSTRACT FROM AUTHOR]