FEMTOSECOND LASER EXPERIMENTS ON ORIENTED OLIVINE SINGLE CRYSTALS:EVIDENCE OF SPACE WEATHERING
A. Fazio1, D. Harries1, G. Matthäus2, H. Mutschke3, S. Nolte2, and F. Langenhorst1 1Institute of Geoscience,Friedrich Schiller University Jena (FSU), Carl-Zeiss-Promenade 10, 07745 Jena, Germany, 2Institute of Applied
Physics, FSU, Albert Einstein-Str. 15, 07745 Jena, Germany, 3Astrophysical Institute and University Observatory, FSU, Schillergässchen 2-3, 07745 Jena, Germany (email to: Agnese.Fazio@uni-jena.de)Introduction: Space weathering (SW) is the most important set of processes that irreversibly change the surfaces of the atmosphere-free bodies [1]. Ion irradiation and laser experiments are considered the most powerful
techniques to simulate the SW effects. The former are a direct reproduction of solar wind irradiation effects. The
latter are assumed to simulate micrometeoroid impacts (e.g., [2-3]). In this work, we present spectroscopic andTEM results of the application of ultra-short laser pulse as a tool to reproduce SW effects on olivine crystals.
Sample and Methods: Laser experiments were performed on oriented slices (300-500 µm thick) of singlecrystal olivine (Fo94.5). Experiments were carried out under vacuum (10-3 mbar) via femtosecond Ti:sapphire laser
at the wavelength of 800 nm. On each sample, grids (2.5 x 2.5 mm) of single shots were produced at 3 mJ. Eachshot had a pulse duration of 120 fs and a spot diameter of 38 µm. These parameters correspond to a maximum
laser intensity of ~1015 W cm-2. Spectral features of irradiated and non-irradiated areas were investigated in theNUV-vis-NIR range. FIB-lamellae were prepared from selected craters and were investigated by TEM in order toevaluate the defect microstructures at the nanometer scale.
Results: Single-shot laser irradiation produced spherical to elliptical microcraters on the surfaces of the olivineslices (diameter ~ 80 µm). Craters are covered by glass showing distinct splash-form morphologies at the rim.
Spectra of irradiated areas show a strong reduction in the absolute reflectance (darkening). Spectra of irradiatedareas scaled at 550 nm show a strong increase in the reflectance with increasing wavelength (reddening).
Crater cross-sections show a layered structure. In the central zone, we report from top to bottom: a glass layer(~300-500 nm), a polycrystalline layer with palisade-like olivine crystals (~400-300 nm), a polycrystalline layer
with very small (~50 nm) polygonal randomly oriented olivine crystals (~350-600 nm), and the olivine substratewith shock effects (>10 µm). Within the substrate numerous planar fractures and c dislocations occur. Layers are
thicker at the core than at the rim. The polygonal olivine layer is missing at the rim. Two layers of iron nanoparticles (npFe0; d110 = 0.206 nm; average diameter 4-7 nm) occur at the interface between the polygonal and thepalisade layers and between the palisade and the glass layers. TEM-EDX maps and analyses show a significant
Mg-loss in the glass layer. The interface between the polygonal and the palisade olivine is depleted in Fe. NpFe0
contain Ni, particularly at the interface between the palisade and the glass layers.
Discussion: Experimental approach. For fs laser pulses, the absorbed energy is transferred from electron to
the lattice by electron-phonon-coupling at the end of the laser pulse. The release of a shock wave and thermalenergy occurs in a nanosecond to microsecond timescale [4]. Compared to previously used laser pulses with nanosecond duration ([2-3]), ultra-short laser pulses have the main advantages to allow the propagation of a shock
wave and to avoid the interaction between the laser light and the ejecta plume, i.e., no additional energy is transferred to the ejecta, and no additional thermal effects are produced.
Contrary to previous experiments, which used mostly powdered samples compressed to pellets [2-3], we performed our experiments on oriented olivine crystals to avoid additional particle-particle interactions and to study
the orientation-dependence of shock effects in this anisotropic material.
Origin of the layered structure and npFe0 formation. The propagation of a shock wave in the irradiated material
results in the formation of a layered structure similar to the one observed in a lunar microcrater [5]. Due to thesplash-form morphology, the topmost glass layer likely formed as a thin sheet of olivine melt, there is no evidence
for vapor deposition. The palisade structure of the topmost polycrystalline layer indicates that this layer crystallized from the melt. The underlying polycrystalline layer, due to the polygonal shape of its crystals and the highdegree of misorientation, might represent an extremely high shock stage, a transition between a molten layer andthe olivine substrate. The in-situ formation of npFe0 is preferred due to the distribution of nanoparticles and thelack of evidence for the vapor deposition of the glass layer (e.g., [3]). The completion of the TEM-EDX analyses
and EELS measurements will allow to constrain the mechanism for the in-situ formation of npFe0.
Conclusions and future works: The results of the spectral measurements and the TEM investigation corroborate that the use of an ultra-short laser pulse is a reliable approach to reproduce the effects of the SW and to checkand study exactly where and how certain modifications occur. Next steps include to constrain the mechanism forthe formation of npFe0 and the characterization of shock-features due to the different crystal orientation (i.e., planar
fractures and dislocations).
References: [1] Chapman C.R. 2004. Annu. Rev. Earth Pl. Sc. 32:539-567. [2] Yamada M. et al. 1999. Earth Planets Space
51: 1255-1265. [3] Sasaki S. et al. 2001. Nature 410:555-557. [4] Gattass R. R. and Mazur E. 2008. Nat. Photonics 2:219-225.
[5] Noble S. K. et al. 2015. Abstract #2034. Space weathering of airless bodies.
Acknowledgements: This work was supported by the Gottfried Wilhelm Leibniz Programme (LA 830/14-1) and the research unit FOR 2285 “Debris Disk in Planetary Systems” of the Deutsche Forschungsgemeinschaft (LA 830/20-1 and NO
462/11-1). A. F. thanks the Alexander von Humboldt Foundation for providing a research fellowship.