Comparison of ion- and laser-weathered spectra of olivines and pyroxenes
K. Chrbolková1,2,3, R. Brunetto4, J. Ďurech2, T. Kohout1,3, K. Mizohata5, P. Malý6, V. Dědič7, C. Lantz4, A. Penttilä8, F.
Trojánek6, and A. Maturilli9
1
Department of Geosciences and Geography, University of Helsinki, Gustaf Hällströmin katu 2, 00560 Helsinki, Finland(katerina.chrbolkova@helsinki.fi)
2
Astronomical Institute of Charles University, V Holešovičkách 2, 18000 Prague 8, Czech Republic
3
Institute of Geology, The Czech Academy of Sciences, Rozvojová 269, 16500 Prague 6, Czech Republic
4
Université Paris-Saclay, CNRS, Institut d'Astrophysique Spatiale, 91405 Orsay, France
5
Department of Physics, Faculty of Science, University of Helsinki, Pietari Kalmin katu 2, 00560 Helsinki, Finland
6
Department of Chemical Physics and Optics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 12116
Prague, Czech Republic
7
Institute of Physics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 12116 Prague, Czech Republic
8
Department of Physics, Faculty of Science, University of Helsinki, Gustaf Hällströmin katu 2, 00560 Helsinki, Finland
9Institute of Planetary Research, DLR German Aerospace Centre, Rutherfordstrasse 2, 12489 Berlin, Germany

Introduction: Irradiation by solar wind ions and impacts of micrometeoroids are the leading processes that weather surfacesof airless planetary bodies in the solar system. As a result, key diagnostic features of their spectra get altered. The mostprominent changes in the silicate-rich bodies in the visible (VIS) and near-infrared (NIR) wavelengths are increase in thespectral slope, reduction of the albedo, and flattening of the mineral absorption bands (see, for example, [1]).
Our laboratory experiments aimed at understanding what are the similarities and differences between the effect of the solar
wind ions and micrometeoroid impacts on the final spectra of the silicate-rich bodies.
Methods: We have used two different terrestrial minerals, olivine and pyroxene, which we ground , dry-sieved, and pressed
into pellets.
Hydrogen (H) irradiations proceeded at the Accelerator Laboratory of the University of Helsinki, using 5 keV ions withvarying fluences from 1014 to 1018 ions/cm2. Helium and argon irradiations were done using the INGMAR set-up (IASCSNSM, Orsay) with 20 and 40 keV ions and with fluences from 1015 to 1017 ions/cm2.
Individual 100-fs laser pulses were shot into a square grid on the pellets’ surface to simulate the micrometeoroid impacts (as in[2]) in the laboratories at Charles University. Various densities of the pulses per cm 2 simulated different weathering stages.
Subsequent spectral measurements covered wavelengths from 0.54 to 13 μm, i.e. VIS to mid-infrared wavelengths. After themeasurements, we evaluated the evolution of the spectral parameters estimated using the Modified Gaussian Model [3, 4].
Results: We found that the variation of the spectra in the VIS range was similar for H+- and laser-irradiated samples, but wehave identified a difference in the NIR wavelength range. Laser irradiation caused greater changes in NIR than any of the ions
we used, see Fig. 1. The reason for such difference in behaviour may be that the penetration depth of the laser pulses is muchlarger than in the case of the ions. The relative contribution of the irradiated material in the spectra is then smaller in the ioncase than in the laser case.
Otherwise, we found that the original mineralogy of the surface is the leading factor influencing the evolution of the spectral
parameters. While olivine and pyroxene showed albedo variations of a similar order, the evolution of pyroxene’s spectral slope
was negligible when compared to olivine. This has implications for olivine-pyroxene mixtures and their evolution. E.g. in thecase of asteroid (433) Eros, the variation of the spectral slope is minor, but other spectral parameters show some variation. As
the surface of Eros is old, we hypothesise that spectral slope changes induced by olivine alteration already saturated and theleading source of the spectral variation is pyroxene, which does not show large variations in the slope. In contrast asteroid(25143) Itokawa is younger and thus still shows variations in the spectral slope as it has not saturated yet.

Figure 1. Spectral differences caused by laser and hydrogen ion (H+) irradiation. Each bar connects at the upper end the reflectance, R, ofthe fresh material and at the lower end R of the material weathered to the level marked in the legend.

References[1] Hapke, B. 2001, J. Geophys. Res., 106, 10039[2] Fazio, A., Harries, D., Matthäus, G., et al. 2018, Icarus, 299, 240[3] Sunshine, J. M., Pieters, C. M., & Pratt, S. F. 1990, J. Geophys. Res., 95, 6955
[4] Sunshine, J. M., Pieters, C. M., Pratt, S. F., & McNaron-Brown, K. S. 1999, in Lunar and Planetary Inst. Technical Report,Vol. 30, Lunar and Planetary Science Conference, 1306