Experimental Investigation of Sputtering on Phobos by Atomic and Molecular Ions in the Martian Wake
Authors: Szabo Paul Stefan , Biber Herbert , Jäggi Noah , Wappl Markus , Stadlmayr Reinhard , Primetzhofer Daniel , Nenning Andreas , Mutzke Andreas , Fleig Jürgen , Mezger Klaus , Lammer Helmut , Galli André , Wurz Peter , Aumayr Friedrich 
Published: October 2020

Experimental quantification of sputtering of planetary surface analogs provides important constraints for the understanding of space weathering [1]. Whereas the Moon and Mars are mostly irradiated by solar wind ions, the Martian moon Phobos is also exposed to atmospheric ions that escape from Mars [2]. In the Martian tail region O+ and O2+ ions at energies of several 100 to several 1000 eV are the predominant contribution to the sputtering of the surface of Phobos [3].Validating and improving such models requires a more detailed experimental investigation of sputtering with relevant analog samples. Therefore, sputtering experiments with O+, C+, O2+ and CO2+ were performed with energies from 1 to 5 keV, corresponding to energies relevant for Martian ions [4]. Augite (Ca, Mg, Fe)2Si2O6 mineral samples were chosen as a Phobos analog since their elemental composition is close to the current understanding of the composition of Phobos&#8217; surface [5]. Thin films were deposited from augite onto Quartz Crystal Microbalances (QCM), which allow in-situ measurements of sputtering yields [6]. SDTrimSP simulations with established input parameters for augite were performed to compare the simulation outcomes with experimental results [7].&#160;&#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160;Figure 1 Sputtering yields of 2 keV (blue) and 5 keV (orange) O+ ions under different angles of incidence. Experimental values (dots) are compared to SDTrimSP (dashed lines) and SRIM simulations (dotted lines).&#160;Measured mass changes during O+ and C+ irradiation are slightly smaller than predicted by the SDTrimSP sputtering simulation, which indicates implantation of projectile ions into the samples (for O+ results, see Figure 1). SRIM simulations are known to overestimate sputter yields for such samples, as is also observed here. Measurements with O2+ and CO2+ show no indication of molecular effects. Their behavior is thus equivalent to sputtering by the sum of their atomic constituents at the same velocity.&#160;&#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160; &#160;Figure 2 Calculated ratio of sputtering by atmospheric O ions and solar wind ions. The result from Nenon et al. (blue dashed line) is compared to rescaled calculations on our new experimental data for sputtering by O ions (red line).&#160;For the sputtering of the surface of Phobos, the new experimental results support previous assumptions that only O+ and O2+ ions have to be considered. The sputtering by CO2+ ions is most likely negligible. Regarding the O ions, the new experimental results suggest lower sputtering yields by about 50%. Nevertheless, sputtering in the Martian magnetotail region will still be dominated by atmospheric O ions, as previously calculated (Figure 2) [2, 3]. Over the whole orbit of Phobos, our results predict that atmospheric O ions account for 10 to 15% of the sputtering of Phobos&#8217; surface.&#160;List of References[1]&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; B. Hapke, J. Geophys. Res.: Planets 106, 10039 (2001).[2]&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; A.R. Poppe, S.M. Curry, Geophys. Res. Lett., 41, 6335 (2014).[3]&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; Q. Nenon, et al., J. Geophys. Res.: Planets 124, 3385 (2019).[4]&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; P.S. Szabo, et al., submitted to J. Geophys. Res.: Planets (2020).[5]&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; F. Cipriani, et al., Icarus 212, 643 (2011).[6]&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; G. Hayderer, et al., Rev. Sci. Instrum. 70, 3696 (1999).[7]&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; A. Mutzke, IPP-Report 2019-02 (2019).