Experimental Investigation of Sputtering on Phobos by Atomic and Molecular Ions in the Martian Wake

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’ 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].

 

                     

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).

 

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.

 

                     

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).

 

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’ surface.

 

List of References

[1]          B. Hapke, J. Geophys. Res.: Planets 106, 10039 (2001).

[2]          A.R. Poppe, S.M. Curry, Geophys. Res. Lett., 41, 6335 (2014).

[3]          Q. Nenon, et al., J. Geophys. Res.: Planets 124, 3385 (2019).

[4]          P.S. Szabo, et al., submitted to J. Geophys. Res.: Planets (2020).

[5]          F. Cipriani, et al., Icarus 212, 643 (2011).

[6]          G. Hayderer, et al., Rev. Sci. Instrum. 70, 3696 (1999).

[7]          A. Mutzke, IPP-Report 2019-02 (2019).