Europlanet Science Congress 2021
Virtual meeting
13 – 24 September 2021
Europlanet Science Congress 2021
Virtual meeting
13 September – 24 September 2021

TP8

Planetary space weather and space weathering on airless bodies

The surfaces of air-less celestial bodies are directly exposed to the environmental radiation, ions, and micrometeoroids. The result of these interactions is an alteration of the surface structure and chemical composition, generally referred to as space weathering. At the same time, these interactions release surface material that refills the surface-bounded exosphere and, directly or indirectly, is a source of planetary ions in the environment. The study of the planetary response to variable external conditions is the broad meaning of planetary space weather.
Over the next decade, the BepiColombo mission to Mercury and JUICE mission to Jupiter’s system, together with the Moon space exploration program, will offer unprecedented opportunities to investigate the interaction processes at airless bodies.
In the present session, we welcome observation-driven, theoretical, and experimental studies
• on all the air-less bodies interacting with solar wind (like Mercury, Moon and asteroids) or with magnetospheric ions (outer planets icy moons);
• on micrometeoroid gardening and impact vaporization effects onto the surface and onto the exosphere;
• on the effects of other agents like photons, electrons, and high-energy particles;
• on laboratory experiments for investigating surface release processes and surface modifications.
• on spectral measurements of various planetary analogous undergone space weathering processes.

Co-organized by OPS
Convener: Anna Milillo | Co-conveners: Sae Aizawa, André Galli, Indhu Varatharajan

Session assets

Discussion on Slack

Oral and Poster presentations and abstracts

Chairpersons: André Galli, Sae Aizawa, Anna Milillo
weathering
EPSC2021-770
|
ECP
Mirza Arnaut, Kay Wohlfarth, and Christian Wöhler

1 Introduction
The current understanding of space-weathering is that spectral changes of space-weathered airless planetary bodies are mainly caused by the gradual formation of submicroscopic-sized iron particles (smFe0) in the upper layer of the regolith. The resulting spectral changes include the darkening and reddening of the spectrum as well as the obstruction of diagnostic features [1,2,3]. Previous research also indicates that very small particles (nanophase iron <10 nm) mainly account for reddening and larger iron particles cause spectral darkening without reddening [3]. This theory has been adopted in a variety of studies such as [3,4,5]. However, this interpretation does not come without issues. The estimated sizes of large particles may deviate from the experimental particle size [3]. Recently, Denevi et al. [6] suggested that agglutinates might be the main cause for substantial reddening even though they are abundant in large iron particles. We add another aspect to the discussion: Previous mathematical treatments of small iron particles implicitly assumed that the distances between particles are large and no interactions occur. However, TEM imagery of lunar soil and laser irradiated samples [1] suggests that cases exist in which the iron particles are close together and form layers or clusters. We apply advanced light scattering theory [7] to simulate clusters and layers of very small iron particles with varying geometries and densities. We find that interparticle interactions between nanophase iron particles can alter the spectral slope and counteract the reddening effect.

2 Methods
To investigate the spectral behaviour of nanophase iron particle layers or clusters, we carry out three steps.
(1) First, we define our baseline. We utilise Mie scattering to simulate the spectral effects that non-interacting smFe0 (radius = 5nm) has on a fresh lunar soil spectrum as described by [4]. A fresh highland spectrum from calibrated data of the Moon-Mineralogy-Mapper (M3) [https://pds-imaging.jpl.nasa.gov/volumes/m3.html] is converted to single scattering albedo via the Hapke model [8]. Mie modelling yields the single scattering albedo w, the phase function p(g) and the extinction/scattering efficiencies Qext and Qsca of smFe0 [9]. We use the optical constants of iron from [10]. The soil albedo and the iron albedo as well as the phase functions are combined via Hapke’s mixing equation [8]. The resulting albedo wsoil, smFe0 is subsequently fed into the Hapke model to generate computationally space-weathered soil spectra.
(2) Secondly, we assume that not all iron particles are present in the form of single particles but many of them are somewhat closely packed and form layers. Now, we simulate these layers. The particle radii are drawn from a normal distribution with μ = 22nm and σ = 10nm and populate a box with dimensions 700nm x 700nm x 100nm (figure 1b). The dimensions of the box represent a small layer of particles that accumulate at the grain rim. The depth (100nm) is largely inspired by TEM imagery that suggest depths of roughly 10–100nm [1]. We utilise the T-matrix method [7] using the CELES framework [11] and compute the interaction between adjacent particles. Again, the resulting quantities (albedo, phase function, efficiencies) are fed into the Hapke model to simulate a computationally weathered spectrum.
(3) Finally, we compare the results of steps (1) and (2).

Figure 1a: TEM image of small iron particles (small dots) occurring in the melt and vapor phases of the soil grain (image from [1]).

Figure 1b: Artificially created particle geometry with varying radius. The size is also colour coded with red particles being the largest, blue the smallest, and other sizes in between.

3 Results
To evaluate the influence of layered smFe0, we chose a fresh highland spectrum near crater Krasovskiy [4], see figure 2a (blue solid line). The spectrum is red-sloped, has no strong features and a significant hydroxyl/water absorption band above 2.8μm. We simulate the spectral behaviour of single isolated iron particles according to (1) and obtain a spectrum with considerable reddening and some darkening (red line in figure 2a). If a mixture of single non-interacting particles and interacting layered particles is included, the spectral slope becomes flatter in the NIR region, i.e., exhibits less reddening as seen in figure 2a (black line). For the sake of comparison, we tuned the abundances of the components such that the weathered spectra have similar reflectance values near 2.5μm.

Figure 2a: Simulated spectra from the fresh spectrum (blue) using non-interacting particles (red) and layered particles (black)

Figure 2b: Normalised spectra at 1μm

4 Conclusion
In figure 2a and 2b one can clearly see that interacting particles organised in layers counteract the reddening effect. Nanophase iron alone can thus mimic the behaviour of microphase iron which has previously been believed to be solely responsible for this effect [3,4]. This results in the following consequences: The distinction between nanophase iron and microphase iron become less clear. Not only the particle size matters but also the geometric alignment and the inter-particle interactions are important if the particles are somewhat dense. To arrive at a comprehensive understanding of space weathering and its spectral characteristics, the geometry, density, and distribution and interaction of smFe0 should be taken into consideration. Further simulation campaigns are necessary to characterise the dependency of the spectral behaviour on these geometric parameters.

[1] Carle M. Pieters and Sarah K. Noble, 2016, JGR Planets, 121(10):1865–1884
[2] Bruce Hapke, 2001, JGR Planets, 106(E5):10039–10073
[3] Paul G.Lucey and Miriam A.Riner, 2011, Icarus, 212(2):451–462
[4] Kay S. Wohlfarth, et al., 2019, The Astronomical Journal, 158(2):80
[5] Antti Penttilä, et al., 2020, Icarus 345:113727
[6] Brett W. Denevi, et al., 2021, LPSC2021
[7] Michael I.Mishchenko, 2006, Cambridge University Press
[8] Bruce Hapke, 2012, Cambridge University Press
[9] Christian Wöhler, et al., 2019, Science Advances, 3(9):1701286
[10] M. R. Querry, 1985, Optical constants
[11] Amos Egel, et al., 2017, eprint.

How to cite: Arnaut, M., Wohlfarth, K., and Wöhler, C.: The interaction between multiple nanophase iron particles changes the slope of lunar reflectance spectra, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-770, https://doi.org/10.5194/epsc2021-770, 2021.

EPSC2021-642
Antti Penttilä, Timo Väisänen, Johannes Markkanen, Julia Martikainen, Tomas Kohout, Gorden Videen, and Karri Muinonen

We present a multi-scale light-scattering model that is capable of simulating the reflectance spectra of a regolith layer. In particular, the model can be applied to a case where the regolith grains have varying amounts of nanophase inclusions due to space weathering of the material. As different simulation tools are employed for different size scales of the target geometry (roughly, nano-, micro-, and millimeter scales), the particle size effects, the surface reflections, and the volume scattering can all be properly accounted for. Our results with olivine grains and nanophase iron inclusions verify the role of the nanoinclusions in the reflectance spectra of space-weathered materials. Together with the simulation results, we give simplified explanations for the space-weathering effects based on light scattering, namely the decrease of albedo, the general increase of the red spectral slope, and the dampening of the spectral bands. We also consider the so-called ultraviolet bluing effect and show how the change in the spectral slope over the ultraviolet-visual wavelengths is due to the decrease of reflectance in the visual wavelengths rather than the increase of reflectance in the ultraviolet part.

How to cite: Penttilä, A., Väisänen, T., Markkanen, J., Martikainen, J., Kohout, T., Videen, G., and Muinonen, K.: Rigorous light-scattering simulations of space-weathering effects on reflectance spectra, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-642, https://doi.org/10.5194/epsc2021-642, 2021.

EPSC2021-172
Jeremie Lasue, Patrick Pinet, Michael Toplis, Pierre Beck, Loise Cao, and Pascal Munsch

Introduction:

Atmosphereless bodies of the Solar System present surface evolution under the harsh conditions of space weathering. Their regolith are exposed to thermal cycling, cosmic and solar rays irradiation, solar wind sputtering and micrometeorites bombardment and vaporization. With time these materials accumulate optically active opaque particles such as nanophase metallic iron particles on the surface or rims of dust grains or larger iron particles. The density increase in iron particles modifies the spectral properties of the material with a lowering of the particles albedo, and of the absorption bands amplitude generally associated with a global reddening of the spectral slope [1, 2].

Previous studies have shown that nanophase iron particles inducing weathering spectral properties will accumulate at the surface of olivine and pyroxene minerals under heating conditions simulating micrometeorite impacts [3]. The skin depth of weathered material depends on the weathering process and minerals.

Context for Phobos studies:

The moons of Mars, Phobos and Deimos, are some of the most enigmatic small bodies of the Solar System. Indeed, it is still debated in the community whether they originate from a large impact on Mars [4, 5], or if they are captured primitive asteroids [6]. The overall spectra of Phobos and Deimos are very red with no obvious diagnostic spectral features readily detected in the visible or near-infrared, similar to very primitive small bodies of the solar system. However, significant variations in the spectral slope indicates inhomogeneities of the material on the surface of Phobos. Comparison with analogues such as Tagish Lake meteorite and basalts suggests a probable mixture of asteroidal primitive material with basaltic materials such as the one originating from the nearby surface of Mars [7]. 

The Mars Moons Explorer (MMX) mission should attempt to bring back samples of the surface of Phobos to Earth in order to decipher the origin and dynamics of the Martian Moons system [8]. The MMX InfraRed Spectrometer (MIRS) instrument on-board the mission is an infrared imaging spectrometer in the range 0.9 to 3.6 μm that will extensively study the spectral properties of the surface of Phobos and provide context for the sample collection region.  

Method:

In this context, we are preparing laboratory weathered analogues samples by submitting basalt (such as the Pic d’Ysson basalt already used for laboratory studies on the lunar regolith optical and spectrophotometric behavior [9]) and meteorite samples to near melting temperature (1000-1200°C) in an atmosphere controlled oven under reducing conditions (partial pressure of CO/CO2 of 96% corresponding to an oxygen fugacity of about 10-17) mimicking the space environment alteration effects for different exposure times (Fig. 1). We then explore the process of development and formation of submicroscopic metallic Fe, also referred to as nanophase iron particles (npFe0) [10] on the surfaces of the weathered samples.

Results:

A first comparison between unaltered Pic d’Ysson basalt grains and grains subjected to 4 hours heating at 1100°C under reducing conditions (fugacity of 10-17) is presented in Figure 2. The unaltered basalt grain present typical crystals of olivine, feldspar and ilmenite on the left. On the right, the onset of heat alteration is clearly seen over a depth of about 30 microns, with the melting of crystals and the apparition of nanophase metallic iron phases (presence of small white grains on the surface and inside the crystals).

Conclusion:

Laboratory simulations by heating samples under reducing conditions can recreate the conditions under which regolith may be weathered in space with surface alteration of grains and apparition of nanophase iron on the altered depth. The effect of the changes on the grain spectral properties depending on the experimental conditions will be further studied for applications to airless bodies in the Solar System. The samples will be analyzed with SEM at IRAP and their reflectance spectra acquired with the SHADOWS instrument at IPAG [12] for future comparisons with the spectra of Phobos and Deimos.

 

Fig. 1: The atmosphere controlled oven at IRAP for heating under reducing conditions [11].

 

Fig. 2: First results of the SEM analysis showing the onset of heat diffusion and iron metal formation on the surface of the basaltic grains. left: crystalline basalt grain unaltered showing the presence of olivine, feldspar crystals and ilmenite. right: onset of alteration with apparition of nanophase iron over a 30 micrometer depth after 4 hours heating at 1100°C under reducing conditions.

Acknowledgements

This study was funded by CNES.

References:

[1] Hapke, B. (2001) JGR, 106(E5), 10039-10073. [2] Pieters, C.M., & Noble, S.K. (2016) JGR, 121(10), 1865-1884. [3] Weber, I., et al. (2020) EPSL, 530, 115884. [4] Canup, R., & Salmon, J. (2018) Science advances, 4(4), eaar6887. [5] Bagheri, A., et al. (2021) Nature Astronomy, 1-5. [6] Hansen, B. M. (2018) MNRAS, 475(2), 2452-2466. [7] Glotch, T. D., et al. (2018) JGR: Planets, 123(10), 2467-2484. [8] Kuramoto, K., et al. (2021) Earth, Planets and Space. Submitted. [9] Souchon, A.L., et al. (2011) Icarus, 215(1), 313-331. [10] Pieters, C.M., et al. (2000) Meteor. And Planet. Sci., 35, 1101 [11] Toplis, M.J., & Corgne, A. (2002) Contributions to Mineralogy and Petrology, 144(1), 22-37. [12] Potin, S., et al. (2018) Applied optics, 57(28), 8279-8296.

How to cite: Lasue, J., Pinet, P., Toplis, M., Beck, P., Cao, L., and Munsch, P.: Regolith weathering through heating samples under reducing conditions for Phobos surface studies, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-172, https://doi.org/10.5194/epsc2021-172, 2021.

EPSC2021-565
|
ECP
Kateřina Chrbolková, Rosario Brunetto, Josef Ďurech, Tomáš Kohout, Kenichiro Mizohata, Petr Malý, Václav Dědič, Cateline Lantz, Antti Penttilä, František Trojánek, and Alessandro Maturilli

Introduction: Solar wind ions and impacts of micrometeoroids are the leading processes that weather the surface of airless planetary bodies in the solar system. As a result, key diagnostic features of their spectra get altered. The most prominent changes in the silicate-rich bodies in the visible (VIS) and near-infrared (NIR) wavelengths are increase in the spectral slope, reduction of the albedo, and subduction of the mineral absorption bands (see, for example, Hapke 2001).

Our work aims 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. Our study is based on laboratory simulations using planetary analogue materials.

 

Methods: We have used two different terrestrial minerals, olivine and pyroxene. Each material was ground and dry-sieved to sizes smaller than 106 μm. Subsequently, we created pressed pellets from potassium bromide, which served as a base, and 100 mg of the mineral, which created the top layer of the pellet.

Ion irradiations were conducted at two different laboratories. Hydrogen irradiations proceeded at the Accelerator Laboratory of the University of Helsinki, using 5 keV ions with varying fluences from 1014 to 1018 ions/cm2. Subsequent spectral measurements were done out of the vacuum chamber after surface passivation of the samples. Helium and argon irradiations were done using the INGMAR set-up (IAS-CSNSM, Orsay) with 20 and 40 keV ions and with fluences from 1015 to 1017 ions/cm2. Spectra were measured in the vacuum chamber, where we irradiated the samples.

Individual 100-fs laser pulses were shot into a square grid on the pellets’ surface to simulate the micrometeoroid impacts (as in Fazio et al. 2018). Various densities of the pulses per cm2 simulated different weathering stages. Spectral measurements were done outside of the vacuum chamber.

The spectral measurements covered, in all the set-ups, wavelengths from 0.54 to 13 μm, i.e. VIS to mid-infrared wavelengths. After the measurements, we evaluated the evolution of the spectral parameters estimated using the Modified Gaussian Model (Sunshine et al. 1990, 1999).

 

Results: Variation of the spectra in the VIS range was similar for H+- and laser-irradiated samples, but we have 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 the different penetration depth of the irradiating ions and laser pulses. While laser penetrates approximately 100 μm under the surface of the pellet, our ions did not penetrate deeper than 150 nm. Spectra of the laser-irradiated samples thus bear information solely from the irradiated material, while spectra of the ion-irradiated samples are a mixture of the top-most altered layers and the unaltered underlying layers. The relative contribution of the irradiated material is then smaller in the ion case.

Otherwise, we found that the original mineralogy of the pellet is more determinative to the evolution of the spectral parameters than the space weathering agent (ions or laser pulses). While olivine and pyroxene showed albedo variations of a similar order, the evolution of pyroxene’s spectral slope was negligible when compared to olivine, see Fig. 2.

   

 

Implications to the solar system studies: Evolution of the spectral parameters of our pyroxene samples agrees with the observed space weathering trends on asteroid (4) Vesta, which shows prominent albedo changes but the spectral slope is stable. Rapid changes of olivine’s spectral slope are in agreement with the high observed slopes in the A-type asteroid population. Our weathering trends also agree with the Q- to S-type asteroid transition.

The difference we found between the influence of the ion and laser irradiation on the NIR wavelengths may help us in estimating the surface exposure ages of different geomorphological features in the solar system bodies. Solar wind ions are known to alter the surface on shorter timescales than the micrometeoroid impacts. At the beginning of the surface’s evolution, the NIR wavelengths will thus not be much influenced and as the contribution of the micrometeoroid impacts will grow, the NIR wavelengths will start to evolve. Based on that we may give an estimate on the surface age by studying the NIR range.

As we have seen, olivine’s spectral slope evolves faster and significantly more than pyroxene’s. This has implications for olivine-pyroxene mixtures and their evolution. E.g. in the case 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 the leading 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.

 

Acknowledgements: This work was supported by the University of Helsinki Foundation and the Academy of Finland project nos 325805, 1335595 and 293975, and it was conducted with institutional support RVO 67985831 from the Institute of Geology of the Czech Academy of Sciences. The authors acknowledge funding from Charles University (Project Progres Q47). Part of the irradiation was performed using the INGMAR set-up, a joint IAS-CSNSM (Orsay, France) facility funded by the French Programme National de Planétologie (PNP), by the Faculté des Sciences d’Orsay, Université Paris-Sud (Attractivité 2012), by the French National Research Agency ANR (contract ANR-11-BS56-0026, OGRESSE), and by the P2IO LabEx (ANR-10-LABX-0038) in the framework Investissements d’Avenir (ANR-11-IDEX-0003-01).

 

References: Fazio, A., Harries, D., Matthäus, G., et al. 2018, Icarus, 299, 240; Hapke, B. 2001, J. Geophys. Res., 106, 10039; Sunshine, J. M., Pieters, C. M., & Pratt, S. F. 1990, J. Geophys. Res., 95, 6955; 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

How to cite: Chrbolková, K., Brunetto, R., Ďurech, J., Kohout, T., Mizohata, K., Malý, P., Dědič, V., Lantz, C., Penttilä, A., Trojánek, F., and Maturilli, A.: Ion- and laser-weathered spectra: How (dis)similar are they?, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-565, https://doi.org/10.5194/epsc2021-565, 2021.

EPSC2021-526
Herbert Biber, Paul Stefan Szabo, Noah Jäggi, Christian Cupak, Johannes Brötzner, Daniel Gesell, Andreas Mutzke, Andreas Nenning, Klaus Mezger, André Galli, Peter Wurz, and Friedrich Aumayr

1. Introduction
Ions from the solar wind impinging on airless planetary bodies eject particles from the very surface. Those sputtering processes can also be caused by ions originating from planetary bodies in the direct vicinity. This type of process occurs on the martian moon Phobos, where molecules from the upper atmosphere of Mars are ionized, accelerated and subsequently impact the surface of Phobos [1, 2]. The particles ejected due to ion sputtering typically have an energy of a few eV [3]. For lighter elements, this leads to a significant fraction of them leaving the gravitational field of bodies like Mercury, where an escape velocity of 4.25 km/s corresponds to about 1.5 eV for oxygen [4]. Those particles that are emitted at lower energies are still coupled to the planet‘s gravitational field and contribute to the formation of a so-called exosphere, i.e. a zone with elevated density around airless bodies.

Modeling of the exosphere requires accurate sputtering data as input. A common way of obtaining these data is through the simulation package SRIM, which can provide an estimate for sputtering yields, but is known for its deviations from experimentally obtained values. Improvements can be made by using the SDTrimSP code, but here input parameters have to be set manually to achieve satisfactory agreement with measurements [5]. In both models, amorphous samples are assumed for the simulation and no effects of crystallinity are taken into account. However, these effects can vary from drastic suppression of sputtering due to channeling in single crystals [6] to an increase in sputtering due to linear collision sequences in polycrystalline materials [7].

Experimental investigations of mineral sputtering also mostly use amorphous films [8, 9]. This is due to the use of Quartz Crystal Microbalances (QCM) in sputtering experiments, which allow determining mass changes in the sub-monolayer range in real time and in situ [10]. For experiments with more realistic crystalline minerals however, different approaches have to be used.

2. A novel setup
The new setup for sputtering investigations for minerals at TU Wien is based on catching the material sputtered from samples rather than measuring mass depletion of an irradiated sample [11, 12]. This is accomplished by the use of a QCM in a catching configuration (Figure 1). With this approach, a much wider range of samples can be investigated. Due to the use of two independent rotary manipulators, the angular dependence of the sputtering
processes (variation of α in Figure 1) as well as the angular distribution of sputtered particles (variation of αC ) can be analyzed.

Figure 1: Geometry of the measurements using a QCM to catch sputtered material. Both angles α and αC can be varied independently. Figure taken from [12].

Surface roughness has to be taken into account in sputtering experiments. It has a significant influence on ion sputtering and it is therefore necessary to characterize the surface morphology of irradiated samples in order to derive the correct conclusions [13] . For this purpose, an Atomic Force Microscope is used to precisely map the topography of the irradiated samples surfaces at the nanometer scale.

3. No effects of crystallinity observed
The combination of both a thin wollastonite CaSiO3 based film on a QCM and a pressed wollastonite pellet as samples allows for direct comparison of the signals measured at the catching QCM [14]. Only minor deviations from flat surfaces were expected due to roughness effects. The samples were irradiated with 2 keV Ar+ ions under an angle of incidence of α = 60°. The angular distribution of sputtered particles was analyzed for both samples by varying the catcher angle αC. The signal of the catcher QCM normalized to the ion current is shown in Figure 2. Within measurement uncertainties, no difference in signal per impinging ion was found. This indicates that crystallinity does not have a significant influence on the sputtering of mineral samples, but further measurements are needed for more conclusive statements about the relevance of the materials phase for sputtering effects on celestial bodies like Mercury.

Figure 2: Signal of the catcher QCM per incident ion measured at different catcher QCM angles αC for irradiations with 2 keV Ar+ on wollastonite CaSiO3 at α = 60°.

References
[1] Nenon Q., et al.: J. Geophys. Res. Planets, 124, 3385, 2019.
[2] Szabo P.S., et al.: J. Geophys. Res. Planets, 125, e2020JE006583, 2020.
[3] Betz G., et al.: Int. J. Mass Spectrom., 140, 1, 1994.
[4] Williams, D.R.: Mercury Fact Sheet, 25 Nov. 2020, https://nssdc.gsfc.nasa.gov/planetary/factsheet/mercuryfact.html, (Accessed 17. May 2020).
[5] Szabo, P.S., et al.: Astrophys. J., 891, 100, 2020.
[6] Onderdelingen D.: Appl. Phys. Lett., 8, 189, 1966.
[7] Schlueter K., et al.: Phys. Rev. Lett., 125, 225502, 2020.
[8] Hijazi H., et al.: J. Geophys. Res. Planets, 122, 1597, 2017.
[9] Biber H., et al.: Nucl. Instrum. Methods Phys. Res. B, 480, 10, 2020.
[10] Hayderer G., et al.: Rev. Sci. Instrum., 70, 3696, 1999.
[11] Berger B. M., et al.: Nucl. Instrum. Methods Phys. Res. B, 406, 533, 2017.
[12] Biber H., et al.: EPSC Abstracts, 14, EPSC2020-264, 2020.
[13] Küstner M., et al.: Nucl. Instrum. Methods Phys. Res. B, 145, 320, 1998.
[14] Jäggi N., et al.: Icarus, 365, 114492, 2021.

How to cite: Biber, H., Szabo, P. S., Jäggi, N., Cupak, C., Brötzner, J., Gesell, D., Mutzke, A., Nenning, A., Mezger, K., Galli, A., Wurz, P., and Aumayr, F.: Comparing sputtering effects of amorphous films and mineral pellets, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-526, https://doi.org/10.5194/epsc2021-526, 2021.

Exospheres
EPSC2021-235
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ECP
Liam Morrissey, Orenthal Tucker, Rosemary Killen, Dennis Herschback, and Daniel W. Savin

Introduction:  Na, Ar and He are some of the most abundant confirmed neutral species in Mercury’s exosphere. Whereas the source of He is from the solar wind (SW), the source of the Na is potentially due to sputtering from silicates on the Hermean surface (1, 2). As SW ions impact the surface, they deposit energy, leading to sputtered atoms from the substrate (3, 4). The yield and energy distributions of the sputtered atoms depends on the energy of the impacting ions and the composition of the impacted surface. Understanding the role SW ions play on surface sputtering of Mercury is critical to any exosphere model (3).

 

The most common sputtering models use the binary collision approximation (BCA) and thus consider sputtering to be a result of binary collision cascades (5). These models can be used to predict the energy distribution and yield of sputtered atoms as a function of incoming ion type, energy, and impacting angle. A fundamental physical parameter for BCA models is the surface binding energy (SBE) of atoms in the substrate (6, 7). The SBE is a user defined value in SDTrimSP (8), a BCA sputtering simulation tool, and in the commonly referenced Thompson energy distribution of the sputtered atoms (9). Despite the clear importance of the SBE, its actual value is not well understood for many substrates. For single component substrates, the SBE is often approximated as the heat of sublimation for the substrate atoms (10). However, previous research has suggested that this approach can underestimate the SBE by 20-40% (7). More importantly for planetary science, there is no universal approach to estimating the SBE for multicomponent substrates where the Na is likely bonded to other atomic species. SDTrimSP recommends using the pure heat of sublimation of each atomic species as the SBE for sputtering from a compound, which is 1.1 eV for Na (8). However, this approach assumes that the SBE is independent of the bonds formed with the other atoms within the substrate. In contrast, Lammer et al. (12) predict a value between 2-2.65 eV but note that this is not well determined due to a lack of experimental data. Given that BCA methods rely on a user defined SBE, this can be a significant source of error for sputtering predictions.

 

To address this issue, we have performed molecular dynamics (MD) simulations to better constrain the SBE of Na from silicates. We then consider the effect these modified inputs have on the predicted yield and energy distributions of sputtered Na due to SW impacts.

 

Methods:  MD simulations were conducted to determine the SBE of Na for various crystalline silicates: sodium metasilicate (Na2SiO3), sodium orthosilicate (Na4SiO4), and albite (NaAlSi3O8). An iterative method was used to determine the minimum energy needed to remove one Na atom completely from the substrate surface. Simulations were conducted using a many-body reactive potential that was previously shown to be suitable for a variety of sodium silicate crystals (13).

 

BCA models were then used to determine how the resulting SBE values affected the predicted yield and energy distribution of sputtered Na. The commonly referenced Thompson distribution was used to determine the energy distribution vs. SBE. SDTrimSP was used to calculate the sputtering yield of Na vs. SBE. To capture the most common components of the SW, 1 keV H+ and He2+ impacts were simulated on sodium silicate surfaces.

 

Results:  The MD simulations yielded a range of SBEs from sodium silicates: 2.6 eV for sodium orthosilicate, 4.4 eV for sodium metasilicate, and 7.9 eV for albite. In contrast, the individual cohesive energy of pure Na is only 1.1 eV. Therefore, SBEs from a compound can be drastically different than their atomistic cohesive energies. These results show that the SBE of a specific atom is a function of the compound in which the atom is bound.

 

The newly predicted Na SBE values were then used to determine the sputtering yield and energy distribution of the sputtered atoms using SDTrimSP and the Thompson energy distribution. We find that increasing the SBE from 1.1 to 7.9 eV had a significant effect on predicted energy distribution (Fig. 1). Therefore, the characteristics of sputtered atoms are highly dependent on the SBE used for the simulations. Similarly, the Na yield from albite was highly dependent on the Na SBE (Fig. 2). For example, the Na yield from albite for a 1keV H impact decreased by a factor of almost 15 when the SBE was increased from 1.1 eV to the SBE for Na from albite (7.9 eV). Overall, this study demonstrates that the SBE within in a compound can be significantly different than the monatomic cohesive energy. The results demonstrate the potential of MD to better understand and constrain these values, though laboratory measurements are still needed to benchmark these calculations. In summary, an accurate SBE is critical to obtaining realistic models of SW sputtering contribution to the Hermean exosphere.

Fig 1. Normalized Energy distribution of sputtered Na atoms as a function of SBE

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Fig 2. Sodium sputtering yield as a function of surface binding energy

 

 

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How to cite: Morrissey, L., Tucker, O., Killen, R., Herschback, D., and Savin, D. W.: Simulating Solar-Wind-Ion Sputtering of Sodium from Mercury’s Surface: The Importance of the Surface Binding Energy , Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-235, https://doi.org/10.5194/epsc2021-235, 2021.