TP8

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 Q_ext and Q_sca 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 w_{soil, 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.

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.

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.

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.

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 α_C ) can be analyzed.

Figure 1: Geometry of the measurements using a QCM to catch sputtered material. Both angles α and α_C 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 α_C. 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 α_C 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.

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

\

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

References:

[1] McCoy TJ, et al. 2018. Mercury. View after MESSENGER, pp. 176–90 [2] McClintock WE, et al. Mercur. View after MESSENGER, pp. 371–406 [3] Killen RM, et al. 2001. J. Geophys. Res. Planets. 106(E9):20509–25 [4] Domingue DL, et al. 2014. Space Sci. Rev. 181(1–4):121–214 [5] Eckstein W, Urbassek HM. 2007. In Sputtering by Particle Bombardment, pp. 21–31. Springer [6] Stepanova M, Dew SK. 2001. J. Vac. Sci. Technol. A Vacuum, Surfaces, Film. 19(6):2805–16 [7] Yang X, Hassanein A. 2014. Appl. Surf. Sci. 293:187–90 [8] Mutzke A, et al. 2019 [9] Thompson MW. 1968. Philos. Mag.18(152):377–414 [10] Kelly R. 1986. Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Materials and Atoms. 18(1–6):388–98 [11] Leblanc F, Johnson RE. 2003. Icarus. 164(2):261–81 [12] Lammer H, et al. 2003. Icarus. 166(2):238–47 [13] Hahn SH, et al. 2018. J. Phys. Chem. C. 122(34):19613–24

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.

Atmosphere-free celestial bodies are constantly irradiated by solar wind or magnetospheric ions. In the case of Mercury, the intrinsic magnetic field, although weak, leads to localized plasma precipitation around the magnetospheric cusps and nightside precipitation below the magnetotail [1, 2, 3]. The material ejected by the impacting ions (sputtering) thereby contributes to the exosphere and magnetosphere surrounding Mercury [4, 5, 6]. Magnetospheric ions originate from ionization of exospheric atoms or ions directly sputtered from the surface. Those can become part of the magnetospheric plasma, and have trajectories that lead them back to Mercury’s surface [7, 8].

We explored different scenarios of the sputtering of Mercury’s surface with a strong focus on dynamic surface alteration and sputtering. This includes time varying inputs from varying solar wind conditions as well as contributions of secondary ions originating from Mercury’s exosphere and magnetosphere. We rely on the dynamic sputter model SDTrimSP [9] and compare the simulation results with more simplistic TRIM [10] simulations as well as recent laboratory results of solar wind ion sputtering on Mercury analogues [11, 12, 13].

[1] Winslow, R.M., et al. (2017). J. Geophys.Res.-Space, 122(5), 4960–4975. 

[2] Winslow, R.M., et al.  (2014). Geophys. Res. Lett., 41(13), 4463–4470.

[3] Schmidt, C.A. (2013). J. Geophys.Res.-Space, 118(7), 4564–4571. 

[4] Killen, R.M., & Ip, W. H. (1999). Rev. Geophys., 37(3), 361–406.

[5] Raines, J.M., et al. (2016). Plasma Sources of Solar System Magnetospheres (pp. 91–144). Springer.

[6] Wurz, P., et al., (2019). J. Geophys. Res., 124, 2603–2612.

[7] Yagi, M., et al. (2017). J. Geophys.Res.-Space, 122(11), 10,990-11,002. 

[8] Delcourt, D.C., et al. (2003). Ann. Geophys., 21(8), 1723–1736.

[9] Mutzke, A., et al. (2019). SDTrimSP Version 6.00. Max-Planck-Institut für Plasmaphysik.

[10] Ziegler, J.F., et al. (2010). Nucl. Instrum. Methods Phys. Res. B, 268, 1818–1823. 

[11] Jäggi, N., et al. (2021). Icarus, 365, 114492. 

[12] Biber H., et al. (2020). Nucl. Instrum. Methods Phys. Res. B, 480, 10. 

[13] Szabo, P.S., et al. (2018). Icarus, 314, 98–105.

How to cite: Jäggi, N., Biber, H., Szabo, P., Vorburger, A., Mutzke, A., Aumayr, F., Wurz, P., and Galli, A.: Dynamic modeling of ion sputter yields in agreement with recent experimental data, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-287, https://doi.org/10.5194/epsc2021-287, 2021.

The study of the meteoroid environment for particles with masses in the 1 μg - 10 g range is relevant to planetary science, space weathering of airless bodies and their upper atmospheric chemistry. For the case of airless bodies as Mercury, meteoroids hit their surfaces directly, producing impact debris and contributing to shape their thin exospheres.

Mercury is a unique case in the solar system: absence of an atmosphere and the weakness of the intrinsic magnetic field. The Hermean exosphere is continuously eroded and refilled by interactions between plasma and surface, so the environment is considered as a single, unified system surface- exosphere-magnetosphere. The study of the generation mechanisms, the compositions and the configuration of the Hermean exosphere will provide crucial insight in the planet status and evolution. A global description of planet’s exosphere is still not available: missions visited Mercury and added a consistent amount of data, but still the actual knowledge about the morphology of this tenuous atmosphere is anyway poor. The ESA BepiColombo mission will study Mercury in details, by orbiting around the planet from 2025. For this reason, it is important to study the planet exospheric density and to develop a modelling tool ready for testing different hypothesis on the release mechanisms and for interpreting future observational data.

In this work we focus the attention on one of the processes responsible of the Mercury’s Ca exosphere formation: micro-meteoroids impact vaporization (MMIV) from the planetary surface. A prototype of the Virtual Activity (VA) SPIDER (Sun-Planet Interactions Digital Environment on Request) services is used as a Monte Carlo three-dimensional model of the Hermean exosphere to simulate the bombardment of Mercury’s surface by micrometeorites from different sources, as Jupiter Family Comets (JFCs), Main Belt Asteroids (MBA), Halley Type and Oort Cloud Comets (HTCs and OCCs), and to analyze particles ejected. We study how the impact vapor varies with heliocentric distance and the high impact velocity of these particles makes them critical for the morphology of Mercury exosphere, demonstrating a persistent enhancement of dust/meteoroid at dawn, which should be responsible of the dawn–dusk asymmetry in Mercury’s Ca exosphere.

_{The Sun Planet Interactions Digital Environment on Request (SPIDER) Virtual Activity of the Europlanet H2024 Research Infrastucture is funded by the European Union's Horizon 2020 research} 

_{and innovation programme under grant agreement No 871149.}

How to cite: Moroni, M., Milillo, A., Mura, A., André, N., Alberti, T., Mangano, V., Massetti, S., Orsini, S., Aronica, A., Plainaki, C., Kazakov, A., De Angelis, E., Rispoli, R., and Sordini, R.: Meteoroids as Source for Mercury’s exosphere, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-120, https://doi.org/10.5194/epsc2021-120, 2021.

Since mid ‘80s the Na exosphere of Mercury has been investigated by means of both ground-based observations and spacecraft measurements, showing a wide range of variability from tens of minutes up to seasonal variations along the planetary orbit. It has been shown that the most common Na distribution is characterized by a high latitude double peak probably related to solar wind ion precipitation through the polar cusps. However, the existence of a single peaked equatorial Na emission has been frequently observed too. Generally, it is not straightforward to recognize the contributions due to different surface release processes that produces the observed Na exospheric global image.

Here we apply the Multivariate Empirical Mode Decomposition (MEMD) to a dataset of images of the exospheric Na emission collected by the THEMIS ground-based telescope with the goal to disentangle the different contributions operating at different scales that are expected to be responsible of the occurrence of single vs. double peaked emissions or exospheric asymmetries. In particular, we found the existence of a wide range of scales characterizing both type of spatial patterns, ranging from small scales (less than 0.5 Mercury radii) up to large scales (about 1-2 Mercury radii). These scale-dependent patterns can be linked to different source mechanisms as the variability of solar wind magnetic field, different surface release mechanisms (thermal desorption, photon-stimulated desorption, micrometeoroid impact vaporization and ion-sputtering), as well as, to the whole Na exosphere surrounding the Hermean environment. Our conclusions are double checked by applying the MEMD both on Na exospheric measurements and on simulations of the Na exosphere as created by the different source mechanisms. The positive results show the great potential of the MEMD technique to study the complex environment of planetary exospheres and recognize the different components/processes that create it.

How to cite: Alberti, T., Moroni, M., Milillo, A., Mangano, V., Mura, A., Massetti, S., Orsini, S., Plainaki, C., Aronica, A., De Angelis, E., Kazakov, A., Noschese, R., Rispoli, R., Sordini, R., and Vertolli, N.: Investigating exospheric Na distributions at different spatial scales to disentangle between single and double peaked global patterns, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-64, https://doi.org/10.5194/epsc2021-64, 2021.

BepiColombo is going to conduct its first Mercury flyby in October 2021. During this flyby,  plasma measurement will be obtained and bring new insights on the Hermean magnetosphere and its interaction with the Sun despite the limited field of view of the instruments during the cruise phase. Unlike Mariner-10 ion measurements will be obtained, and unlike MESSENGER, low energy electrons and ions will be measured simultaneously. In this study, we have revisited Mariner 10 and MESSENGER observations with the help of the global hybrid model LatHyS in order to understand the influence of time-variable solar wind and to constraint the plasma environment. We are able to reproduce the magnetic field observations of Mariner 10 along its trajectory with in particular two distinct signatures consisting of a quiet and disturbed state of the magnetosphere. In addition, the plasma spectrogram is also collected in the model and this enables us to detail the properties of the charged particles observed during the flyby. We will discuss all these signatures both in term of an interaction with a time-variable solar wind and localized processes occurring in the magnetosphere. We will then present the virtual sampling of both the magnetic field and plasma spectrogram along BepiColombo’s first Mercury flyby trajectory and discuss the possible signatures to be observed at that time.

How to cite: Aizawa, S., André, N., Modolo, R., Werner, E., Slavin, J., Boardsen, S., Leblanc, F., Chaufray, J.-Y., and Raines, J.: Influence of time-variable solar wind on the response of Mercury’s magnetosphere, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-531, https://doi.org/10.5194/epsc2021-531, 2021.

The magnetosphere of Mercury is relatively small and highly dynamic, mostly due to the weak planetary magnetic field. Varying solar wind conditions principally determine the location of both the Hermean bow shock and magnetopause. In 2011 – 2015 MESSENGER spacecraft completed over 4000 orbits around Mercury, thus giving a data of more than 8000 crossings of bow shock and magnetopause of the planet, this makes it possible to study in detail the bow shock, the magnetopause and the magnetosheath structures.

In this work we determine crossings of the bow shock and the magnetopause of Mercury by applying machine learning methods to the MESSENGER magnetometer data. We attempt to identify the crossings during the whole duration of the orbital mission and model the average three-dimensional shapes of these boundaries. The results are compared with those previously obtained in other works.

This work may be of interest for future Mercury research related to the BepiColombo spacecraft mission, which will enter the orbit around the planet in December 2025.

How to cite: Lavrukhin, A., Parunakian, D., Nevsky, D., Julka, S., Granitzer, M., Windisch, A., and Amerstorfer, U.: Determination of magnetopause and bow shock shape based on convolutional neural network modelling of MESSENGER data, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-651, https://doi.org/10.5194/epsc2021-651, 2021.

The H2020 Europlanet-2020 programme, which ended on Aug 31^st, 2019, included an activity called PSWS (Planetary Space Weather Services), which provided 12 services distributed over four different domains (A. Prediction, B. Detection, C. Modelling, D. Alerts) and accessed through the PSWS portal (http://planetaryspaceweather-europlanet.irap.omp.eu/):

A1. 1D MHD Solar Wind Prediction Tool – HELIOPROPA,

A2. Propagation Tool,

A3. Meteor showers,

A4. Cometary tail crossings – TAILCATCHER,

B1. Lunar impacts – ALFIE,

B2. Giant planet fireballs – DeTeCt3.1,

B3. Cometary tails – WINDSOCKS,

C1. Earth, Mars, Venus, Jupiter coupling- TRANSPLANET,

C2. Mars radiation environment – RADMAREE,

C3. Giant planet magnetodiscs – MAGNETODISC,

C4. Jupiter’s thermosphere, D. Alerts.

In the framework of the ongoing Europlanet-2024 programme, SPIDER will extend PSWS domains (A. Prediction, C. Modelling, E. Databases) services and give the European planetary scientists, space agencies and industries access to 6 unique, publicly available and sophisticated services in order to model planetary environments and solar wind interactions through the deployment of a dedicated run on request infrastructure and associated databases.

C5. A service for runs on request of models of Jupiter’s moon exospheres as well as the exosphere of Mercury,

C6. A service to connect the open-source Spacecraft-Plasma Interaction Software (SPIS) software with models of space environments in order to compute the effect of spacecraft potential on scientific instruments onboard space missions. Pre-configured simulations will be made for Bepi-Colombo and JUICE missions,

C7. A service for runs on request of particle tracing models in planetary magnetospheres,

E1. A database of the high-energy particle flux proxy at Mars, Venus and comet 67P using background counts observed in the data obtained by the plasma instruments onboard Mars Express (operational from 2003), Venus Express (2006–2014), and Rosetta (2014–2015);

E2. A simulation database for Mercury and Jupiter’s moons magnetospheres and link them with prediction of the solar wind parameters from Europlanet-RI H2020 PSWS services.

A1. An extension of the Europlanet-RI H2020 PSWS Heliopropa service in order to ingest new observations from Solar missions like the ESA Solar Orbiter or NASA Solar Parker Probe missions and use them as input parameters for solar wind prediction;

The developments performed during the second year of the project will be discussed in the presentation.

The Europlanet 2020 Research Infrastructure project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 654208.

How to cite: André, N. and Spider, T.: Sun Planet Interactions Digital Environment on Request (VESPASPIDER) for Europlanet RI H2024: status after 2 years, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-629, https://doi.org/10.5194/epsc2021-629, 2021.