# Dynamics of electrostatically lofted dust on airless planetary bodies

Yeo Li Hsia, Xu Wang, Jan Deca, Hsiang-Wen Hsu, Mihály Horányi

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Yeo Li Hsia, Xu Wang, Jan Deca, Hsiang-Wen Hsu, Mihály Horányi. Dynamics of electrostatically lofted dust on airless planetary bodies. Icarus, 2021, 366 (September), pp.114519. 10.1016/j.icarus.2021.114519. insu-03221315

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Distributed under a Creative Commons Attribution 4.0 International License
Dynamics of Electrostatically Lofted Dust on Airless Planetary
Bodies

Yeo Li Hsia<sup>a,b,c,*</sup>, Xu Wang<sup>a,b</sup>, Jan Deca<sup>a,b,d</sup>, Hsiang-Wen Hsu<sup>a,b</sup> and Mihály Horányi<sup>a,b,c</sup>

<sup>a</sup>Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA.

b Institute for Modeling Plasma, Atmospheres and Cosmic Dust, NASA/SSERVI, California 94035, USA.

<sup>c</sup>Department of Physics, University of Colorado, Boulder, Colorado 80309, USA.

d Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), Université de Versailles à Saint Quentin, 78280 Guyancourt, France.

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ARTICLE INFO

Keywords:

Moon, surface

Asteroids, surfaces

Regoliths

Solar radiation

ABSTRACT

The dynamics of lofted dust on airless bodies have been the subject of much study since the 1960s, when the lunar horizon glow was first observed. Lofted dust dynamics have important implications for the evolution of an airless body's surface properties. To date, most of these studies have had to rely on assumptions of initial conditions of the lofted dust grains due to a lack of experimental results. In this paper, we present a study of lofted dust grains, making use of initial conditions obtained from recent laboratory results. Additionally, we distinguish between photoemission rates from isolated dust grains and from the bulk regolith. We explore dust lofting dynamics across a large size range of dust grains (0.1 to 10 µm) and the surface gravity of their parent bodies (0.0001 to 1 lunar g). Charging and lofting results reveal a complex interplay between the grain size, gravity, initial charge, and the various charging currents. It is shown that the sheath electric field becomes increasingly significant in the lofted dust dynamics as the gravitational field decreases. Due to the initial negative charge of dust grains lofted from a positively charged regolith surface, their lofting heights are found to be generally reduced compared to uncharged grains with similar initial speeds. We show that micron-sized dust grains can be lofted to high altitudes and even escape from smaller bodies, leading to the loss of fine-grained materials from the surface.

# 1. Introduction

Since the first observations of the lunar horizon glow by the Surveyor 5, 6, and 7 missions in the 1960s, electrostatic dust lofting has been suggested to be responsible for generating a hovering dust cloud that appears shortly after sunset (Criswell, 1973; Rennilson and Criswell, 1974; Colwell et al., 2007). In addition, high-altitude dust lofting above the lunar surface was indicated from the sketches of the Apollo 17 astronauts (McCoy, 1976; Zook and McCoy, 1991) as well as the LADEE/UVS spectral data (Wooden et al., 2016). Contradicting these observations, the Clementine mission, the LRO/LAMP and LADEE/LDEX measurements did not find supporting evidence for this phenomenon (Glenar et al., 2014; Feldman et al., 2014; Szalay and Horányi, 2015). Recently, the Chang'e-3 rover found a layer of dust deposition on lunar rocks as high as ~28 cm, indicating possible electrostatic dust lofting operating at least in the near-surface environment of the Moon. (Yan et al., 2019). There have been indications of sunrise-driven transport of lunar dust from the Apollo 12 Dust Detector Experiment (O'Brien and Hollick, 2015) and a suggestion on dust lofting near twilight craters based on possible interpretation of LADEE/LDEX data (Xie et al., 2020).

Dust charging, mobilization, and transport across the surface of an airless body has important implications for the evolution of the physical and spectral properties of its surface. For example, high-albedo markings on the lunar surface, so-called lunar swirls, have been thought to be the result of dust sorting by near-surface electric fields created by solar wind interactions with lunar magnetic anomalies. High-albedo patterns are visible due to the increased relative brightness of fine dust (Garrick-Bethell et al., 2011).

In addition to the Moon, evidence of electrostatic dust transport has also been observed on other airless bodies. Deposits of fine dust in crater beds, known as dust ponds, have been found on the asteroid 433 Eros (Robinson et al., 2001; Colwell et al., 2005; Hughes et al., 2008) as well as potentially on comet 67P (Thomas et al., 2015). Further, electrostatic dust lofting mechanisms may be responsible for the escape of dust grains from small bodies with weaker gravity (Hsu et al., 2020). On the asteroid Ryugu, an absence of fine dust observed by the Hayabusa 2 spacecraft

<footnote>*Corresponding author: li.yeo@colorado.edu
ORCID(s): 0000-0001-6713-1521 (Y.L. Hsia)</footnote>
(Jaumann et al., 2019) suggests that electrostatic dust removal mechanisms could be active. Additionally, electrostatic dust lofting could contribute to the observations by the Rosetta spacecraft of 'fluffy' dust grains being ejected from comet 67P beyond 3 AU (Schulz et al., 2015).

These observations have motivated laboratory experiments to understand the details of dust charging and lofting processes acting on fine-grained regolith surfaces. Dust lofting has been demonstrated in various plasma conditions in the laboratory (Flanagan and Goree, 2006; Wang et al., 2009, 2010). However, measured dust charges were found to be too small for dust to be lofted (Wang et al., 2007; Ding et al., 2013). Lee (1996) suggested preferential removal of small grains from the regolith of asteroids via the coulomb force, and subsequently several theories have been developed to explain enhanced dust charging and subsequent lofting (Flanagan and Goree, 2006; Sheridan and Hayes, 2011; Heijmans and Nijdam, 2016), but as of now, none of them can fully explain the laboratory and space observations discussed above.

A recently proposed patched charge model (Wang et al., 2016) describes the charging and lofting of dust grains exposed to ultraviolet (UV) irradiation or an electron beam via the accumulation of photo or secondary electrons, respectively, inside microcavities within the regolith. This large accumulation of negative charges on selected dust grains results in a repulsive force strong enough to loft dust grains. The accumulated charge is estimated as

$$Q \approx -0.5 \times 4\pi\epsilon_0 r\phi_c, \qquad (1)$$

where $\epsilon_0$ is the permittivity of free space, $r$ is the radius of a dust grain, and $\phi_c$ is the electric potential on the dust grains inside a microcavity. The factor of 0.5 represents that half of the dust surface is exposed to the microcavity. The large negative charge of lofted dust grains under UV irradiation was subsequently verified in a laboratory experiment (Schwan et al., 2017). Since then, the patched charge model has been further supported by more experiments (Dove et al., 2018; Hood et al., 2018; Orger et al., 2019).

A recent study shows that lofted dust grains have a broad velocity distribution likely due to a large variability in inter-particle cohesive forces (Carroll et al., 2020). The relationship between the dust size and its initial lofting velocity $v$ was observed to follow

$$v \approx \gamma \frac{|\phi_c|}{r} \sqrt{\frac{3\epsilon_0}{\rho}}, \qquad (2)$$

where $\rho$ is the dust mass density and $\gamma$ is an empirical coefficient that mainly accounts for the shape of dust grains partially controlling the cohesive force between the grains. For irregularly-shaped grains, $\gamma = 5$ gives a good fit to the measured velocities as a function of the dust size (Carroll et al., 2020).

The investigation of lofted dust dynamics in various electrical environments has thus far been a mostly theoretical and modeling-based endeavor (Lee, 1996; Nitter et al., 1998; Stubbs et al., 2006; Wang et al., 2008; Poppe et al., 2012; Hirata and Miyamoto, 2012; Hartzell and Scheeres, 2013; Nordheim et al., 2015; Piquette and Horányi, 2017). To date, these studies have mainly relied on assumptions of initial conditions of lofted dust grains due to the lack of experimental results. One prevailing assumption has been that the lofted dust grain's initial charge has a positive polarity due to photoemission from the grain itself. Instead, in light of the recent laboratory experiments and the predictions based on the patched charge model (Eq. 1), lofted grains are expected to have a large negative initial charge (Schwan et al., 2017). Additionally, the previous studies did not have knowledge of the recently found relationship between the size of a lofted dust grain and its initial velocity (Eq. 2).

Solar UV irradiation of the lunar surface causes photoemission, which produces a dominant current to the lunar surface. This photoemission current results in an upward pointing electric field and an electron-rich region, forming a photoelectron sheath above the surface (Guernsey and Fu, 1970; Grard and Tunaley, 1971; Walbridge, 1973). The electrostatic interaction of lofted dust grains with this sheath, in combination with their initial lofting speed and charge, determines their dynamics. Measurements of the photoelectric yield from Apollo 14 and 15 dust samples (Willis et al., 1973; Feuerbacher et al., 1972) have been used to estimate the profile of this photoelectron sheath.

There is experimental evidence that photoelectron emission rates from individual dust grains may be up to an order of magnitude higher than these previous measurements on bulk regolith samples (Abbas et al., 2007). This is consistent with the phenomenon of microcavity absorption of photoelectrons as described by the patched charge model (Wang et al., 2016) and several later experimental results (Wang et al., 2016; Dove et al., 2018). To date, this distinction
between photoemission from solitary dust grains and from the bulk regolith has not been considered when modeling lofted dust dynamics (Nitter et al., 1998; Poppe and Horányi, 2010).

The patched charge model has been used to identify the initial conditions of lofted dust grains and predict their dynamics in the lunar terminator region, where the surface is expected to be charged to negative potentials (Orger et al., 2018) due to the weak photoelectron flux compared to the flux of solar wind electrons. A recent experimental study found that the majority of dust particles lofted by the patched charge effect had diameters of < 10 µm (Hood et al., under review). In this paper, we present a study of lofted dust grains in a typical photoelectron sheath where the surface is positively charged. We apply initial conditions from the patched charge model and recent laboratory results. Our simulations consider a range of photoemission rates from lofted individual dust grains. In addition, we explore lofting dynamics across a large size range of dust grains smaller than 10 µm and under a wide range of gravity conditions.

## 2. Test Particle Simulations

A test particle code was developed for this study to track the charging and trajectories of individual dust grains as they are launched from the surface of an airless body. A range of gravitational environments is investigated, from lunar $g = 1.62 \text{ ms}^{-2}$ to $10^{-4} \times \text{ lunar g}$, which is comparable to that of small asteroids like Ryugu and Bennu.

We assume a typical photoelectron sheath on an airless body at 1AU at zero solar zenith angle. The characterization of the electrostatic sheath is based on a 1D Particle-in-Cell simulation (Poppe and Horányi, 2010) to determine the electric potential, as well as the electron and ion densities above the body's surface. Photoelectrons follow a Maxwellian velocity distribution (Poppe and Horányi, 2010). The solar wind flow speed is $v_{SW} = 400 \text{ km s}^{-1}$, the electron and ion temperatures are $kT_{SW} = 10 \text{ eV}$, and the solar wind density is $n_{SW} = 7.7 \times 10^{6} \text{ m}^{-3}$. The photoelectron temperature is $kT_{p} = 2.2 \text{ eV}$, and the photoemission current density from the surface of an airless body at 1AU is $J_{d} = 4.5 \times 10^{-6} \text{ Am}^{-2}$, which was measured from lunar samples (Willis et al., 1973). These parameters result in a Debye length of $\lambda_{D} \approx 1 \text{ m}$.

The photoemission current density from a lofted individual dust grain ranges from $J_{d} = 4.5 \times 10^{-6} \text{ Am}^{-2}$ to $9 \times J_{d}$ (Abbas et al., 2007), dust grain radii range from 0.1 µm to 10 µm. The initial charge on the dust grains is determined using Eq. 1. According to the patched charge model (Wang et al., 2016), the net change of a lofted dust grain is dominated by the negative charge deposited on half of the surface area of the dust grain in the microcavity, with a surface potential of $\phi_{c}$. To simplify the charging simulation, we assume a uniform distribution of charge across the surface of the dust grain with an initial surface potential $\phi_{0}$. Therefore, $\phi_{0}$ is half of $\phi_{c}$ for the same amount of charge. Hence, based on experimental results (Wang et al., 2016), the initial dust potential $\phi_{0}$ is in the range of $\phi_{0} = -1 \text{ V}$ to $\phi_{0} = -10.5 \text{ V}$. Dust grains are assumed to be launched vertically upwards with initial velocities estimated using Eq. 2 with $\gamma = 5$ for irregularly shaped grains (Carroll et al., 2020). As shown in Carroll et al., 2020, the grain's initial velocity is mainly determined by overcoming the cohesive force, which depends on dust shape, composition, cleanliness, compaction and how grains come into contact (Perko et al., 2001; Hartzell and Scheeres, 2011; Hood et al., 2018; Orger et al., 2019). Recent laboratory results provide baseline initial parameters of lofted dust. All initial parameters are applied from laboratory experiments with lunar or Martian simulants, and charging conditions simulated using UV light or electron beam (Wang et al., 2016; Schwan et al., 2017; Carroll et al., 2020).

To follow the charging of lofted dust grains, we include: a) both photo and secondary electron emission from the grain; b) electron collection currents from both the photoelectron sheath and the solar wind; and c) the solar wind ion current (Horányi, 1996). The evolution of the charge and the trajectories of the dust grains are calculated based on the UV and plasma environment, and forces due to gravity and the sheath electric field. For comparison, trajectories are also calculated without the electric field in order to quantify the effect of the photoelectron sheath on the dynamics of charged dust grains.

## 3. Results of Dust grain Lofting

Figure 1 shows sample trajectories and charging histories of dust grains of 10, 1, and 0.1 µm radii lofted on the Moon. Dotted lines in parts a), b) and c) indicate local equilibrium potentials - the potential at which a dust grain will settle at given infinite time at a certain height. A dust grain with larger size more closely follows this equilibrium potential due to larger charging currents. Though the grains are launched with the same initial potential of $\phi_{0} = -4 \text{ V}$, the larger grains have lower initial velocities (Eq. 2), such that the 10 and 1 µm grains stay within the photoelectron sheath through their trajectories. The 0.1 µm grain is launched to a much greater height and is less affected by the
<figure><img src="image_2.png" /><figcaption>**Figure 1:** Dust grain trajectories and charging. The trajectories, real-time potentials and equilibrium potentials of 10 (a), 1 (b) and 0.1 (c) µm radius dust grains launched vertically upwards under lunar gravity with initial potentials of $\phi_0 = -4$ V, photoemission current densities of $J_d = 4.5 \times 10^{-6}$ Am$^{-2}$, and the following charging currents (d, e, f): photoelectron emission from dust, $I_p$; secondary electron emission, $I_{sec}$; the collection of electrons from the photoelectron sheath, $I_s$; and the collection of solar wind ions and electrons, $I_{SWi}$, $I_{SWe}$, respectively.</figcaption></figure>

sheath. Differences due to varying sheath effects on the charging currents can be seen in parts d), e), and f) of Fig. 1. Close to the surface, the electron current to the dust grain from the surface-emitted photoelectrons in the sheath $I_s$ is relatively high due to the high density of photoelectrons, but this falls off quickly with increasing height as the photoelectron density diminishes rapidly. Solar wind electrons with a relatively hot temperature, on the other hand, have a more constant current $I_{SWe}$ to the dust grain. The high-energy tail solar wind electrons are energetic enough to produce a small secondary electron current $I_{sec}$ from the dust grain. The solar wind ion current $I_{SWi}$ is low due to the large mass and thus lower mobility of the ions. Finally, photoelectron emission from the dust grain results in a constant current $I_p$ as the dust grain is negatively charged throughout the trajectories, such that all the photoelectrons emitted from its surface are repelled into the ambient plasma.

An overall view of the maximum lofted heights of dust grains of various sizes and initial potentials under different gravitational fields is shown in Figure 2. Here, $J_d = 4.5 \times 10^{-6}$ Am$^{-2}$. The effect of $J_d$ variations will be shown in Fig. 3. As expected, a weaker gravitational field results in higher dust lofting. Also, given the same initial dust potential, a smaller grain is lofted to a higher altitude because of its higher initial velocity (Eq. 2). When dust grains are launched with electric potentials of larger magnitude, they have larger initial velocities (Eq. 2) and are therefore lofted higher than dust grains with smaller electric potentials. Where data points are ‘missing’ at lower gravitational strengths and/or higher magnitude $\phi_0$, dust grains have effectively escaped the gravitational pull of the airless body (Fig. 2).

At lunar g, 10 µm radius grains are lofted to a height of ~30 cm. At the lunar terminator, plasma conditions and hence the sheath profile (Stubbs et al., 2014) are different from the photoelectron sheath in this study. A different lofting height but on the same order of magnitude may be expected (Orger et al., 2018), which is consistent with observations of the lunar horizon glow (Criswell, 1973; Rennilson and Criswell, 1974). Grains of 0.1 µm in radius are lofted to as high as 3 km, which coincides with the height suggested for the global-scale horizon glow (Zook and McCoy, 1991). For small bodies with 0.001 lunar g and less, grains of 10 µm and smaller mostly escape from these bodies. This electrostatic dust escape process may be responsible for dust removal on small asteroids, as it was recently reported from the observations of asteroids Ryugu and Bennu (Jaumann et al., 2019; Hsu et al., 2020).
<figure><img src="image_3.png" /><figcaption>**Figure 2: Maximum lofting heights.** Overview of maximum lofting heights of dust grains of various sizes and initial potentials in different gravitational fields. Horizontal dashed lines are added to guide the eye for the 10 µm and 0.1 µm grains with $\phi_0$ = -10.5V under lunar g. Here, $J_d$ = $4.5 \times 10^{-6}$ Am$^{-2}$. The effect of changing $J_d$ is shown in Figure 3.</figcaption></figure>

To examine the effect of the sheath electric field on dust lofting, Figure 3 shows the maximum lofting heights of grains launched with $\phi_0$ = -1 V including and excluding the sheath electric field. The rate of photoemission from dust grains was varied from $J_d$ = $4.5 \times 10^{-6}$ Am$^{-2}$ to $9 \times J_d$. Dust grains with larger negative values of $\phi_0$ (-4 V or -10.5 V) have higher launch velocities, and spend less time in the photoelectron sheath. Their lofting heights are thus less affected by the sheath electric field, showing less than 5% difference compared to the gravity-only condition.

For the majority of simulation cases, the dust grains reach a lower maximum lofting height than the gravity-only cases. This is because the dust grains are launched with a negative charge and are initially attracted to the positively charged surface. We find that the reduction in lofting height is more significant as the gravitational field weakens, due to the increasing effects of the sheath electric field. As shown in Fig. 3, under lower gravitational conditions, the sheath electric field can reduce the lofting height by as much as 25% compared to the height determined by gravity alone.

Notice that the r = 10 µm dust grain with $J_d$ = $9 \times 4.5 \times 10^{-6}$ Am$^{-2}$ actually exceeds the gravity-only lofting height under lower gravitational conditions. This is because the dust grain becomes positively charged during flight in the photoelectron sheath and is lifted higher as a result of a combination effect of a larger $J_d$, a faster charging time for larger dust grains and a longer time spent in the photoelectron sheath in which the electric field force is effective. In addition, non-monotonic trends shown in Fig. 3 also result from a combination effect of the grain size, gravitational field, initial dust potential, and charging currents.

Figure 4 shows the electric potentials $\phi_f$ on dust grains as they return to the surface of their parent bodies after being lofted. We observe that larger grains, regardless of $J_d$, have generally more negative $\phi_f$ than smaller ones. This is because larger grains spend a longer time deep in the photoelectron sheath, where the grains collect more photoelectrons. We also observe that, as expected, for the same grain size and $\phi_0$, a higher $J_d$ results in a more positive $\phi_f$. This is especially true under a weaker gravitational field, where the grains have a longer flight time.

To illustrate how dust charging behaviour is the result of a complex interplay of several factors, we examine a unique confluence of factors that results in dust grains of vastly different sizes having little variation in $\phi_f$. At lunar g, it is observed that when a dust grain is launched with a very negative $\phi_0$ and small $J_d$, its $\phi_f$ is virtually independent of its size. These cases are indicated by the red and blue triangles in Fig. 4. Such an observation is a direct consequence of the dust grain's strongly negative electric potential and the significant role of gravity in the its trajectory. The small $J_d$ does little to shift the dust potential away from strongly negative, meaning that sheath and solar wind electrons are overwhelmingly repelled from the grain. The result is a near constant and small influence from mainly the photoemission process.

This allows one to estimate the total change in dust charge as a product of two terms: the current to the grain and its total time in flight ($\delta Q = I_{dust} \times t_f$). The current to the dust is proportional to its surface area and thus proportional to $r^2$ ($I_{dust} \propto r^2$). For the second term $t_f$, we note that at lunar g under the plasma conditions used in this study, the trajectory of a lofted dust grain is mainly determined by the gravitational force, as described above. This means that the flight time follows that of a simple parabolic trajectory in a gravitational field ($t_f = 2v_0/g$). According to Eq.
<figure><img src="image_4.png" /><figcaption>**Figure 3:** Effect of sheath electric field on dust lofting height. Plots of maximum lofted heights of dust grains of different sizes under a range of gravitational conditions. Trajectories are calculated with multiples of $J_d = 4.5 \times 10^{-6} \text{ Am}^{-2}$. All grains plotted here started with the same initial potential of $\phi_0 = -1\text{V}$. A reference plot is provided in black of the dust height when only gravitational forces are considered. The difference in height of various trajectories as compared to this reference plot is also displayed on the bottom row of graphs.</figcaption></figure>

2, this means that the flight time is inversely proportional to the radius of the grain ($t_f \propto v_0 \propto 1/r$). The change in charge on the dust grain is a product of these two terms and is hence proportional to $r (\delta Q \propto (r^2 \times 1/r) \propto r)$. We use the relationship between the electric charge and potential of a spherical dust grain ($\phi = Q/(4\pi\epsilon_0 r)$) to find that the change in dust potential is ultimately independent of $r (\delta\phi \propto \delta Q/r \propto r/r = 1)$.

## 4. Conclusion

The dynamics of charged dust above the surfaces of airless bodies was studied over large ranges in the sizes of dust grains and their parent bodies, using initial conditions from recent laboratory results. With these new initial conditions, dust grains under UV irradiation were launched with negative electric charges as described by the patched charge model, rather than the customarily assumed positive charges used in previous studies. Additionally, we have incorporated the recently found relationship between the size of a lofted dust grain and its initial velocity.

The maximum lofting heights on the Moon were found to be about 30 cm for 10 µm radius dust grains and 3 km for 0.1 µm radius grains, which are consistent with previous observations of the local and global-scale lunar horizon glow. Smaller grains tend to escape as the gravitational field weakens. A previous study (Lee, 1996) shows that grains smaller than 0.01 µm can escape a 0.01 lunar g body. However, our study finds that, given initial velocity distributions observed in the laboratory, larger dust grains up to 0.1 µm can be lost to space. Grains of 10µm in radius and smaller mostly escape from bodies as small as asteroids Ryugu and Bennu, which may contribute to the removal of fine dust on these bodies. Charging and lofting results shown in this work also reveal a complex interplay between the grain size,
<figure><img src="image_5.png" /><figcaption>**Figure 4:** Effect of $J_d$ on dust charges. Plots of final dust potentials $\phi_f$ of grains of different sizes under different gravitational conditions. Dust grains with varying initial potentials and photoemission rates are differentiated on the plots.</figcaption></figure>

gravitational field, initial dust potential, and various charging currents. It was shown that the sheath electric field effect becomes more significant as the gravitational field decreases, causing a reduced maximum height of lofted grains. The reduction is as much as 30% of the height determined by gravity only. For larger bodies (>0.1 lunar g) under typical photoelectron sheath conditions, electrostatically lofted grains larger than 10 µm can be modeled as following ballistic trajectories with initial velocity determined from the surface potential and equation 2. It is also found that higher photoemission rates from dust grains can significantly alter their lofting trajectories. Ambient plasma conditions and subsequently, the sheath profile, can vary over a large range to differ from the photoelectron sheath shown in this study, for example in magnetic anomaly or terminator regions, thereby causing variations in lofted dust dynamics. In addition, variable gravitational acceleration due to aspherical shapes of small asteroids and the radiation pressure force will be considered in future studies on dust dynamics on the small bodies.

These results on the dynamics of charged dust are important for the understanding of surface processes due to electrostatic dust transport as well as their implications for the surface physical and spectral properties of airless bodies, such as the Moon and asteroids. Furthermore, our findings are critical for the design of dust instruments to measure electrostatically lofted dust, and for mitigating dust hazards on future surface exploration missions on airless bodies like the Moon.

## Acknowledgements

This work was supported by the NASA/SSERVI's Institute for Modeling Plasma, Atmospheres and Cosmic Dust (IMPACT) and by the NASA Solar Systems Working Program (Grant number: NNX16AO81G).

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