# Expected effect of atomic oxygen of Europa's exosphere on the MAss Spectrometer for Planetary EXploration

Alexis Bouquet, Ryan Blase, Tim Brockwell, J. Hunter Waite

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Alexis Bouquet, Ryan Blase, Tim Brockwell, J. Hunter Waite. Expected effect of atomic oxygen of Europa's exosphere on the MAss Spectrometer for Planetary EXploration. Planetary and Space Science, 2020, 188, 10.1016/j.pss.2020.104918. insu-03667059

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# Expected effect of atomic oxygen of Europa's exosphere on the MAss Spectrometer for Planetary EXploration

Alexis Bouquet<sup>a,b,c,*</sup>, Ryan Blase<sup>c</sup>, Tim Brockwell<sup>c</sup>, J. Hunter Waite<sup>c,b</sup>

## Abstract

The future Europa Clipper NASA mission to Europa will carry the MAss Spectrometer for Planetary EXploration (MASPEX), which will analyze the compounds making up Europa's exosphere, and plumes, if present. Europa's exosphere is likely to feature abundant atomic oxygen, a product of sputtering of Europa's surface by energetic particles from Jupiter's radiation belts, or photolysis of oxygen-bearing molecules in the exosphere. Due to its very high reactivity, this atomic oxygen could induce chemical processes within MASPEX. These processes could involve materials composing the instrument, or other compounds from Europa's exosphere. In both cases, the possible effect on the instrument's measurements is a concern. In this work we first review previous relevant experiments on the effect of atomic oxygen on several candidate metals and conclude that stainless steel with a gold coating is the most satisfactory choice for MASPEX' antechamber walls, and alumina for the impact plate. We then perform simulations of adsorption/desorption processes within the instrument's antechamber to bound the effect of atomic oxygen on other compounds from Europa's exosphere. We find that the accumulation of atomic oxygen during a flyby would lead to a reduction of the apparent abundance of water by as much as 0.125 % (Δ[H<sub>2</sub>O]/[H<sub>2</sub>O] = -1.25 × 10<sup>-3</sup>). Evaluating the exact extent of this effect during data post-processing would require constraining the atomic

<footnote><sup>a</sup>Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France</footnote>

<footnote><sup>b</sup>Department of Physics and Astronomy, University of Texas at San Antonio</footnote>

<footnote><sup>c</sup>Space Science and Engineering Division, Southwest Research Institute, San Antonio</footnote>

<footnote>*Corresponding author
Email address: alexis.bouquet@gmail.com (Alexis Bouquet)</footnote>
oxygen abundance, which would have to be achieved with other instruments,
such as the ultraviolet spectrometer.

Keywords: Europa, MASPEX, exosphere, atomic oxygen

# 1. Introduction

Europa is among the most promising objects in our solar system for astro-
biological considerations, with a liquid water ocean under its icy crust [1, 2, 3]
and evidence of communication between this ocean and the surface [4], even
including possible plumes [5, 6], similar to those observed at Saturn's moon
Enceladus by the Cassini mission [7]. Europa is situated within Jupiter's very
active radiation belts, exposing its surface to an intense bombardment by en-
ergetic particles [8], which has the effect of inducing chemical processes on the
surface and creating a sputtered exosphere [9, 10]. This exosphere creates an
opportunity for the future Europa Clipper mission to analyze material sput-
tered from the surface, including from species resulting from exchange with the
ocean. To this effect, the Europa Clipper spacecraft will be equipped with the
MAss Spectrometer for Planetary EXploration (MASPEX). We refer the reader
to Brockwell et al. [11] for details on the instrument and will only present here
the aspects relevant to our study.

Due to the high velocity of the spacecraft relative to the gas to be sampled,
MASPEX' design includes an antechamber with an impact plate (Figure 1) to
thermalize the sample before it can reach the ion source. There, the gas is ion-
ized by electron impact and the ions are extracted and sent to the Multi Bounce
Time-Of-Flight (MBTOF) analyzer. The ion beam does not contact the walls
nor is dense enough for ion-ion interaction to have a noticeable effect, but the
neutral gas sample present in the antechamber prior to ionization can interact
with the antechamber walls (wall-sample interactions), and sample compounds
colliding with the walls may react with other sample compounds adsorbed on
the walls (intra-sample interactions). The presence of atomic oxygen, which
is very reactive, in Europa's exosphere [12], raises the question of the effect it
<figure><img src="image_2.png" /><figcaption>Figure 1: Schematic of the antechamber, impact plate and outlet leading to the ion source of MASPEX. The yellow layer on the antechamber wall represents the coating of gold that this work has shown to be a good candidate to mitigate atomic oxygen effects on measurements.</figcaption></figure>

could have on measurements. We focus our work on finding the most appro-
priate material to build the antechamber and impact plate to avoid alteration
by atomic oxygen, and minimize intra-sample reactions between atomic oxygen
and other compounds within the antechamber.

Section 2 reviews the available experimental information on the effect of
atomic oxygen on materials that could be used for building the antechamber and
the impact plate. Section 3 presents the model of antechamber chemistry we use
for bounding the effects of intra-sample reactions, and explains the results we
obtain with input corresponding to the current state of knowledge on Europa's
exosphere and the expected trajectory of Europa Clipper's flybys. Section 4
discusses the implications of our findings and how to address the effects we have
quantified.

## 2. Atomic oxygen interaction with the instrument: selection of the materials

Atomic oxygen is a di-radical neutral in its electron configuration, having two
lone electrons in two valence orbitals. The di-radical is very unstable and hence
very reactive. The reactivity of atomic oxygen compared to molecular oxygen has been studied in the field of catalysis with investigation of oxidation rates of metal surfaces. Slezak et al. [13] found the oxidation rate for atomic oxygen to be two orders of magnitude higher than for molecular oxygen on a Ni(111) surface, "Atomic oxygen induced oxidation occurs about 100 times faster than that for molecular oxygen induced oxidation in all regimes of oxygen uptake." The reason for this is that with atomic oxygen, a bond does not need to be broken at the surface, giving 498 kJ mol⁻¹ additional energy to the reactants. Similarly, Raspopov et al. [14] found the rate constant of nickel oxidation to be higher by orders of magnitude for atomic oxygen versus molecular oxygen and reported the activation energy of oxidation to be lowered to 76 kJ mol⁻¹ for atomic oxygen versus 182 kJ mol⁻¹ for molecular oxygen [14]. Studies with copper metal surfaces showed atomic oxygen to be a much stronger oxidizer as well with initial stages of oxidation being 10⁶ times larger (higher reaction probability) than that of molecular oxygen [15]. For these reasons, the interaction of atomic oxygen with the metals present in the instrument and other compounds of the sample are a primary concern for future measurements.

## 2.1. Interaction of atomic oxygen with metals

Atomic oxygen effects on materials have been an interest to the space science community for a number of years. Due to the large flux of atomic oxygen in low Earth orbit, many studies have deployed spacecrafts with a variety of materials to study the effects of atomic oxygen. Studies include but are certainly not limited to results from the Long Term Duration Exposure Facility (LDEF) [16] [17] [18], STS 1-3 in the Space Shuttle Orbiter payload [19] and STS-8 and 41-G (STS-17) [20] [21]. Materials investigated include metals, metal alloys, polymers (composite materials), paints, lubricants, structural materials, thermal control coatings and optical coatings. Materials were divided into the following categories: organic, metallic, nonmetallic, optical coatings and thermal control coatings in reference [16]. For the purposes of this study, we are most concerned with the results of metals and nonmetals suitable for selection
of materials in the antechamber of MASPEX.

The effect of atomic oxygen on metals is usually measured in two ways: mass loss or mass gain of the metal sample, and change in its optical properties, using Rutherford Backscattering and ellipsometry. Measurements of mass loss or gain [16] show that copper, gold, nickel, niobium, and tantalum do not undergo an important mass change. Among these, copper shows a modest change in optical properties [20]. Gold appears as the material of choice for the antechamber because of this absence of mass loss or gain, combined with considerations of cost and the ease to deposit a gold film over stainless steel. Moreover, multiple publications have confirmed desorption of molecular oxygen from recombination of atomic oxygen leading to a clean gold surface at a temperature of 550 K [22, 23, 24]. Heating of the antechamber to 550 K enables the antechamber gold surface to be cleaned after each flyby, eliminating atomic oxygen buildup over time.

## 2.2. Interaction of atomic oxygen in the ion source

Intra-sample interactions of atomic oxygen pose two measurement challenges: quantification of atomic oxygen itself, and the creation or destruction of other compounds due to atomic oxygen. It is estimated that in situ atomic oxygen measurements in Low Earth Orbit are too low by a factor of 2.5 [25, 26], which is attributed mainly to metal surface reactions with other compounds. We investigate the metal surface reactions possible in MASPEX in Section 3. The present section focuses on the interaction of atomic oxygen in the ion source.

An in-depth study of atomic oxygen effects in the ion source [27] showed an enrichment in key masses 18, 28, 32 and 44 due to the following chemical reactions in the ion source:
$$H_2 + O \rightarrow H_2O \quad (1)$$

$$C + O \rightarrow CO \quad (2)$$

$$O + O \rightarrow O_2 \quad (3)$$

$$CO + O \rightarrow CO_2 \quad (4)$$

With prolonged exposure of the instrument to the beam, atomic oxygen accumulates on its surfaces, including in the ion source. This offers more opportunities for other compounds to encounter it, therefore increasing the importance of these reactions. This is illustrated by the sharp increase in CO₂ production seen in the experiments [27]. This concern has led to several design compromises in mass spectrometers to limit the effects of ion source chemistry [28, 29, 30, 31]. For MASPEX, the chosen route is to limit the amount of atomic oxygen entering the ion source. This can be achieved by using materials in the antechamber on which atomic oxygen will chemisorb before reaching the ion source. By contrast, covering the antechamber with a material totally inert to atomic oxygen (such as alumina) would let the atomic oxygen bounce and find its way into the ion source. To prevent accumulation of atomic oxygen over flybys, it is desirable that this material allows for thermally stimulated desorption. Gold presents both characteristics, with a temperature of 550K being sufficient to remove the accumulated atomic oxygen after a flyby.

## 2.3. Material selection for MASPEX’ antechamber

Antechamber materials need to be selected for the impact plate and the walls. The impact (or striker) plate allows impacting neutral gas molecule to reduce their energy while the walls act as collision surfaces for the neutral molecules. The population of neutral molecules will eventually leave both through the antechamber outlet (towards the ion source) and the antechamber inlet (to the outside of the instrument). Both the striker plate and the walls need to be resistant to atomic oxygen attack that would cause mass loss and formation of surface oxides.
### 2.3.1. Impact/striker plate

The impact plate will be made out of alumina, $Al_2O_3$, a ceramic which has proven to be a good protective barrier of polyimide substrates from atomic oxygen attack. A film of 35 Å was shown to protect the polymer from erosion [32] and no evidence of atomic oxygen reaction with the $Al_2O_3$ surface was seen (Minton, personal communication), indicating that scattering reflection of incoming oxygen atoms is the most likely.

### 2.3.2. Antechamber walls

The antechamber walls will be made of stainless steel, with the region of specular impact from the impact plate to be coated with a thin wall of gold. Gold does not suffer mass loss from atomic oxygen attack [16], and its optical properties are relatively unchanged, indicating the gold surface characteristics are not affected by atomic oxygen exposure [33].

Molecular oxygen does not adsorb on gold surfaces, but atomic oxygen does [34]. The literature shows a range of sticking coefficients for atomic oxygen on gold. Linsmeier and Wanner (2000) [22] found a value decreasing from $5 \times 10^{-3}$ at low coverage to $1.5 \times 10^{-3}$ at higher coverage of $1.6 \times 10^{15}$ atoms/cm$^2$. This is a very low value when considering the high reactivity of atomic oxygen, and that other experiments showed a high coverage of gold by atomic oxygen [23, 24], albeit at a low temperature (77K) compared to the conditions expected in MASPEX (300K). Another set of experiments shows a sticking coefficient decreasing from $4.5 \times 10^{-2}$ at zero coverage to $2.5 \times 10^{-2}$ at $0.8 \times 10^{15}$ atoms/cm$^2$, and to less than $1 \times 10^{-2}$ above $1 \times 10^{15}$ atoms/cm$^2$[35]. We refer to these values for modeling the adsorption of atomic oxygen in MASPEX' antechamber in Section 3. Another piece of information necessary for this modeling is the adsorption/desorption energy of atomic oxygen on the gold surface, which governs the probability for a physisorbed atom to overcome the energy barrier and leave the surface at any time. Adsorption and desorption energy are assumed to be the same throughout this study, as supported by experimental evidence [36]. It is assumed that a physisorbed oxygen atom will chemisorb to the
surface. The desorption energy would then be the opposite of this chemisorption energy, which is reported to be 155 kJ/mole [37]. This very high energy ensures that desorption of atomic oxygen is negligible on the timescale of a flyby.

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### 3. Model of antechamber chemistry-Expected effect of atomic oxygen on MASPEX observations at Europa

We use an antechamber surface chemistry model of MASPEX to quantify the effects that atomic oxygen will have on the measured composition. This model calculates the evolution of the compounds population in the antechamber by using a statistical representation of the interactions producing or removing the molecules. This model was originally developed for the Cassini INMS investigation [38], to study the effect of the release of pure titanium by ice grain impacts [39].

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#### 3.1. Parameters of the antechamber model

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Here, this INMS antechamber model has been adapted to MASPEX and the environment at Europa. The model requires the sticking coefficient and desorption energy (from gold) of every compound to be included. We simulate the evolution of the three most abundant neutral compounds for each flyby: water ($H_2O$), oxygen ($O_2$), and hydrogen ($H_2$). The fourth compound for the simulation is atomic oxygen. The important parameters for atomic oxygen as detailed in Section 2.3.2 are: sticking coefficient of $4.5 \times 10^{-2}$ and desorption energy of 155 kJ/mole. For water, the most abundant compound in most flybys, the sticking coefficient is assumed to be 1.0, similar to the experimental value for water on $TiO_2$ [40]. The water desorption energy on gold used in the simulation is 51.5 kJ/mole [41]. The water sticking coefficient of 1.0 creates the worst-case scenario for atomic oxygen effects with water by increasing the probability that an atomic oxygen, upon entering the antechamber (and after specular reflection by the impact plate), would land near an adsorbed water molecule. If atomic

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<table><caption>Table 1: Values of compound-specific constants: sticking coefficients and desorption energies considered in this study.</caption><thead><tr><th>Parameter(all w/r to gold)</th><th>Value</th><th>Source/commments</th></tr></thead><tbody><tr><td>H<sub>2</sub>O sticking coefficient</td><td>1.0</td><td>Based on value for TiO<sub>2</sub> [40]</td></tr><tr><td>H<sub>2</sub>O desorption energy</td><td>51.5 kJ/mole</td><td>[41]</td></tr><tr><td>O<sub>2</sub> sticking coefficient</td><td>0</td><td>[34, 22]</td></tr><tr><td>O<sub>2</sub> desorption energy</td><td>N/A</td><td>Irrelevant since sticking coefficient is zero</td></tr><tr><td>O sticking coefficient</td><td>4.5× 10<sup>-2</sup></td><td>[35](their Figure 7) experiment at 300K, near zero coverage</td></tr><tr><td>O desorption energy</td><td>155 kJ/mole</td><td>[37] Ensures no desorption during simulation</td></tr><tr><td>H<sub>2</sub> sticking coefficient</td><td>0</td><td>[35]</td></tr><tr><td>H<sub>2</sub> desorption energy</td><td>N/A</td><td>Irrelevant since sticking coefficient is zero</td></tr></tbody></table>

oxygen were to access a gold surface site near a bound water molecule or vice versa then the following reaction is assumed to occur

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$$H_2O_{(ad)} + O_{(ad)} \rightarrow 2OH_{(ad)} \quad (5)$$

where two hydroxide molecules are then bound to the gold surface. This binding is assumed to be strong and OH to desorb from the surface only with excess heating [24, 42, 36]. For molecular oxygen (O₂) the sticking coefficient is assumed to be zero [34, 22]. As such, with a sticking coefficient of zero there is no need for a value of desorption energy for O₂. Similarly, for hydrogen (H₂) the sticking coefficient is zero [35] and no value of desorption energy is needed. A summary of these parameters is provided in Table 1.

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## 3.2. Model description

The antechamber chemistry model used for MASPEX is based on the model developed and used for Cassini-INMS at Enceladus [39]. This model takes into account ram enhancement (material entering the antechamber as a function of the spacecraft's velocity), physisorption, desorption, and escape through the main aperture to describe the population of compounds of interest in gas phase

190
<figure><img src="image_3.png" /><figcaption>Figure 2: Interactions considered in the antechamber chemistry model: Ram enhancement (sample entering the antechamber), escape through the main inlet, adsorption in the form of physisorption or chemisorption (reaction with atomic oxygen) and desorption.</figcaption></figure>

and in the surface phase (on the walls). It calculates the evolution of the popu-
lations through a statistical treatment of interactions. The processes considered
are summarized in Figure 2.

The model relies on the following core assumptions:

* Desorption is governed by the Temkin isotherm [43], that is, the adsorption energy decreases linearly with the surface coverage. The Temkin isotherm is appropriate for a surface with low coverage (below 30%), which has been verified to be the case in the study of INMS at Enceladus [39], where the plumes induce higher densities than expected at Europa.

* The reactions we consider are instant if the two reactants meet. Reactions have a 100% probability.

* The gas sample is at the temperature of the antechamber wall; this tem-
perature is 300 K, due to the heating of the instrument.

* All reactions of interest happen on the surface of the antechamber (molecule in gas phase making contact with an adsorbed molecule); there are no re-
210

actions in gas phase. Once again, the INMS study showed the gas stayed non-collisional through the crossing of the highest density regions of Ence-ladus' plumes.

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* The geometry is not taken into account: a molecule can have equal probability to hit any site on the surface. Surface populations are homogeneously distributed. Since reflection is non specular on a metal surface that has not been highly polished [44] and most species can undergo multiple bounces or adsorption and desorption, this assumption is justified.

The required inputs are:

* External densities of each compound of interest. These densities can be varied with time to simulate a flyby.

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* Properties of each compound: sticking coefficient and desorption energy on the surface material (here, gold), molecular weight, Temkin coefficient (very low impact as in this study coverage stays well below a percent). For use at Europa, the model has been adapted as follows:

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* The surface is covered in gold. We ignore the impact plate as no compounds are expected to stick on it (reflection of molecules due to suprathermal arrival velocity).

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* The compounds of interest are H₂O, O₂ (most abundant neutrals expected at Europa) and atomic oxygen (most abundant reactive expected at Europa). The reaction between water and atomic oxygen produces two OH. The reaction between O₂ and O may produce ozone (O₃) but experimental evidence ([45] and Minton, personal communication) leads us to believe this ozone would dissociate on contact with the surface; even if it were not the case, the mass spectrum of ozone after EI ionization has its main peak at mass 32 [46] as ozone dissociate into O₂ and O.
### 3.3. Flyby conditions

To determine the conditions for the simulated flybys, we use an estimate of compound densities at Europa by Teolis et al. [47], who considered separately a sputtered exosphere model and a (south polar) plume model. In the case of a sputtered only exosphere (no plume) the model shows water is expected to be the most abundant compound on the dayside, while nightside and polar flybys will see a higher amount of $O_2$. A possible factor in the densities to consider is spacecraft outgassing of water. Previous studies [48] have found that spacecraft outgassing could create a water background of $1\text{E5}$ to $1\text{E7 cm}^{-3}$. These values are comparable to the ones that Europa Clipper may encounter over a flyby [47]. However, the spacecraft will have a velocity relative to Europa's exosphere (5 km/s in this study) while the outgassed water will be at a rest relative to the spacecraft. This ensures that the density inside of the antechamber is driven by ramming of exospheric gas rather than equilibration with the outgassed water. For this reason we elect not to include outgassed water in our calculations. The Teolis et al. [47] profile does not include atomic oxygen so since it is produced by the dissociation of $O_2$, we implemented atomic oxygen density as a function of $O_2$ density, following the profile calculated in the study of Vorburger and Wurz (2018) [49]. The O density they obtained is within an order of magnitude of most major studies about Europa's exosphere.

We investigated two flyby profiles, leading to different evolutions of $O_2/H_2O$; a south polar flyby at 50 km (maximum $O_2/H_2O$ ratio) and a dayside flyby at 25 km (minimum $O_2/H_2O$ ratio for a sputtered exosphere). We simulated the antechamber population over a timespan extending from -250s to +300s, corresponding to the $O_2$ density increasing above $10^9$ molecules/m$^3$ (Figures 3 and 4). We introduced two compounds "X" and "Y" at 1% abundance relative to water, and gave them different sticking coefficients and desorption energies (Table 1). Component X had the properties of water while component Y had the properties of molecular hydrogen (stickiness is negligible). We then simulated the same flyby without any atomic oxygen present and compared the antechamber population for all compounds of interest. The antechamber population of a
compound at a given time dictates the population in the ion source and there-
fore how many counts the instrument returns over one integration period for
the related mass peaks. As with Cassini INMS, a quantity of interest will be
integrated counts, i.e., the sum of the counts over the whole flyby, indicative of
the total quantity of a compound encountered. We therefore also integrated the
antechamber populations over time and calculated the effect of atomic oxygen
on this value, which is a proxy for the effect on integrated counts.

3.4. Results of flyby simulations at Europa

We find the effect of reaction with atomic oxygen on the water population
to be very limited in both flybys: Δ[H₂O]/[H₂O] = -4 × 10⁻⁵ and -9 × 10⁻⁵
for the integrated populations for the dayside flyby and the south pole flyby,
respectively. The maximum effect on instant population is Δ[H₂O]/[H₂O] = -
1.25 × 10⁻³ at most during the dayside flyby, and -6 × 10⁻⁴ for the south pole
flyby (Figures 5 and 6). The effect is the same on both compounds X and Y.
This points to the surface coverage of atomic oxygen being the driving factor
in reactions. The atomic oxygen surface population grows steadily during the
flybys, as water-O reactions do not compensate for the incoming O flow from the
exosphere. The consequence is that (i) measurements in the first half (before
closest approach) of the flyby are relatively unaffected by atomic oxygen; (ii)
the effect of atomic oxygen is maximum in the later part of the flyby, due
to the combination of accumulated O with lower exospheric density (iii) if no
step is taken to clean up the antechamber after every flyby, atomic oxygen
could accumulate in the instrument and alter subsequent measurements as the
mission progresses. Assuming linear growth of the O surface population and
of its effect on H₂O measurements with the number of flybys, the readings on
water population could be offset by several percents by the end of the mission
(for measurements in the less dense part of the exosphere).
<figure><img src="image_4.png" /><figcaption>Figure 3: Simulated densities of H₂O and O₂ for a south polar flyby, closest approach at 50 km (sputtered atmosphere only).</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 4: Simulated densities of H₂O and O₂ for a dayside flyby, closest approach at 25 km (sputtered atmosphere only).</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 5: Simulations results for a dayside flyby at 25 km (minimum O/water ratio). Top: Evolution of H₂O population in gas phase in the antechamber without atomic oxygen (red) versus with atomic oxygen (blue). Middle: Instant difference in water vapor population between the case with and without oxygen, expressed as a percentage of the instant water vapor population. Bottom: Evolution of the atomic oxygen surface population during the flyby.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 6: Same as Figure 5, but for the south polar flyby with closest approach at 50 km.</figcaption></figure>
## 3.5. Discussion

### 3.5.1. Other reactive neutrals

There are other reactive neutrals in Europa's exosphere that are susceptible to cause alteration in the antechamber. Table 2 shows, along with H₂O, O₂ and atomic oxygen, expected averaged densities at 100 km for some other reactive neutrals. These include atomic hydrogen (H), hydroxyl (OH), sodium (Na), and sulfur dioxide (SO₂). Atomic hydrogen density is expected to be consistently one order of magnitude below atomic oxygen [49] and its effect on measurements will therefore be more limited than atomic oxygen. The hydroxide neutral is reactive but will most likely adsorb to the surface of the antechamber and stay there, much like OH formed by atomic oxygen - water reaction; OH will be desorbed only through subsequent heating. The abundance of sodium below 400 km found by Cassidy et al. [50] is similar to the abundance expected of atomic oxygen (above 1 × 10³ cm⁻³). Therefore its effect on the water population in the antechamber is expected to be of the same order of magnitude as atomic oxygen's: a fraction of a percent. Sulfur dioxide, being present on Europa's surface[51], is expected to be found in the exosphere due to desorption and sputtering[50]. Desorbed SO₂ should then be less abundant than atomic oxygen in most of the exosphere (above 100 km), but present a steep profile below this limit [50], and could be as much as 50 times more abundant than O at 25 km. However the behavior of SO₂ in the instrument is likely to be different. Experiments of adsorption of SO₂ onto films of gold indicate that SO₂ is not retained at temperatures above 200 K [52] and thus would not build up any significant surface population on the antechamber's wall. This raises the possibility that SO₂ would reach and affect the ion source of the instrument. However other adsorption experiments show that SO₂ binds onto an Al₂O₃ [53]: indicating that sulfur dioxide from Europa's exosphere could build up on the impact plate. These experiments cover the relevant condition of temperature (30 °C) but have been performed at SO₂ partial pressure (several mbars) significantly above anything that could be expected in MASPEX. The behavior of
<table><caption>Table 2: Globally averaged densities at 100 km altitude. [12, 50, 54, 49]</caption><tr><td>H<sub>2</sub>O</td><td>10<sup>4</sup> - 2 &times; 10<sup>5</sup> cm<sup>-3</sup></td></tr><tr><td>O<sub>2</sub></td><td>4 &times; 10<sup>6</sup> - 10<sup>7</sup> cm<sup>-3</sup></td></tr><tr><td>H</td><td>&lt; 100 cm<sup>-3</sup></td></tr><tr><td>OH</td><td>10<sup>2</sup>- 9 &times; 10<sup>4</sup> cm<sup>-3</sup></td></tr><tr><td>Na</td><td>60-1600 cm<sup>-3</sup></td></tr><tr><td>SO<sub>2</sub></td><td>110-600 cm<sup>-3</sup></td></tr></table>

$SO_2$ in the instrument and its effect on measurements are out of the scope of this paper and should be the object of future experiments.

### 3.5.2. Path forward

We have found the expected value of loss of compounds due to atomic oxygen for different conditions. For every compound reactive to atomic oxygen, the loss fraction is the same, therefore the ratios of integrated counts between these compounds will be unaffected. $O_2$ is not expected to be subject to loss as the product of the reaction (ozone) would dissociate on the surface of the instrument, releasing $O_2$. The driving factor of the process is the quantity of atomic oxygen. Atomic oxygen accumulates in the instrument during the flyby; as a consequence the losses are not equally distributed during the flyby (e.g., Figure 6). Local densities measurement can be offset by as much as 0.125 % in our chosen cases. This offset is much lower at lower altitudes in the exosphere. However, it could increase over the course of the mission if atomic oxygen is allowed to accumulate in the instrument, which can be prevented by heating the antechamber to 550K after each flyby. During the mission, measurements of atomic oxygen are required to apply, if necessary, a correction to deduce both density at a given point in the trajectory, and the total amount of a compound encountered over the flyby (density integrated along the spacecraft trajectory). However, direct measurement of atomic oxygen is not within the capabilities of MASPEX. Obtaining the $O/O_2$ ratio over the whole flyby trajectory through UVS limb measurements (if possible) and considering a modeled $O/O_2$ profile
would allow for corrections. Laboratory experiments are recommended to validate the values of compounds losses given by the model before using it in data post-processing, and to characterize the effect of other reactive neutrals such as SO₂.

4. Conclusion

We have reviewed the available information to decide the best materials to use in constructing the MASPEX antechamber to reduce the instrument interaction with atomic oxygen, and modeled the effect of atomic oxygen reactions with other compounds of the sample in a Europa flyby.

We found that an impact plate made of ceramic Al₂O₃, and a coating of gold on the region of specular reflection of the antechamber offer the best properties. In particular, atomic oxygen chemisorbs onto the gold coating, preventing it from reaching the ion source, but without affecting the antechamber surface's properties.

During a flyby, atomic oxygen is expected to affect the precision of abundance measurements in the MASPEX instrument. The impact is found to be rather limited as the abundance of water measured is reduced by Δ[H₂O]/[H₂O] = -1.25 × 10⁻³ at most. The bulk perturbation (reduction of total counts of water over the flyby) is even lower: Δ[H₂O]/[H₂O] = -9 × 10⁻⁵. This change in abundance was also seen for other compounds which undergo reaction with atomic oxygen. Instrument calibration accuracy will provide a benchmark for whether or not this perturbation in measured abundance is to be considered significant. Atomic oxygen accumulates on the antechamber walls during the flyby, so its impact on measurements can grow over the duration of the mission. Sodium, being present in comparable quantities, could induce a similar offset in measurements. For spatial measurements across the flyby, results from the antechamber surface chemistry model can be utilized to determine instantaneous changes in abundance measurements. Corrective actions for abundance measurements using MASPEX alone seem unattainable. It is suggested that collaboration with
other instruments on the payload be pursued to determine the column density ratio of atomic oxygen to molecular oxygen prior to flybys. At the conclusion of each flyby, the antechamber can be heated to drive off all adsorbed atomic O on the antechamber walls to ensure a clean gold surface for the start of the next flyby.

Acknowledgments

This work is funded through JPL subcontract 1531256. Thanks to Peter Wurz for sending the calculation results from Vorburger & Wurz (2018). AB acknowledges funding by CNES.

5. Appendix: governing equations of the antechamber chemistry model

At each timestep, the number of adsorbing particles of compound i is given by:

$$N_{ad,i} = A_c \tau s_{0,i} (1 - \theta) P_i J_i \quad (6)$$

where $N_{ad,i}$ is the number of molecules of compound i adsorbed in the timestep $\tau$, $A_c$ is the area considered (ie the antechamber surface), $s_{0,i}$ is the sticking coefficient of compound i for a zero surface coverage, $\theta$ is the total coverage of the surface (by all adsorbed compounds), $P_i$ is the partial pressure of compound i in gas phase and

$$J_i = \frac{c_i}{4k_B T} \quad (7)$$

where $c_i$ is the thermal speed of compound i, $k_B$ is the Boltzmann constant, and T the gas temperature (assumed to be the same as the surface's), so that $J_i P_i$ is a collision rate per unit of surface and time.

The number $N_{des,i}$ of molecules of compound i desorbing during a given timestep is given by:

$$N_{des,i} = -\nu_0 N_i \tau \exp(-\frac{E_{des,i}(1-\alpha\theta)}{RT_s}) \quad (8)$$
where $v_0$ is a prefactor assumed to be $10^{13} \text{s}^{-1}$ for most surfaces including gold [55, 56], $N_i$ the number of molecules of compound $i$ currently adsorbed on the surface, $E_{des}$ is the adsorption/desorption energy, R is the ideal gas constant, and $T_s$ the temperature of the surface (K).

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