A photochemical model of Pluto's atmosphere and ionosphere

A photochemical model of Pluto's atmosphere and ionosphere

Accepted Manuscript A photochemical model of Pluto's atmosphere and ionosphere Vladimir A. Krasnopolsky PII: DOI: Reference: S0019-1035(19)30231-3 h...

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Accepted Manuscript A photochemical model of Pluto's atmosphere and ionosphere

Vladimir A. Krasnopolsky PII: DOI: Reference:

S0019-1035(19)30231-3 https://doi.org/10.1016/j.icarus.2019.07.008 YICAR 13374

To appear in:

Icarus

Received date: Revised date: Accepted date:

29 March 2019 4 July 2019 11 July 2019

Please cite this article as: V.A. Krasnopolsky, A photochemical model of Pluto's atmosphere and ionosphere, Icarus, https://doi.org/10.1016/j.icarus.2019.07.008

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ACCEPTED MANUSCRIPT A Photochemical Model of Pluto’s Atmosphere and Ionosphere Vladimir A. Krasnopolsky

1. Introduction

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Abstract. The model for Titan by Krasnopolsky (2014, Icarus 236, 83-91) has been adjusted to Pluto’s conditions during the New Horizons flyby. The model includes 419 reactions of 83 neutrals and 33 positive ions plus 10 reactions of CO+ and HCO+ that were missing for Titan because of the low CO abundance. The model has 289 altitude steps up to the exobase at 1600 km with thermal escape for neutrals and diffusion velocities for ions as the upper boundary conditions. The model involves condensation on the haze and the surface with sticking coefficient γ = 0.002 for hydrocarbons, H2O, and CO2, γ = 0.01 for nitriles except HC3N with γ = 0.1. The photochemistry is driven by the solar EUV and UV radiation, the interplanetary Lymanalpha emission, and the galactic cosmic rays. The adopted eddy diffusion K = 3×104 cm2 s-1 facilitates transport of C2H4 and C2H6, their condensation on the surface, and does not require the revision of the laboratory data on saturated vapor densities of C2H4 and C2H6 by orders of magnitude to fit the New Horizons observations. The CH4 homopause is at 110 km for this K, and the CH4 vertical profile is mostly controlled by molecular diffusion and agrees with the NH observations. Productions and losses of major hydrocarbons, nitriles, oxygen species, and hydrogen are briefly discussed. The daytime ionosphere is predicted with a maximum electron density of 800 cm-3 at 750 km. The most abundant ions are HCNH+ and C9H11+ above and below 600 km, respectively. Chemical effects of the ion reactions on some neutral species are significant. The predicted ion densities are measurable by an analog of the Cassini ion-neutral mass spectrometer. Evolution of the atmosphere includes thermal escape of H2 + H, CH4 + CH3, and N2 + N (92, 203, and 1 g cm-2 Byr-1, respectively), condensation of hydrocarbons and nitriles (306 and 101 g cm-2 Byr-1, respectively), and polymerization (30 g cm-2 Byr-1). (Annually mean values are halves of those.) The composition and photochemistry of Pluto during the New Horizons flyby are very different from those of Triton during the Voyager 2 flyby and controlled by the methane mole fraction near the surface. Seasonal variations of the atmospheric methane should cover both states of the atmospheres on both Pluto and Triton.

Photochemical modeling is an effective tool to study chemical structures of planetary atmospheres. Using a couple of densities of parent species, this makes it possible to calculate vertical profiles of a few dozens of photochemical products throughout the atmosphere. The first photochemical models for Pluto were published two decades ago to respond to the atmospheric properties deduced from the first definitive stellar occultation (Hubbard et al. 1988, Elliot and Young 1992, Millis et al. 1993), detection and measurement of N2:CH4:CO in Pluto’s ice (Owen et al. 1993), its temperature (Tryka et al. 1994), and detection of atmospheric methane (Young et al. 1997). Studies of Triton based on the Voyager 2 flyby in 1989 stimulated modeling of Pluto as well because of the significant similarity between both bodies.

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First models for Pluto were created by Summers et al. (1997). Their basic model was Triton’s analog with mixing ratios fCH4 = 4×10-5 and fCO = 5×10-4, smaller than the current CH4 abundance by two orders of magnitude and equal to that for CO. It is not clear why atomic nitrogen is less abundant in the model than that observed and modeled for Triton by three orders of magnitude. One of their “high methane” models had the current values fCH4 = 4×10-3 and fCO = 5×10-4. However, condensation rates (150 g cm-2 Byr-1 total) and the ionospheric composition with C2H5+ as the most abundant ion were only published from that model. Lara et al. (1997) made a photochemical model for Pluto’s neutral atmosphere that involved 111 reactions of 29 neutral species. The adopted methane abundance was 0.74%, within a factor of 2 of the present value. Their condensation times correspond to a haze visible optical depth τ = 0.07/γ; here γ is the sticking coefficient. Krasnopolsky and Cruikshank (1999) developed a rather detailed model of Pluto’s atmosphere and ionosphere near perihelion that includes 191 reactions of 44 neutral and 23 ion species and accounts for a slow hydrodynamic escape calculated by Krasnopolsky (1999). One of the model versions is for fCH4 = 0.9%, near a factor of 2 of the present value. Heterogeneous loss at the surface with γ = 0.1 was adopted for all photochemical products except H2. The calculated condensation rate is 416 g cm-2 Byr-1 near perihelion. Significant progress in the studies of Pluto is related to the close flyby of the New Horizons mission (hereafter NH) in July 2015 and ground-based high-resolution spectroscopy in the infrared (VLT/CRIRES, Lellouch et al. (2011)) and submillimeter (ALMA, Lellouch et al. (2017)) ranges. Vertical profiles of N2, CH4, C2H2, C2H4, C2H6, and haze were retrieved from the NH UV solar occultations (Gladstone et al. 2016, Young et al. 2018). The derived temperature profile is a combination of the NH UV solar and radio occultation data (Hinson et al. 2017) and stellar occultations using VLT (Dias-Oliveira et al. 2015, Sicardi et al. 2016). Abundances of CO and HCN were measured as well (Lellouch et al. 2011, 2017). These new data require updated photochemical modeling, and that was made by Wong et al. (2017). Their model has 40 levels up to 1300 km and includes 1600 reactions of 88 neutral species. The results are focused on the four observed hydrocarbons and the HCN data from the ALMA measurements. To fit the observed profiles of ethylene C2H4 and ethane C2H6, Wong et al. (2017) adopted saturated vapor densities of these species equal to those of acetylene C2H2, though they are greater by orders of magnitude. Sticking coefficients of the three hydrocarbons and HCN in collisions with the haze were fitting parameters in the model. Various aspects of the model will be compared with our model below. Luspay-Kuti et al. (2017) created a model with irreversible sticking of the three hydrocarbons on the haze. They fitted the observed hydrocarbon abundances using sticking coefficients variable with altitude. However, the calculated total loss by aerosol trapping was extremely high, ≈3×1011 cm-2 s-1 for C2H2 in their figure 9, while total losses of hydrocarbons are ≈108 cm-2 s-1 in the other models. Furthermore, the hydrocarbon densities at the surface are fitting parameters in the model, while they should be predicted by the model. Finally, the calculated peak total ion density of 30 cm-3 is too low compared with those in the other models. 2

ACCEPTED MANUSCRIPT Both Wong et al. (2017) and Luspay-Kuti et al. (2017) compared their models with the results from the first analysis of the NH UV solar occultations by Gladstone et al. (2016). Here we will present a model that does not require the revision of the laboratory data on saturated vapor densities of C2H4 and C2H6 by orders of magnitude. Our model also involves ion chemistry that affects the neutral composition and was neglected by Wong et al. (2017). 2. Model

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Table 1. Some input data of the model Heliocentric distance 32.9 AU Surface radius 1190 km Surface gravity 61.4 cm s-2 Surface temperature 40 K Surface pressure 11.5 μbar Upper boundary 1603 km Number of levels 289 Step near surface 2.5 km Step near upper boundary 10.5 km

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Here we apply the model by Krasnopolsky (2012, 2014) that was developed for the atmosphere and ionosphere of Titan. Titan was studied in detail by the Cassini/Huygens mission, as well as the Voyager 1, ground-based and earth-orbiting observations. The better is studied an atmosphere, the more difficult is its modeling to fit numerous observational constraints. The model by Krasnopolsky (2012, 2014) is in reasonable agreement with the observations of Titan and presents therefore rather adequately chemistry of the nitrogen-methane atmospheres. The model is adjusted to the observed conditions during the NH flyby (Table 1).

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The main input data are the atmospheric pressure of 11.5 μbar near the surface and temperature profile that is a combination of the radio and UV solar occultations and stellar occultation data (Hinson et al. 2017, Young et al. 2018, Dias-Oliveira et al. 2015). This profile shown in Figure 1 is mostly copied from figure 19 in Young et al. (2018). The N2 densities are calculated using this temperature profile and shown in Figure 1 as well. The N2 densities are required to convert mole fractions calculated and discussed below to species number densities. The calculated [N2] = 2×108 cm-3 at 1000 km agrees with that extracted from the UV solar occultations (Young et al. 2018). Using a calculated profile for CH4 (see below), the abundances of N2 and CH4 at 1600 km fit the condition for the exosphere: Here σi ≈ 3×10-15 cm2 is the collisional cross section, Ni is the column abundance, and 2 is the mean airmass factor. Therefore the upper boundary of the model at 1600 km is the exosphere. With a general trend to remove unimportant species and reactions, the model for Titan involves 419 reactions of 83 neutrals and 33 positive ions. Though the conditions of Pluto suppose further reductions, we apply this chemistry unchanged. The major lists of the reactions, 3

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their rate coefficients and references may be found in Krasnopolsky (2009) with some minor additions in Krasnopolsky (2012). Coefficients of molecular diffusion are similar to those in Krasnopolsky (2009). The same refers to coefficients of ambipolar diffusion of ions Di that, however, should be corrected for electronic temperature Te on Pluto. We have not found these data in the literature and adopt Te = 300 K.

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Figure 1. The observed temperature and calculated N2 number density profiles. The upper boundary conditions for neutral species are their bulk velocities of thermal

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escape:

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Here k is the Boltzmann constant, γ is the gravitational constant, M and mi are masses of Pluto and molecule i, re = 2790 km is the exobase radius, and T = 68 K is the exobase temperature. These velocities are near zero for mi ≥ 28 (N2 and heavier species). Upper boundary conditions for ions are their velocities cm s-1.

Here Hi is the ion scale height, μ0 and μi are the mean and ion masses in atomic units. Chemistry in Pluto’s atmosphere is initiated by the solar EUV and UV radiation, interplanetary background Lyman-alpha emission, and the galactic cosmic rays. The solar radiation and absorption and ionization cross sections of the atmospheric species are similar to those in Krasnopolsky (2009). The solar Lyman-alpha emission is scaled by a factor of 1.43 to account for the background emission, similar to that in Wong et al. (2017). Pluto’s reflectivity is A ≈ 0.25 at 200-255 nm (Krasnopolsky 2001), and photolyses of species are increased in this range by a factor of 1 + 2 A = 1.5. 4

ACCEPTED MANUSCRIPT Using the calculations by Molina-Cuberos et al. (1999), the production rate of N2+ by galactic cosmic rays in the N2 atmosphere can be approximated by [N2]/1.1e17 cm-3 s-1 at the low densities typical of Pluto’s atmosphere. Similar to Krasnopolsky (2009), we adopt the proportion N2+ : N+ : N(4S) : N(2D) = 1 : 0.22 : 0.95 : 0.73 in the production by the cosmic rays. 3. Condensation and Haze

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Condensation of 24 photochemical products is considered in our models for Titan. Titan’s tropopause is at 70 K, while the surface of Pluto is even colder at 40 K. We updated the data on saturated vapor pressures in Krasnopolsky (2009) using the data for C2H4, C2H6, C3H4, C6H6, HCN, HC3N, C2N2, and C4N2 from Fray and Schmitt (2009).

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Figure 2. Volume surface area of the haze. The point at 45 km is shown for its equal-volume radius and skipped in the final plot (solid curve).

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Analysis and interpretation of the NH haze observations were made by Gladstone et al. (2016), Cheng et al. (2017), Gao et al. (2017), Bertrand and Forget (2017), Zhang et al. (2017), and Krasnopolsky (2018). To model the loss of species by condensation, we need a vertical profile of volume surface area of the haze (Figure 2). Here we adopt the approach to the problem by Zhang et al. (2017), who specified equal-volume particle radii and number densities at five breakpoints with interpolation between them. Points at 0 and 20 km are for spherical particles with r = 500 nm. Points at 350 and 700 km are for spherical monomers with r = 10 nm. The intermediate point at 45 km is for fractal particles with the equal-volume radius of 150 nm. The mean free path in Pluto’s atmosphere is greater than the aerosol particle radius, and an equal projected area radius is appropriate to aerosol scavenging. This radius is greater than the equalvolume radius; therefore the point at 45 km is uncertain and skipped in the final plot. To calculate the plot in Figure 2, the particle number densities are taken from Zhang et al. (2017) that approximate the NH UV solar occultations of the haze near 180 nm (Young et al. 2018). The 5

ACCEPTED MANUSCRIPT total column surface area is 0.57, smaller than that in Gao et al. (2017) by a factor of 15. They adopted surface areas of the monomers to model the condensation, and this approach is invalid for large mean free paths. 4. Model Results: Hydrocarbons

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The model is a set of the second order nonlinear ordinary differential equations, which number is equal to the number of species in the system (118 in our case). The equations are solved numerically using a method described by Krasnopolsky and Cruikshank (1999).

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Figure 3. Observed (dashed curves, Young et al. 2018) and calculated (solid curves) hydrocarbons. Horizontal bars on the CH4 profile show the scatter of the observational points.

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Methane and the three C2H2X hydrocarbons that are the most abundant and have been measured by the NH UV solar occultations (Young et al. 2018) are of the greatest interest (Figure 3). Acetylene C2H2 condensation occurs at 280-620 km in our model, and sticking coefficient γ = 0.002 provides a reasonable agreement with the observed profile. However, ethylene C2H4 and ethane C2H6 can condense only near the surface, and their profiles are almost insensitive to adopted sticking coefficients. Using eddy diffusion K = 1000 cm2 s-1 similar to that in Wong et al. (2017), the calculated C2H6 mole fraction is 10-4 at 50 km, significantly greater than the observed value of 1.5×10-5. However, the model value near the surface is much smaller than 10-4, the downward flux is proportional to their difference and therefore insensitive to adopted sticking coefficient. Here the only effective means to increase the downward flux and reduce the C2H6 mole fraction is to increase eddy diffusion. The observed profile of methane was considered by Young et al. (2018) and Wong et al. (2017) as the major constraint to eddy diffusion. According to Young et al. (2018), K is between 550 and 4000 cm2 s-1. We have tested our model and conclude 6

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that a CH4 mole fraction fCH4 = 0.45% at the surface with K = 3×104 cm2 s-1 still fit the observed methane profile (Figure 3), while the calculated C2H6 mole fraction at 50 km agrees with that observed. Actually the homopause of methane is at 110 km for K = 3×104 cm2 s-1, and the profile of CH4 is controlled mostly by molecular diffusion and weakly sensitive to the chosen eddy diffusion above the homopause. The predicted methane mole fraction is ≈0.6 at the exobase, so that methane is more abundant here than nitrogen by a factor of ≈1.5. The chemistry and escape reduce the CH4 densities, the latter by a factor of 1.5 in the upper atmosphere.

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Figure 4. Major reactions of production (left) and loss (right) of methane on Pluto.

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Major reactions of production and loss of methane are shown in Figure 4. The production is mostly by the reactions CH3 + C2H3, CH3 + NH, and ion reactions. The total column production rate is 4.67×107 cm-2 s-1 scaled to the surface (Table 2). The main chemical loss processes are photolysis and the reactions with CH and metastable 1CH2. The total column chemical loss of 6.58×108 cm-2 s-1 exceeds the production by a factor of 14. Thermal escape with a rate of 4.2×107 cm-2 s-1 at the exobase and 2.3×108 cm-2 s-1 scaled to the surface adds significantly to the chemical loss, and the required sublimation rate of methane from the surface is 706 g cm-2 Byr-1 (Table 2). The mean chemical lifetime of methane in the atmosphere is 570 yr, that is, 2.3 Pluto’s years. Our model is calculated for the NH flyby conditions for Pluto’s heliocentric distance of 32.9 AU, and the annually mean sublimation and precipitation rates should be smaller than the calculated data by a factor of ≈2 (see below). Main sources of acetylene C2H2 (Figure 5, left panel) are photolysis of C2H4 with some contribution from other hydrocarbons and the reactions C2H3 + H and C2H3 + CH3. Loss of acetylene (Figure 5, right panel) is mostly by condensation that peaks at 310 km and extends from 280 to 620 km with the adopted sticking coefficient γ = 0.002. Photolysis, the reaction with C2H radical, and ion reactions are significant in the loss of C2H2. The calculated condensation rate is 167 g cm-2 Byr-1, and the mean lifetime of C2H2 is 4.5 yr (Table 2).

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Figure 5. Major reactions of production (left) and loss (right) of acetylene on Pluto.

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Production of ethylene C2H4 (Figure 6, left panel) is mostly by the reactions CH + CH4, CH2 + CH3, and ion reactions. Photolysis, the reaction with CN, and ion reactions dominate in the loss of ethylene (Figure 6, right panel). Ethylene can condense only near the surface, its condensation as well as those of other hydrocarbons are not constrained by the observations; therefore we adopt sticking coefficient γ = 0.002 for all hydrocarbons in our model. Then the C2H4 condensation rate is very low, 0.19 g cm-2 Byr-1, and the ethylene lifetime in the atmosphere is 1.5 yr (Table 2).

Figure 6. Major reactions of production (left) and loss (right) of ethylene on Pluto. The association of two methyl radicals and dissociative recombination of C 2H7+ dominate in the production of ethane C2H6 below and above 800 km, respectively (Figure 7, left panel). Other reactions that form C2H6 are weaker by orders of magnitude. There are a few reactions with the comparable contributions to the loss of ethane: photolysis and the reactions with C 2N, 8

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CH, C2H, CN, and C2 radicals and metastable N(2D) (Figure 7, right panel). Lifetimes of radicals are rather short, and we do not consider their loss on the haze particles (sticking is zero in the model). The column production of ethane exceeds its photochemical loss almost by a factor of 2, and the excess is condensed at the surface with a rate of 66 g cm-2 Byr-1. The ethane lifetime is 36 yr in Pluto’s atmosphere (Table 2). Overall, our model reproduces rather well the observed vertical profiles of the four hydrocarbons.

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Figure 7. Major reactions of production (left) and loss (right) of ethane on Pluto.

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Table 2. Column production and loss rates, escape/precipitation flows, and mean chemical lifetimes of some species Species H H2 CH3 N CH4 N2 C2H2 C2H4 C2H6 C3H4 -2 -1 Production (cm s ) 8.72+8 6.25+8 5.00+8 7.26+7 4.67+7 2.44+7 2.17+8 1.87+8 8.69+7 2.07+7 Loss (cm-2 s-1) 3.64+8 2.73+6 4.87+8 7.19+7 6.58+8 4.96+7 9.47+7 1.87+8 4.51+7 1.70+7 Flow (g cm-2 Byr-1) 26.7 65.3 10.2 0.51 706a 37.5b -167 -0.19 -66 -7.8 Lifetime (yr) 0.04 2.0 0.06 0.0052 570 1.9+6 4.5 1.5 36 1.9

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Species C3H8 C4H2 C4H4 C4H6 C6H6 HCN CH3CN HC3N C2H3CN H2O -2 -1 Production (cm s ) 1.83+6 1.01+8 3.43+7 2.91+6 4.38+6 8.84+7 2.79+5 1.47+7 4.02+7 8866c -2 -1 Loss (cm s ) 3.20+5 8.06+7 3.30+7 1.45+6 3.98+6 6.51+7 3.72+4 6.84+5 2.97+7 584 Flow (g cm-2 Byr-1) -3.5 -52 -3.5 -4.1 -1.6 -33 -0.5 -37.5 -29 -0.008 Lifetime (yr) 19 0.15 0.04 1.3 0.12 0.11 1.5 0.1 0.11 0.6 Production and loss refer to photochemical reactions; positive flows are escape or sublimation from the surface, negative flows are condensation; all values are reduced to the surface; 8.72+8 = 8.72×10 8 cm-2 s-1. a Net photochemical loss of 513 g cm-2 Byr-1 plus escape of 193 g cm-2 Byr-1. b Net photochemical loss of 37 g cm-2 Byr-1 plus escape of 0.48 g cm-2 Byr-1. c 8500 cm-2 s-1 from meteorite ablation (Poppe and Horanyi 2018) plus chemical production of 366 cm-2 s-1.

Calculated vertical profiles of the C3H2X hydrocarbons are shown in Figure 8a. Propyne C3H4 and propane C3H8 are the most abundant below 500 km, and their mole fractions are ≈10-6 near 200 km. Their condensation rate is 11 g cm-2 Byr-1 (Table 2).

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C4H2X hydrocarbons and benzene C6H6 are shown in Figure 8b. Diacetylene C4H2 is the most abundant. It is formed by photolysis of C4H4, by the reaction of C2H2 + C2H, and recycles in C4H2 + H → C4H3 + hν C4H3 + H → C4H2 + H2 C4H3 + CH3 → C4H2 + CH4 . Loss of diacetylene is by photolysis and reactions of polymerization: C4H2 + C6H → polymer, production rate 10.2 g cm-2 Byr-1 C4H2 + C3N → polymer, production rate 2.4 g cm-2 Byr-1. Another important reaction of polymerization is C6H2 + C6H → polymer, production rate 9.4 g cm-2 Byr-1. Beside the production of polymers, diacetylene condenses with a rate of 52 g cm-2 Byr-1. Its lifetime in the atmosphere is 0.15 yr (Table 2). Condensation rate of the other C4H2X hydrocarbons plus benzene is 11 g cm-2 Byr-1. Benzene C6H6 is the simplest aromatic hydrocarbon. It is formed by 2 C3H3 + M and dissociative recombination of C7H7+ below and above 500 km, respectively.

Figure 8. Vertical profiles of C3 hydrocarbons (a), C4 hydrocarbons and benzene (b), nitriles and H2 (c), and oxygen species (d). 10

ACCEPTED MANUSCRIPT 5. Nitriles and H2

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The N2 dissociation limits to N(4S) + N(4S) and N(4S) + N(2D) states are 9.76 and 12.14 eV, that is, 127 and 102 nm, respectively. The bands at 80-100 nm predissociate to N(4S) + N(2D). However, intervals between the lines and bands are significant and lower the absorption effect. Ionization to N2+ begins below 80 nm and returns N2 in the charge exchange reactions. Therefore predissociation at 80-100 nm, dissociative ionization below 51 nm, and photoelectron dissociation are only effective in the loss of N2. The metastable N(2D) is quenched by N2 and reacts with hydrocarbons and H2; the ground state N(4S) reacts mostly with hydrocarbon radicals. Nitriles and amines are the products of these reactions. The C≡N triple bond is strong (7.85 eV) in nitriles and does not dissociate. Therefore all CN once formed finally precipitate to the surface. The N-H bonds are ≈3.5 eV in amines and comparable to the C-H bonds in hydrocarbons. Therefore amines return N in their reactions and are much less abundant than nitriles. Total loss of N2 is 37.5 g cm-2 Byr-1. It exceeds very much the N2 thermal escape of 0.48 g cm-2 Byr-1 (6×104 and 3.3×105 cm-2 s-1 at the exobase and scaled to the surface, respectively). The total escape is 5.8×1022 s-1, in accord with (3-7)×1022 s-1 from Young et al. (2018). It is much smaller than hydrodynamic escape predicted for Pluto before the New Horizons flyby. It is not ruled out that conditions on some other KBOs may be favorable for hydrodynamic escape. Lifetime of N2 in the atmosphere of Pluto is 1.9 Myr (Table 2). However, the reservoir of N2 ice in Sputnik Planitia on Pluto is estimated using the NH observations as a global-equivalent layer of 100 m thick (McKinnon et al. 2017), that is, 0.85×104 g cm-2. The photochemical lifetime of this reservoir exceeds very much the age of the Universe.

Figure 9. Major reactions of production and loss of HCN on Pluto. Vertical profiles of some nitriles are shown in Figure 8c. Hydrogen cyanide HCN is the most abundant. Its main reactions of production and loss are in Figure 9. The major source of the nitrile chemistry is 11

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N + CH3 → H2CN + H H2CN + H → HCN + H2 (Figure 9, left panel). The ion reactions dominate in both production and loss of HCN above 800 km and almost compensate each other. A significant part of these reactions involves HCNH+, which is the most abundant ion at 700-1000 km. Photolysis of nitriles forms CN radicals that return HCN in the reactions with ethylene and ethane (Figure 9, left panel). The main loss of HCN is the reaction with CH (Figure 9, right panel). Condensation of HCN begins near 100 km and reduces HCN above this altitude. The calculated HCN densities are close to saturation at 100-140 km and exceed those saturated by a factor of 1000 at 200 km and even more above (Rannou and West 2018). A chosen sticking coefficient γ = 0.01 agrees the calculated column abundance of HCN with the value of 1.6×1014 cm-2 observed using ALMA (Lellouch et al. 2017). Column condensation rate of HCN is 33 g cm-2 Byr-1 (Table 2). Cyanoacetylene HC3N is another nitrile that was searched for using ALMA; no signal has been detected with an upper limit of 2×1013 cm-2 to the HC3N column abundance (Lellouch et al. 2017). Our model (Figure 8c) predicts two maxima in the HC3N mole fraction: 3.7×10-8 at 55 km and 3.1×10-6 at 525 km. Though the mole fraction at 525 km is greater by a factor of ~100, the proper number densities of HC3N are 5.8×107 cm-3 and 1.7×105 cm-3 at 55 and 525 km, respectively. The difference is a factor of 350, and almost all column abundance of HC 3N refers to the layer below 150 km (Figure 8c). This layer originates from the temperature peak near 30 km (Figure 1). The calculated HC3N density matches the saturation density at 5-15 km and 50140 km. The column abundances are 3.2×1013 cm-2 below 150 km and 3.9×1013 cm-2 throughout the atmosphere for sticking coefficient γ = 0.1, exceeding the ALMA upper limit by a factor of 2. They are weakly sensitive to its value. Condensation rate of HC3N is significant (37.5 g cm-2 Byr-1) and comparable to that of HCN. Production and loss of HC3N are almost completely by the reaction CN + C2H2 and condensation, respectively. Acrylonitrile C2H3CN is another abundant nitrile predicted by the model (Figure 8c). Its production is by the reaction of CN + C2H4 below 400 km and by CHCN + CH3 above 400 km. The loss is by photolysis below 600 km and by the reactions with HCNH+ and C2H5+ ions above 600 km. Sticking coefficient for condensation of all nitriles except HC3N is adopted equal to 0.01. The calculated condensation rate is 29 g cm-2 Byr-1, close to that of HCN. Condensation rates of other nitriles, CH3CN, C2N2, and C4N2 (Figure 8c), are much smaller (0.8, 0.5, and 0.4 g cm-2 Byr-1, respectively). Atomic and molecular hydrogen is formed by photolysis of hydrocarbons that peaks at 400 km. 60% of the production of H escapes thermally, and the remaining 40% reacts mostly with radicals CH2, C2H3, C4H3, H2CN, and CHCN and form H2. All production of H2 escapes, and photolysis of hydrocarbons constitutes 54% of the production. Diffusion coefficient of H2 is large, therefore the homopause of H2 is at 15 km, and the H2 mole fraction steeply increases up to 400 km by diffusive separation. Further increase above 400 km is not steep and determined by a balance between the photochemical production and the flow of the escaping molecules. Total

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Carbon monoxide CO is the main source of oxygen on Pluto. CO was detected in Pluto’s ice (Owen et al. 1993, Schmitt et al. 2017), and its share in the ice is ≈0.5 % (Owen et al. 1993). The atmospheric pressure of 11.5 μbar corresponds to the N2 saturated vapor at 37 K, and the saturated vapor density ratio is [CO]S/[N2]S ≈ 0.12 at 37 K (Fray and Schmitt 2009). According to the Raoult law, the expected CO mole fraction in the atmosphere is 0.12×5×10-3 = 6×10-4, in accord with the VLT/CRIRES (Lellouch et al. 2011) and ALMA (Lellouch et al. 2017) observations. The surface temperature on Titan is 94 K, too warm for CO ice to exist. Therefore the rather high CO mole fraction of 50 ppm is controlled by photochemistry and delivery of water by the meteorite ablation and O+ ions from the magnetosphere of Saturn (Hӧrst et al. 2008, Krasnopolsky 2012). The CO mole fraction of 500 ppm in Pluto’s atmosphere is determined by sublimation/condensation processes and therefore fixed as the lower boundary condition in our model. Poppe and Horanyi (2018) developed an interplanetary dust dynamics model and calculated the delivery of water into the atmospheres of Pluto and Triton with rates of 8.5×103 and 6.2×105 cm-2 s-1 scaled to the surface, respectively. The difference by two orders of magnitude is caused by the huge Neptune gravity. The value for Triton is close to that used for Pluto by Wong et al. (2017). We apply the H2O flux and its altitude distribution on Pluto from Poppe and Horanyi (2018) in our model. The CO mole fraction in Pluto’s atmosphere is greater than that on Titan by a factor of 10. Therefore we add some ion reactions that look helpful on Pluto but negligible on Titan (Table 3). The calculated column reaction rates significantly exceed those induced by the water chemistry.

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Table 3. Reactions of CO+ and HCO+ added to the model # Reaction Rate Coefficient + + 1 N2 + CO → CO + N2 7.4×10-11 2 N+ + CO → CO+ + N 8.3×10-10 3 CO+ + CH4 → CH4+ + CO 8×10-10 4 CO+ + CH4 → HCO+ + CH3 4.6×10-10 5 CO+ + HCN → HCN+ + CO 3.4×10-9 (300/Te)0.5 6 CO+ + H2 → HCO+ + H 1.5×10-9 7 HCO+ + C2H2 → C2H3+ + CO 1.4×10-9 8 HCO+ + C2H4 → C2H5+ + CO 1.4×10-9 9 HCO+ + HCN → HCNH+ + CO 3.1×10-9 (300/Te)0.5 10 HCO+ + e → CO + H 2.4×10-7 (300/Te)0.7 13

CR 1.79+4 4.91+4 4.16+4 2.39+4 661 835 6705 1.23+4 5347 409

h (km) 611 611 611 611 629 553 574 595 667 839

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The calculated vertical profiles of the major oxygen species are shown in Figure 8d. Condensation of water on the haze and the surface dominates in the water loss (Table 2). The adopted sticking coefficient is 0.002, similar to that for hydrocarbons. Precipitation of water is extremely low (Table 2), especially compared with the water ice mantle above Pluto’s rocky core with radius of 850 km (McKinnon et al. 2017).

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7. Ionosphere

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There are two sources of ionization on Pluto: the solar EUV radiation and the galactic cosmic rays. The latter is proportional to atmospheric density (see section 2) and therefore prominent in the lowest 300 km. The calculated profile of electron density is shown in Figure 10. Our model does not involve negative ions, whose expected abundances are very low, and the electron density is the sum of the ion densities. The ionospheric peak is at emax = 800 cm-3 near 750 km. The calculated ionosphere is close to dayside mean. Using the observations of Titan, the ionosphere near the terminator is less dense by a factor of ≈ 2.5, that is, emax ≈ 300 cm-3. The search for the ionosphere by the NH radio occultations near terminator resulted in an upper limit of emax < 1000 cm-3 (Hinson et al. 2018). The most abundant hydrocarbon ion below 650 km is C9H11+ (Figure 10a), which mass is 119 atomic units. C2H5+ dominates above 800 km, while C5H5+ and C3H5+ have comparable densities near the ionospheric peak. The most prominent nitrile ions are HCNH+, C2H3CNH+, and C3H3CNH+ (Figure 10b). The predicted ion densities are well within the range that was accessible to the Ion-Neutral Mass Spectrometer at the Cassini mission. The Cassini INMS measurements of the ion composition significantly contributed to both ion and neutral composition and structure of Titan’s upper atmosphere and ionosphere.

Figure 10. The most abundant hydrocarbon (a) and nitrogen-bearing (b) ions on Pluto. 14

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8. Discussion 8.1. Initial data and comparison with observations

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Our model is aimed to study the chemical composition, structure, and photochemistry of Pluto’s atmosphere and ionosphere at the conditions of the New Horizons flyby in July 2015. The observed surface atmospheric pressure, the temperature profile, and the CO mole fraction are initial data of the model. The adopted eddy diffusion K = 3×104 cm2 s-1 with the CH4 homopause at 110 km and its mole fraction of 0.45% near the surface agree with the observed CH4 profile that is mostly controlled by molecular diffusion, photochemistry, and thermal escape. We adopted a vertical profile of the haze surface area using the related published data and chose sticking coefficient γ = 0.002 for all hydrocarbons, H2O, and CO2, and γ = 0.01 for all nitriles except HC3N with γ = 0.1. Then our photochemical model for Titan was adjusted using these data to model Pluto’s photochemistry. The calculated vertical profiles of CH4, C2H2, C2H4, and C2H6 are in reasonable agreement with the NH UV solar occultations and do not require artificial assumptions on their saturated vapor pressures. The model is in accord with HCN observed using ALMA, while the predicted column HC3N exceeds the ALMA upper limit by a factor of 2.

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8.2. Pluto and Triton

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It is interesting to compare the atmospheres of Pluto and Triton, because these bodies are twins with the similar size and ice composition. Furthermore, they were studied by New Horizons in July 2015 and Voyager 2 in August 1989 at the rather similar heliocentric distances using the similar flyby tools: radio and UV solar occultations and images of the haze. Methane is the most active parent species in the atmospheres of Pluto and Triton. Methane dissociates mostly by the solar Lyman-alpha radiation, and the process peaks at τ = 2σCH4NCH4 ≈ 1, that is, NCH4 ≈ 3×1016 cm-2. Here σCH4 = 1.6×10-17 cm2 is the cross section at Lyman-alpha, NCH4 is the column abundance, and the mean airmass is 2. If the CH4 mixing ratio is low, then N2 is abundant at this level, molecular diffusion is therefore weak, and the photolysis and chemical reactions deplete methane in the upper atmosphere to a ppm level. This is the case of Triton during the Voyager 2 flyby with fCH4 ≈ 100 ppm near the surface and 1 ppm at 200 km (Krasnopolsky and Cruikshank 1995). The upper atmosphere consists in this case of N2, CO, H2, and atomic species with atomic nitrogen and hydrogen densities reaching ≈109 cm-3. This environment is favorable for atomic ions C+ and N+ in the ionosphere. Their radiative recombination is slow, and the peak electron density reaches 3×104 cm-3. Methane is rather abundant on Pluto, N2 is comparatively low near the level of CH4 photolysis (≈450 km on Pluto), and molecular diffusion and diffusive enrichment of CH4 as a light species overcomes the losses by photolysis and chemical reactions (Figure 3). The hydrocarbon and nitrile chemistries extend to the upper atmosphere, atomic species react with radicals and are significantly depleted (by four and two orders of magnitude for N and H relative 15

ACCEPTED MANUSCRIPT to those on Triton, respectively). Molecular ions with fast dissociative recombination dominate in the ionosphere, which density is lower on Pluto than on Triton by a factor of 40. According to Krasnopolsky (2012), a transition from the Voyager-type chemistry of Triton to the NH-type chemistry of Pluto occurs near fCH4 ≈ 5×10-4 at the surface. Seasonal variations at both Triton (Lellouch et al. 2010) and Pluto (Stern et al. 2017) should induce these transitions.

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8.3. Budgets of species

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Column production and loss rates of species by chemical reactions, their escape and sublimation/precipitation flows and mean lifetimes are given in Table 2. The atmospheric chemistry is supplied by sublimation of methane and nitrogen ices with rates of 706 and 37.5 g cm-2 Byr-1, respectively. These values for Titan are 7132 and 460 g cm-2 Byr-1, respectively (Krasnopolsky 2014). The rates look proportional to the solar radiation, that is, R-2. Here R is heliocentric distance that was equal to 32.9 AU during the NH flyby and 9.58 AU for Titan. Orbit-average data are preferable for evolution of Pluto, and scaling to the mean heliocentric distance of 40 AU for Pluto, we expect that the orbit-average values are smaller than those during the NH flyby by a factor of 1.5. The atmosphere may be extremely thin near aphelion, and this factor is ≈2 to account for this anticipation. Thermal escape is significant for the first six species in Table 2. Its total rate is 296 g cm 2 Byr-1, and CH4 + CH3 and N2 + N contribute 203 and 1 g cm-2 Byr-1, respectively. Their abundances are comparable at the exobase, and the great difference reflects the strong dependence of thermal escape on mass. Escape of H + H2 is 92 g cm-2 Byr-1 may be compared with the CH4 total sublimation of 706 g cm-2 Byr-1. Precipitation rates are 306 and 101 g cm-2 Byr-1 for hydrocarbons and nitriles, respectively, in Table 2. Adding these values and the three reactions of polymerization (section 4), the precipitation plus escape are 725 g cm-2 Byr-1. It is slightly smaller than the sublimation of methane and nitrogen 706 + 37.5 = 743.5 g cm-2 Byr1. The difference is caused by species with low escape, precipitation, polymerization, and ion escape that are not included in Table 2. Dividing the precipitation and escape rates by the factor of 2 to get the orbit-average value, the expected precipitation and escape are global-mean layers of 2 m and 1.5 m thick per billion years, respectively, that is, total ≈15 m thick per the age of the Solar System. Most of the surface features are rather young, indicating significant mixing and resurfacing, and the precipitating species may be strongly diluted. 8.4. Precipitation of the haze The predicted photochemical precipitation rate of 400 g cm-2 Byr-1 can be compared with precipitation of the observed haze (section 3, Figure 2). The aerosol in the lowest ≈30 km may contain condensation clouds, and we adopt the data at 45 km to calculate the photochemical precipitation: the mean aerosol equal-volume radius is 0.15 μm, the particle number density is 16

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4.5 cm-3 (Zhang et al. 2017). If the mean free path is much larger than the particle radius, then the particle precipitation velocity is V = 0.367 ρgrl/η (Krasnopolsky et al. 1992). Here ρ is the particle material density, g = 61 cm s-2 is the gravity acceleration, r is the particle radius, l is the mean free path, and η is the gas viscosity. The effective mean radius for precipitation of fractal particles is greater than the equal-volume radius and close to the equal projected area radius that is greater for the fractal particles on Tiran by a factor a = 2.8 (Tomasko et al. 2008). We adopt this factor as a fitting parameter and bear in mind that this means a reduction of the material density of solid hydrocarbons and nitriles of ≈0.8 g cm-3 by a factor of a3, and the precipitating flow is proportional to a-2. The atmospheric parameters at 45 km are p = 2.4 μbar and T = 103 K and result in l = 1.4 cm and η = 6.9×10-5 Poise. Then the calculated photochemical haze precipitation of 400 g cm-2 Byr-1 is fitted with a = 4.7, greater than 2.8 on Titan. The haze precipitation rate in our model is close to those in the previous models (416 and 497 g cm-2 Byr-1 in Krasnopolsky and Cruikshank (1999, their model 2) and Wong et al. (2017), respectively). Furthermore, 400 g cm-2 s-1 equals 1.3×10-14 g cm-2 s-1, which is similar to 10-14 g cm-2 s-1 retrieved from the NH optical observations of the haze (Cheng et al. 2017).

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9. Conclusions

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The photochemical model of Titan’s atmosphere and ionosphere (Krasnopolsky 2014) has been adjusted to Pluto’s conditions during the New Horizons flyby. The model includes 419 reactions of 83 neutrals and 33 positive ions plus 10 reactions of CO+ and HCO+ that were missing for Titan because of the low CO abundance. The model has 289 altitude steps up to the exobase at 1600 km with thermal escape for neutrals and diffusion velocities for ions as the upper boundary conditions. The model involves condensation on the haze and the surface with sticking coefficient γ = 0.002 for hydrocarbons, H2O, and CO2, γ = 0.01 for nitriles except HC3N with γ = 0.1. The photochemistry is driven by the solar EUV and UV radiation, interplanetary Lyman-alpha emission, and the galactic cosmic rays. The adopted eddy diffusion K = 3×104 cm2 s-1 facilitates transport of C2H4 and C2H6, their condensation on the surface, and does not require the revision of the laboratory data on the saturated vapor densities of C2H4 and C2H6 by orders of magnitude to fit the New Horizons observations. The CH4 homopause is at 110 km for this K, and the CH4 vertical profile is mostly controlled by molecular diffusion, photochemistry, and thermal escape and agrees with the NH observations. Productions and losses of major hydrocarbons, nitriles, oxygen species, and hydrogen are briefly discussed. The daytime ionosphere is predicted with a maximum electron density of 800 cm-3 at 750 km. The most abundant ions are HCNH+ and C9H11+ above and below 600 km, respectively. Chemical effects of the ion reactions on some neutral species are significant. The predicted ion densities are measurable by an analog of the Cassini ion-neutral mass spectrometer. 17

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Evolution of the atmosphere includes thermal escape of H2 + H, CH4 + CH3, and N2 + N (92, 203, and 1 g cm-2 Byr-1, respectively), condensation of hydrocarbons and nitriles (306 and 101 g cm-2 Byr-1, respectively), and polymerization (30 g cm-2 Byr-1). (Annually mean values are halves of those.) The composition and photochemistry of Pluto during the New Horizons flyby are very different from those of Triton during the Voyager 2 flyby and controlled by the methane mole fraction near the surface. Seasonal variations of the atmospheric methane should cover both states of the atmospheres on both Pluto and Triton.

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References

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Bertrand, T., Forget, F., 2017. 3D modeling of organic haze in Pluto’s atmosphere. Icarus 287, 72-86. Cheng, A.F., et al., 2017. Haze in Pluto’s atmosphere. Icarus 290, 112-133. Dias-Oliveira, A. , Sicardy, B. , Lellouch, E. , et al. , 2015. Pluto’s atmosphere from stellar occultations in 2012 and 2013. Astrophys. J. 811, 53. Elliot, J.L., Young, L.A., 1992. Analysis of stellar occultation data for planetary atmospheres. I – Model fitting, with application to Pluto. Astron. J. 103, 991–1015 Fray, N., Schmitt, B., 2009. Sublimation of ices of astrophysical interest: A bibliographic review. Planet Space Sci. 57, 2053-2080. Gao, P., et al., 2017. Constraints on the microphysics of Pluto’s photochemical haze from New Horizons observations. Icarus 287, 116-123. Gladstone, G.R., et al., 2016. The atmosphere of Pluto as observed by New Horizons. Science 351 (6279), aad8866. Hinson, D.P., et al., 2017. Radio occultation measurements of Pluto’s neutral atmosphere with New Horizons. Icarus 290, 96-111. Hinson, D.P., et al., 2018. An upper limit on Pluto’s ionosphere from radio occultation measurements with New Horizons. Icarus 307, 17-24. Hörst, S.M., Vuitton, V., Yelle, R.V., 2008. The origin of oxygen species in Titan’satmosphere. J. Geophys. Res. 113, E10006. Hubbard,W.B., et al., 1988. Occultation evidence for an atmosphere on Pluto. Nature 336, 452454. Krasnopolsky, V.A., 2001. Middle ultraviolet spectroscopy of Pluto and Charon. Icarus 153, 277–284. Krasnopolsky, V.A., 2009. A photochemical model of Titan’s atmosphere and ionosphere. Icarus 201, 226-256. Krasnopolsky, V.A., 2012. Titan’s photochemical model: Further update, oxygen species, and comparison with Triton and Pluto. Planet. Space Sci. 73, 318-326. Krasnopolsky, V.A., 2014. Chemical composition of Titan’s atmosphere and ionosphere: Observations and the photochemical model. Icarus 236, 83-91.

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Krasnopolsky, V.A., 2018. Some problems in interpretation of the New Horizons observations of Pluto’s atmosphere. Icarus 301, 152-154. Krasnopolsky, V.A., Cruikshank, D.P., 1995. Photochemistry of Triton’s atmosphere and ionosphere. J. Geophys. Res. 100, 21,271-21,286. Krasnopolsky, V.A., Cruikshank, D.P., 1999. Photochemistry of Pluto’s atmosphere and ionosphere near perihelion. J. Geophys. Res. 104, 21979–21996. Krasnopolsky, V.A., Sandel, B.R., Herbert, F., 1992. Properties of haze in the atmosphere of Triton. J. Geophys. Res. 97, 11,695–11,700. Lara, L.M. , Ip, W.-H. , Rodrigo, R. , 1997. Photochemical models of Pluto’s atmosphere. Icarus 130, 16–35. Lellouch, E., de Bergh, C., Sicardy, B., Ferron, S., Kaufl, H.U., 2010. Detection of CO in Triton’s atmosphere and the nature of surface-atmosphere interactions. Astron. Astrophys. 512, L8. Lellouch, E., de Bergh, C., Sicardy, B., Kaufl, H.U., Smette, A., 2011a. High resolution spectroscopy of Pluto’s atmosphere: Detection of the 2.3-lm CH4 bands and evidence for carbon monoxide. Astron. Astrophys. 530, L4. Lellouch, E., et al., 2017. Detection of CO and HCN in Pluto’s atmosphere with ALMA. Icarus 286, 289-307. Luspay-Kuti, A., et al., 2017. Photochemistry on Pluto – I. Hydrocarbons and aerosols. MNRAS 472, 104-117. McKinnon, W.B., et al., 2017. Origin of the Pluto-Charon system: Constraints from the New Horizons flyby. Icarus 287, 2-11. Millis, R.L., Wasserman, L.H., Franz, O.G., Nye, R.A., Elliot, J.L., Dunham, E.W., Bosh, A.S., Young, L.A., Slivan, S.M., Gilmore, A.C., 1993. Pluto’s radius and atmosphere: Results from the entire 9 June 1988 occultation data set. Icarus 105, 282–297. Molina-Cuberos, G.J., et al., 1999. Ionization by cosmic rays of the atmosphere of Titan. Planet. Space Sci. 47, 1347-1354. Owen, T.C., Roush, T.L., Cruikshank, D.P., et al., 1993. Surface ices and atmospheric composition of Pluto. Science 261, 745–748. Poppe, A.R., Horanyi, M., 2018. Interplanetary dust delivery of water to the atmospheres of Pluto and Triton. Astron. Astrophys. 617, L5. Rannou, P., West, R., 2018. Supersaturation on Pluto and elsewhere. Icarus 312, 36-44. Sicardy, B. , Talbot, J. , Meza, E. , et al. , 2016. Pluto’s atmosphere from the 2015 June 29 ground-based stellar occultation at the time of the new horizons flyby. Astrophys. J. 819, L38. Stern, S.A., et al., 2017a. Past epochs of significantly higher pressure atmospheres on Pluto. Icarus 287, 47-53. Summers, M.E. , Strobel, D.F. , Gladstone, G.R. , 1997. Chemical models of Pluto’s atmosphere. In: Stern, S., Tholen, D. (Eds.), Pluto and Charon, p. 391-434.

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Tomasko, M.G., et al., 2008a. A model of Titan’s aerosols based on measurements made inside the atmosphere. Planet. Space Sci. 56, 669-707. Tryka, K.A., Brown, R.H., Cruikshank, D.P., et al., 1994. Temperature of nitrogen ice on Pluto and its implications for flux measurements. Icarus 112, 513–527. Wong, M.L., et al., 2017. The photochemistry of Pluto’s atmosphere as illuminated by New Horizons. Icarus 287, 110-115. Young, L.A., Elliot, J.L., Tokunaga, A., de Bergh, C., Owen, T., 1997. Detection of gaseous methane on Pluto. Icarus 127, 258-262. Young, L.A., et al., 2018. Structure and composition of Pluto’s atmosphere from the New Horizons solar ultraviolet occultation. Icarus 300, 174-199. Zhang, X., Strobel, D.F., Imanaka, H., 2017. Haze heats Pluto’s atmosphere yet explains its cold temperature. Nature 551, #7680, 352-355.

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Photochemical model of Pluto’s atmosphere and ionosphere includes 429 reactions of 83 neutrals and 35 ions The model has 289 altitude steps up to exobase at 1600 km Condensation on the haze and the surface with sticking of 0.002 for hydrocarbons and 0.01 for nitriles Revision of laboratory data on hydrocarbon condensation is not required Dayside ionosphere peaks with emax = 800 cm-3 at 750 km and affects neutral composition Evolution of the atmosphere by escape of hydrogen and methane, condensation and polymerization of hydrocarbons and nitriles Seasonal variations of atmospheric methane should greatly affect photochemistry of Pluto and Triton

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