Investigating the optical constants of Pluto’s aerosols

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Orbital and physical parameters of Pluto

Pluto is located more than 30 AU (Astronomical Unit) from the Sun and makes a revolution around the Sun in 247.688 Earth years. Being beyond the orbit of Neptune, it is therefore a Trans-Neptunian Object (TNO), as is the case for all the small bodies of the Kuiper belt (KBO, Kuiper Belt Object). With the discovery of Eris in 2005 (Brown and Schaller, 2005), the IAU decided in 2006 to redefine the notion of “planet”. Pluto being part of a large population of small bodies of smaller or similar sizes, it lost its status of “planet” in favor of that of “dwarf planet” (International Astronomical Union, 2006).
Pluto being inclined at 119.59° with respect to the plane of its orbit (see Figure I.4.right), the rotation around its pole axis is thus done in a retrograde way (i.e., clockwise), in 6.387 Earth days. Also, because of this important obliquity, Pluto presents unusual climate zones characterized by strong seasonal variations in solar illumination versus latitude (Earle and Binzel, 2015), and thus an overlap of tropical and arctic zones (Binzel et al., 2017). On Pluto, the seasons are very marked, with polar nights several Earth-decades long (Binzel et al., 2017). In addition, the strong eccentricity of Pluto’s orbit (e = 0.2488, see Figure I.4.left) causes an asymmetry of the seasons, with a northern summer more intense and shorter than the southern summer. At aphelion (49.31 AU), the energy available from sunlight is only ~40% of that available when Pluto is at perihelion (29.66 AU) (Earle and Binzel, 2015). Pluto’s obliquity oscillates between ~103° and ~128° and Pluto’s eccentricity between 0.222 and 0.266 on million-year timescales (Binzel et al., 2017; Earle et al., 2017), creating Milanković-type insolation cycles influencing Pluto’s past climate northern summer more intense and shorter than the southern summer. At aphelion (49.31 AU), the energy available from sunlight is only ~40% of that available when Pluto is at perihelion (29.66 AU) (Earle and Binzel, 2015). Pluto’s obliquity oscillates between ~103° and ~128° and Pluto’s eccentricity between 0.222 and 0.266 on million-year timescales (Binzel et al., 2017; Earle et al., 2017), creating Milanković-type insolation cycles influencing Pluto’s past climate.
With its radius of 1,188.3 ± 1.6 km (Nimmo et al., 2017), Pluto is smaller than Mercury as well as seven satellites in the Solar System: Ganymede, Titan, Callisto, Io, the Moon, Europa, and Triton. Pluto is nearly spherical with an oblateness of less than 0.6% (Nimmo et al., 2017).

Origin of the Pluto system

The formation of the Pluto-Charon binary system, as well as that of the small satellites is still widely debated (e.g., Stern et al., 2018; Canup, Kratter and Neveu, 2021; McKinnon et al., 2021). Among the different mechanisms proposed to explain the formation of the Pluto system, the one for which there are the most favorable arguments relates to the formation of the Pluto-Charon binary system from a massive collision between two precursors of similar size (radius ≈ 103 km), which would have occurred rather early in the history of the Solar System (McKinnon, 1989; Canup, 2005, 2011; McKinnon et al., 2017, 2021; Sekine et al., 2017; Arakawa, Hyodo and Genda, 2019; Canup, Kratter and Neveu, 2021). Indeed, the large masses of Pluto and Charon, the large specific angular momentum of the pair, and the coplanar orbits of Pluto’s small satellites argue in favor of a giant-impact origin, like the giant collision which created our Moon. This impact should have occurred at low velocity (McKinnon et al., 2017, 2021), between partially differentiated precursors (Canup, 2011; Desch, 2015; Arakawa, Hyodo and Genda, 2019). After the giant impact, Pluto and Charon could have accreted some of the debris, the rest being ejected at high velocity. During this ejection, high-velocity collisions among the debris particles could have occurred, leading either to the ejection of the debris out of the Pluto-Charon binary system, or to the retention of the debris in stable orbits around the binary system (Kenyon and Bromley, 2014; Bromley and Kenyon, 2020).

The New Horizons mission

At the end of the 1980s, with the detection of an atmosphere surrounding Pluto (discussed in more details below), the idea of a spacecraft reconnaissance of the Pluto system emerged. This idea was reinforced with the discovery of Pluto’s evolving surface and atmosphere, the presence of small satellites, and the discovery of the Kuiper Belt (for more details about the genesis of the New Horizons mission, see e.g., Stern (1993, 2008) Guo and Farquhar (2008), Weaver and Stern (2008), and Neufeld (2016)).
The highest priority objectives set out by the National Aeronautics and Space Administration (NASA) for this mission were (Young et al., 2008):
Characterization of the global geology and morphology of Pluto and Charon by obtaining hemispheric panchromatic maps and color maps, acquired at moderate and high phase angles, with sufficient dynamic range and signal/noise ratio.
Mapping of the surface composition of Pluto and Charon by obtaining hemispheric infrared spectroscopic maps with sufficient spectral resolution, to determine the spatial distribution of the volatile ices on Pluto’s and Charon’s surfaces.
Characterization of the neutral atmosphere of Pluto and its escape rate, by determining the precise composition of Pluto’s atmosphere, its thermal structure, and the properties of Pluto’s aerosols.
Secondary and tertiary objectives included (Young et al., 2008):
Characterization of the time variability of Pluto’s surface and atmosphere, Pluto’s ionosphere and solar wind interaction, and the energetic particle environment of Pluto and Charon.
Imaging of Pluto and Charon in stereo.
Mapping of the terminators of Pluto and Charon and of the surface composition of selected areas on Pluto and Charon with high resolution, and of the surface temperature of Pluto and Charon. Searching for neutral species including H, H2, HCN, CxHy, and other hydrocarbons and nitriles in Pluto’s upper atmosphere, for an atmosphere around Charon, for magnetic fields of Pluto and Charon, and for additional satellites and rings.
Determination of the bolometric Bond albedos for Pluto and Charon.
Refining of the bulk parameters (radii, masses, densities) and orbits of Pluto and Charon.

Physical and chemical properties of Titan-simulated atmosphere

The electrons of the plasma discharge are sufficiently energetic to dissociate and ionize N2 and CH4 continuously injected into the reactor (Pintassilgo, Cernogora and Loureiro, 2001; Szopa, Cernogora et al., 2006). In situ measurements performed by Quadrupole Mass Spectrometry (QMS) showed that the first reactions induced by the electrons of the discharge have a slow kinetics. The CH4 concentration is reduced by more than half and reaches steady state in ~100 seconds (Sciamma-O’Brien et al., 2010; Wattieaux et al., 2015). The dissociation of CH4 releases atomic hydrogen H in the plasma, and this release follows a linear law with the percentage of injected CH4 (Carrasco et al., 2012).
For Titan experiments, the main neutral molecule produced, detected by QMS, is hydrogen cyanide HCN, which is one key precursor to the formation of Titan aerosol analogues (Gautier et al., 2011; Carrasco et al., 2012; Wattieaux et al., 2015; Dubois, Carrasco, Petrucciani, et al., 2019). Furthermore, the formation of HCN is enhanced with increasing initial CH4 concentration (Gautier et al., 2011; Carrasco et al., 2012; Dubois, Carrasco, Petrucciani, et al., 2019), as well as when CO is added to the N2:CH4 reactive gas mixture (Fleury et al., 2014). Nevertheless, CO was shown to play an inhibiting role in the formation of Titan’s aerosols, by disturbing the CH4 consumption kinetics (Fleury et al., 2014). In situ analyses of the gas phase composition of Titan-simulated atmosphere revealed that N-bearing molecules, such as nitriles, amines and imines favorable to polymerization, play a crucial role in the formation of Titan’s aerosols (Gautier et al., 2011, 2012; Carrasco et al., 2012). Numerous neutral molecules up to m/z 100 with raw formulae CxHyNz were detected by QMS (Gautier et al., 2011; Carrasco et al., 2012; Dubois, Carrasco, Petrucciani, et al., 2019), as well as various positive and negative ions (Dubois, Carrasco, Bourgalais, et al., 2019; Dubois et al., 2020). In particular, the detection of heteroaromatic molecules in Titan-simulated atmosphere supported the hypothesis of chemical pathways involving Nitrogenous Poly-Aromatic Heterocycles (NPAH) instead of Poly-Aromatic Hydrocarbons (PAH) for the growth of Titan’s aerosols (Gautier et al., 2011). In addition, methanimine CH2=NH and ethanamine CH3CH=NH (Carrasco et al., 2012), as well as ethylene C2H4 (Dubois, Carrasco, Petrucciani, et al., 2019) were suggested to be major precursors to Titan tholin formation. Moreover, C2 cations7, such as HCNH+ and C2H5+, are thought to play a key role in efficient gas-to-solid conversion, i.e., tholin growth (Dubois et al., 2020). Finally, the detection of N-bearing anions such as CN-, CHNN-, C3N-, C3HNN- and C2N3- highlighted the important role of negative ion chemistry in Titan’s aerosol growth (Dubois, Carrasco, Bourgalais, et al., 2019). These results are in agreement with observations made on Titan by INMS (Ion and Neutral Mass Spectrometer) (e.g., Waite et al., 2005, 2007; Cravens et al., 2006; Vuitton, Yelle and Anicich, 2006; Vuitton, Yelle and McEwan, 2007) and by CAPS-ELS (Cassini Plasma Spectrometer – Electron Spectrometer) (e.g., Coates et al., 2007; Vuitton et al., 2009), onboard the Cassini orbiter (e.g., Waite et al., 2004; Young et al., 2004). The time required for the formation of Titan tholins is several tens of seconds after turning on the plasma, and this delay depends on the experimental conditions (e.g., plasma pressure, composition of the reactive gas mixture, etc.) (Alcouffe et al., 2010). Especially, the efficiency of tholin production depends on the N2:CH4 mixing ratio in the reactive gas mixture, with two competitive chemical regimes. The production of Titan tholins in PAMPRE is particularly favored when the initial CH4 percentage is comprised between 4% and 6%, depending on the operating plasma pressure (0.9 or 1.7 mbar) (Sciamma-O’Brien et al., 2010). At low CH4 concentration, the tholin production rate is proportional to the percentage of consumed CH4. In contrast, the tholin production is inhibited for initial CH4 percentage significantly higher than 5%, when the atomic H concentration is too high in the reactive plasma (Sciamma-O’Brien et al., 2010; Carrasco et al., 2012). Once formed, the presence of tholins in the plasma affects the physical and chemical properties of the plasma. In particular, free electrons from the plasma discharge attach to the solid particles, rendering tholins electronegative. Therefore, the electron density decreases in the plasma, which is compensated by the increase in the energy of these electrons (Alcouffe et al., 2010; Wattieaux et al., 2015).

Physical and chemical properties of Titan aerosol analogues

Electronegative tholins form and are maintained in levitation in the plasma between the two electrodes, the solid particles are therefore spherical in shape. When the grains reach a limit size, they are ejected out of the plasma, the spherical particles therefore have a relatively uniform size distribution (Hadamcik et al., 2009). Scanning Electron Microscopy (SEM) measurements revealed the filamentous growth of the spherical particles, observed on the surface of the particles (Hadamcik et al., 2009). The particle size depends on the experimental conditions, such as N2:CH4 mixing ratio in the reactive gas mixture, gas flow rate injected into the reactor, plasma duration, etc. (Hadamcik et al., 2009). In particular, the addition of CO in a N2:CH4 reactive gas mixture leads to the formation of averagely larger solid particles (Fleury et al., 2014).
The optical constants (n and k indices) of Titan tholins produced with the PAMPRE experimental setup were determined by spectroscopic ellipsometry (Mahjoub et al., 2012, 2014; Sciamma-O’Brien et al., 2012). These optical constants depend on the N2:CH4 mixing ratio in the reactive gas mixture (Mahjoub et al., 2012) and on the synthesis temperature (Mahjoub et al., 2014). n index (real part of the complex refractive index) is increased, whereas k index (imaginary part of the complex refractive index) is lowered when the initial CH4 concentration increases. This observation agrees with the color of Titan tholins ranging from dark brown for tholins produced in a N2:CH4 = 99%:1% gas mixture to light yellow for tholins produced in a N2:CH4 = 90%:10% gas mixture. This result was attributed to the proportion of N-bearing molecules that differ between Titan tholins synthesized with different N2:CH4 mixing ratios (Mahjoub et al., 2012). Regarding the effect of synthesis temperature, it was observed that the tholin thin films produced at cryogenic temperature (105 K) are characterized by higher n-values and lower k-values than thin films produced at ambient temperature (300 K) over the whole analyzed [350-1,000 nm] wavelength range. This observation was interpreted as different chemical composition of the reactive gas mixture at low temperature, due to condensation effects on the cold walls of the plasma confining cage (Mahjoub et al., 2014).
Fourier-Transform InfraRed (FTIR) analyses performed on Titan tholins showed that tholins produced with a low concentration of CH4 (1%) are polymer-kind materials containing more amine functional groups (–NH and –NH2) than tholins produced with high methane concentration (10%), whereas tholins produced with a high concentration of CH4 have a higher contribution of aliphatic carbon (–CH, –CH2 and –CH3) (Gautier et al., 2012). Nitrile –C≡N, isocyanide –N≡C, aromatic and aliphatic –C=C and –C=N functional groups were also detected in the mid-IR (Quirico et al., 2008; Gautier et al., 2012). Absorption bands around 2,900 cm-1 (3.4 μm) found in Titan tholin spectrum are in agreement with observations made by VIMS (Visual and Infrared Mapping Spectrometer) instrument onboard Cassini orbiter (e.g., Brown et al., 2004). This confirms an aerosol contribution in Titan’s atmosphere, which is well reproduced by Titan tholins synthesized in the laboratory (Gautier et al., 2012). In the far-IR, Titan tholin spectrum displays many similarities with observations made by Cassini-CIRS (Composite InfraRed Spectrometer) instrument (e.g., Flasar et al., 2004), namely absorption bands at 325 cm-1 (30.76 μm), 515 cm-1 (19.41 μm), 1,380 cm-1 (7.24 μm) and 1,450 cm-1 (6.89 μm) (Gautier et al., 2012). An aromatic signature in the [4.1-5.3 μm] spectral range was also identified in tholins synthesized with the PAMPRE experimental setup in N2:CH4, N2:CH4:C5H5N and in N2:C5H5N gas mixtures (Mathé et al., 2018). This aromatic signature is in agreement with the detection of aromatic fragments released by Titan tholins after being submitted to thermal degradation (pyrolysis) and analysis by ThermoGravimetric Analysis Differential Scanning Calorimetry-Mass Spectrometry (TGA-DSC-MS) (He et al., 2015). Elemental composition analyses indicated that elemental mass percentages in Titan tholins depend on the N2:CH4 mixing ratio in the reactive gas mixture in which the tholins are synthesized. When the initial CH4 concentration increases, the carbon mass percentage stays roughly constant, whereas the nitrogen mass percentage significantly decreases and the hydrogen mass percentage significantly increases. This result regards Titan tholins as spherical grains (Sciamma-O’Brien et al., 2010; Carrasco et al., 2016). For Titan tholins as thin films, when the initial CH4 concentration increases, only the N mass percentage decreases, whereas the C and H mass percentages remain constant (Carrasco et al., 2016). A comparative study of the chemical compositions of Titan tholins as solid spherical grains and thin films concluded that these two sorts of samples are chemically different. Thin films are less rich in nitrogen and hydrogen than solid grains, probably due to efficient etching occurring on the thin films staying in the plasma discharge for 2-3 hours (Carrasco et al., 2016).
High-Resolution Mass Spectrometry (HRMS) measurements were made on both soluble in methanol and insoluble fractions of Titan tholins (e.g., Carrasco et al., 2009; Pernot et al., 2010; Hörst et al., 2012; Gautier et al., 2014, 2016; Maillard et al., 2018). The soluble fraction – which represents up to 35% in mass of the solid samples (Carrasco et al., 2009) – is significantly different from the insoluble fraction – which represents 65% of the global mass sample (Maillard et al., 2018). Mass spectra of both fractions exhibit regularly-spaced peak clusters, suggesting a polymeric nature for the molecules constituting Titan tholins (Carrasco et al., 2009; Pernot et al., 2010; Gautier et al., 2014; Maillard et al., 2018). However, the soluble fraction of Titan tholins is identified as an ideal (CH2)m-(HCN)n polymer (Pernot et al., 2010; Gautier et al., 2014), whereas the insoluble fraction is composed of a set of polymers with an average formula (C4H3N2)n (Maillard et al., 2018). HRMS analyses and Gas Chromatography coupled to Mass Spectrometry (GC-MS) measurements confirmed the presence of amino acids (alanine C3H7NO2, glycine C2H5NO2 and histidine C6H9N3O2) and nucleotide bases (adenine C5H5N5, cytosine C4H5N3O, guanine C5H5N5O, thymine C5H6N2O2 and uracil C4H4N2O2) in Titan tholins (Hörst et al., 2012).

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Adapting the PAMPRE experimental setup to the study of Pluto

Relying on these earlier studies for Titan, we were able to develop a robust experimental protocol for the study of Pluto’s aerosols, from their formation in the upper atmosphere, to their deposition on the surface, through their interactions with the atmosphere during their sedimentation. In particular: Previous studies on Titan performed with the PAMPRE experimental setup helped us to identify the optimal experimental conditions (gas flow rate, pressure, temperature, etc.) to be used for the simulation of Pluto’s atmospheric chemistry and the synthesis of Pluto aerosol analogues. For instance, in order not to be biased by heterogeneous chemistry due to condensation effects as discussed in Mahjoub et al. (2014), we chose to conduct our simulation experiments at ambient temperature.
Observations made in 2015 by the Alice spectrograph onboard New Horizons showed that CH4 concentration varied from ~0.3% near the surface to 50% at 1,450 km of altitude above the surface (Young et al., 2018). Similarly, Global Circulation Model (GCM) results showed that the mean atmospheric concentration of CH4 varies from 0.5% to 5% during Pluto’s seasons/epochs (Bertrand and Forget, 2016; Bertrand et al., 2019). As said before, previous studies on Titan showed that the CH4:N2 mixing ratio has a significant effect on Titan’s atmospheric chemistry and on the chemical composition and the optical constants of Titan’s aerosols. In the scope of this Ph.D. thesis, to account for the variability in terms of chemical composition in Pluto’s atmosphere at different altitudes or at different seasons/epochs, we therefore systematically conducted our studies for at least two different gas mixtures/samples.
The atmosphere of Pluto being similar to that of Titan (N2 and CH4 as main components), with nevertheless a greater proportion of CO (~500 ppm), the previous studies on Titan can be used as a reference “without oxygen” to identify and understand the effect of CO on the chemical system N2:CH4:CO which constitutes Pluto’s atmosphere.

Table of contents :

Résumé
I. Introduction
II. Matériel et Méthodes
III. Résumé des résultats et implications pour Pluton
III.1. Concernant la composition chimique de l’atmosphère de Pluton
III.2. Concernant la composition chimique des aérosols de Pluton
III.3. Concernant les indices optiques des aérosols de Pluton
III.4. Concernant la matière organique à la surface de Pluton
I. Pluto and laboratory simulations of planetary atmospheres
I.1. Historical background
I.1.1. Discovery of the Pluto system
I.1.2. Orbital and physical parameters of Pluto
I.1.3. Origin of the Pluto system
I.1.4. The New Horizons mission
I.2. Pluto’s interior and geology
I.3. Pluto’s surface ices, atmosphere and aerosols
I.3.1. Volatile ices on Pluto’s surface
I.3.2. Pluto’s atmosphere
I.3.3. Pluto’s aerosols
I.4. Laboratory simulations of planetary atmospheres and objectives of this Ph.D.
II. Experimental section
II.1. The PAMPRE experimental setup
II.1.1. History in brief – From Titan to Pluto
II.1.2. Technical characteristics
II.1.3. Summary of selected previous studies involving the PAMPRE experimental setup
II.1.4. Experiments performed during this Ph.D., using the PAMPRE experimental setup
II.2. Swift heavy ion irradiation at GANIL
II.2.1. Ion-matter interaction
II.2.2. Experiments performed on the IRRSUD beamline
II.2.3. Experiments planned on the ARIBE beamline
II.3. Analytical techniques employed
II.3.1. Analytical techniques relative to Chapter III – Investigating the chemical composition of Pluto’s atmosphere
II.3.2. Analytical techniques relative to Chapter IV – Investigating the chemical composition of Pluto’s aerosols
II.3.3. Analytical techniques relative to Chapter V – Investigating the optical constants of Pluto’s aerosols
II.3.4. Analytical techniques relative to Chapter VI – Investigating the organic matter on Pluto’s surface
II.3.5. Summary of the complementary analytical techniques used during this Ph.D.
III. Investigating the chemical composition of Pluto’s atmosphere
III.1. Neutral molecular composition of Pluto-simulated atmosphere
III.1.1. Organic growth in Pluto-simulated atmosphere
III.1.2. Identification of neutral molecules in Pluto-simulated atmosphere
III.2. Cations in Pluto-simulated atmosphere
III.2.1. Identification of cations in Pluto-simulated atmosphere
III.3. Implications for Pluto
IV. Investigating the chemical composition of Pluto’s aerosols
IV.1. Study of the chemical composition of Pluto aerosol analogues
IV.1.1. Global aspect of Pluto tholin high-resolution mass spectrum
IV.1.2. Importance of N2 and CO chemistries
IV.1.3. Investigating the effect of the atmospheric composition (and therefore the altitude of aerosol formation) on the chemical composition of Pluto’s aerosols
IV.1.4. Discussion on the chemical composition of Pluto’s aerosols
IV.2. Search for molecules of prebiotic interest in Pluto aerosol analogues
IV.2.1. Identification by APPI/Orbitrap of molecular formulae potentially corresponding to molecules of prebiotic interest
IV.2.2. Structural information inferred from GC-MS analysis
V. Investigating the optical constants of Pluto’s aerosols
V.1. Determination of the optical constants of Pluto tholins from UV to near-IR
V.2. Impact of the altitude (or epoch) of aerosol formation on the optical constants and implications for radiative transfer
V.2.1. Effect on real part n of the complex refractive index
V.2.2. Effect on imaginary part k of the complex refractive index
V.3. New input parameters for Pluto atmospheric and surface models
V.3.1. Comparison of my optical constants with those of Titan tholins from the study by Khare et al. (1984)
V.3.2. Application of my optical constants to Pluto’s surface modeling
V.3.3. Application of my optical constants to Pluto’s atmosphere modeling
VI. Investigating the organic matter on Pluto’s surface
VI.1. Pluto tholins as analogues of Pluto’s surface material
VI.2. Irradiations at GANIL/IRRSUD
VI.2.1. Evolution of morphology of Pluto tholins
VI.2.2. Evolution of spectral properties of Pluto tholins
VI.2.3. Evolution of molecular composition of Pluto tholins
VI.2.4. Volatiles released by Pluto tholins under swift heavy ion irradiation
VII. Conclusion and Perspectives
VII.1. Summary of results
VII.2. Perspectives
List of publications related to this Ph.D. thesis
Appendix A1: Detailed description of the different parts of the Hiden Analytical EQP 200 Quadrupole Mass Spectrometer (QMS)
Appendix A2: Chemical composition of Pluto aerosol analogues inferred from HRMS (Orbitrap technique)
Appendix A3: Optical constants of Pluto aerosol analogues from UV to near-IR, characterized with spectroscopic ellipsometry and “λ-by-λ” numerical inversion method
Appendix A4: Deconvolution of the IGLIAS background mass spectrum
Table of illustrations (Figures and Tables)
References

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