Investigating the chemical composition of Pluto’s atmosphere

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Pluto’s interior and geology

The mean density of 1,854 ± 11 kg.m-3 (Nimmo et al., 2017) (see Table I.1) suggests that Pluto is a differentiated body (Stern, 1989). ⅔ of the internal structure (i.e., ~66%) would correspond to a rocky core on which probably lies a 100km-thick layer of liquid water (H2O), above which would lie a ~300km-thick water-ice-dominated mantle bedrock (Barr and Collins, 2015; Johnson et al., 2016; Nimmo et al., 2016; McKinnon et al., 2017; Bierson, Nimmo and McKinnon, 2018; Kamata et al., 2019; Kimura and Kamata, 2020). Recent modeling studies supported the hypothesis of a subsurface ocean composed of water with probably a small amount of ammonia (Nimmo et al., 2016; Kimura and Kamata, 2020). The presence of ammonia in this water ocean (1-5%wt relative to H2O) can significantly depress its melting temperature and thus increase the possibility of existence of this subsurface ocean (Lewis, 1971; Desch et al., 2009). To prevent this ocean from completely freezing, methane clathrates were also proposed as thermal insulator (Kamata et al., 2019). Pluto’s interaction with the solar wind is consistent with an unmagnetized body. The magnetic field measured at the surface of Pluto by the SWAP instrument is lower than 30 nT (McComas et al., 2016).

Pluto’s atmosphere

Ground-based spectroscopic observations conducted in 1992 allowed to strictly identify gaseous methane from Pluto’s atmosphere (Young et al., 1997), and its average mixing ratio was estimated to be ~0.5% between 2010 and 2015 (Lellouch et al., 2009, 2011, 2015). Carbon monoxide was detected in 2010 through ground-based high-resolution spectroscopic measurements (Lellouch et al., 2011) and the CO mixing ratio was quantified in 2015 at 515 ± 40 ppm by submillimeter observations with the Atacama Large Millimeter/submillimeter Array (ALMA) interferometer (Lellouch et al., 2017). ALMA observations also showed that hydrogen cyanide HCN is present in Pluto’s atmosphere. HCN mixing ratio is higher than 1.5×10-5 above 450 km of altitude, and equal to 4×10-5 near 800 km of altitude above the surface (Lellouch et al., 2017).
New Horizons studied Pluto’s atmosphere with radio, solar, and stellar occultations, airglow observations and imaging (Gladstone et al., 2016; Cheng et al., 2017; Hinson et al., 2017; Young et al., 2018; Steffl et al., 2020). The plasma environment and Pluto’s interaction with solar wind were studied as well (Bagenal et al., 2016; McComas et al., 2016).
The structure of Pluto’s atmosphere was determined from radio occultation data measured by the REX instrument and solar ultraviolet occultation data recorded by the Alice spectrograph (Gladstone et al., 2016; Hinson et al., 2017; Young et al., 2018). The pressure at the surface of Pluto was estimated by Gladstone et al. (2016) and Hinson et al. (2017) to be 10.2 ± 0.7 and 12.8 ± 0.7 μbar, respectively, whereas the near-surface temperature ranges from 38.9 ± 2.1 K (at ingress) to 51.6 ± 3.8 K (at egress) (see Table I.1). Note that earlier ground-based observations showed a significant increase in pressure on the surface of Pluto by a factor of 3 between 1988 and 2015 (e.g., Elliot et al., 1989; Lellouch et al., 2011; Dias-Oliveira et al., 2015; Sicardy et al., 2016). Regarding the temperature profile on Pluto (shown in Figure I.12), a strong temperature inversion was observed for both ingress and egress REX measurements, at altitudes lower than 20 km (Gladstone et al., 2016). This strong temperature inversion is qualitatively consistent with profiles retrieved from ground-based stellar occultation measurements (e.g., Sicardy et al., 2003; Elliot et al., 2007; Dias-Oliveira et al., 2015). However, two notable differences were observed between the REX profiles obtained at ingress and egress. First, the temperature inversion inferred from REX measurements at ingress (6.4 ± 0.9 K.km-1) is much stronger than at egress (3.4 ± 0.9 K.km-1). Second, the temperature inversion at ingress ends abruptly at an altitude of ~4 km, marking the top of a distinctive boundary layer, whereas the temperature inversion at egress appears to extend to the surface, indicating the absence of this boundary layer. These notable differences between ingress and egress retrievals indicate the presence of horizontal variations in temperature (Gladstone et al., 2016).

Laboratory simulations of planetary atmospheres and objectives of this Ph.D.

Given all these effects mentioned above, the study of the formation processes and the physical and chemical properties of Pluto’s aerosols is essential for a better understanding not only of Pluto’s atmospheric chemistry, but also of its climate (modulated by interactions between radiation, the constituents of its atmosphere and its aerosols), and of its heterogeneous surface.
In addition to direct observations of Pluto, from the ground or with the instruments onboard the New Horizons spacecraft, there are different approaches to study the atmospheric chemistry and the formation of Pluto’s aerosols, as well as their physical and chemical properties. For example, through numerical modeling, theoretical mechanisms can be confronted with observations. For my Ph.D. thesis, I used another approach based on experimental laboratory simulation to simulate the atmospheric chemistry of Pluto. This was done using a representative gas mixture and an energy source, to form aerosol analogues, which were then subjected to various physicochemical analyses using state-of-the-art analytical instruments.
These three approaches – observations, modeling, laboratory simulation – are linked, since laboratory experiments provide, for instance, inputs used in numerical models explaining the observations. For example, the optical constants of aerosol analogues are particularly used in numerical modeling, both to understand the radiative transfer in Pluto’s atmosphere (e.g., Zhang, Strobel and Imanaka, 2017), and at its surface (e.g., Protopapa et al., 2017, 2020; Grundy et al., 2018). However, due to a lack of both observational and experimental data on these optical properties, the study by Zhang, Strobel and Imanaka (2017) was done using those of Titan aerosol analogues produced on Earth. The modeling studies of Pluto New Horizons data across the full [0.4-2.5 μm] wavelength range by Protopapa et al. (2020) also used optical constants determined for Titan aerosol analogues by Khare, Sagan, Arakawa et al. (1984) and Tran et al. (2003). To explain the different colors on Pluto’s surface, Grundy et al. (2018) used multiple-scattering radiative transfer models with optical constants determined for Titan aerosol analogues by several groups: Khare, Sagan, Arakawa et al. (1984), Ramírez et al. (2002), Imanaka et al. (2004), Vuitton et al. (2009), and Sciamma-O’Brien et al. (2012), here again given the lack of optical constants for Pluto aerosol analogues.

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History in brief – From Titan to Pluto

PAMPRE stands for “Production d’Aérosols en Microgravité par Plasma REactif” (French acronym), i.e., “Production of Aerosols in Microgravity by REactive Plasma” (Szopa, Cernogora et al., 2006; Alcouffe et al., 2010). This experimental setup, inspired from a dusty plasma reactor dedicated to the study of microelectronic processes and located at GREMI5 laboratory (e.g., Bouchoule and Ranson, 1991; Boufendi and Bouchoule, 1994), was developed at Service d’Aéronomie (Verrières-le-Buisson, France, ancestor of LATMOS), initially to simulate the chemistry occurring in Titan’s atmosphere, and to synthesize analogues of Titan’s aerosols, also called “tholins”. Note that the term “tholins”, from the Ancient Greek word θολός (tholós) meaning “muddy”, was introduced by Carl Sagan and Bishun N. Khare in 1979 to designate a complex organic solid produced from cosmically abundant molecules irradiated by UV photons or spark discharge (Sagan and Khare, 1979). The development of an experimental reactor such as PAMPRE was motivated by the NASA/ESA Cassini-Huygens mission exploring the Saturnian system (e.g., Lebreton and Matson, 1992; Matson, 1996; Matson, Spilker and Lebreton, 2003; Spilker, 2019), and especially to help interpret the data acquired by the Aerosol Collector and Pyrolyser (ACP) instrument developed at Service d’Aéronomie (Israel et al., 1999, 2003), onboard the Huygens probe that landed on Titan’s surface (e.g., Lebreton and Matson, 1997).

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
Références bibliographiques
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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