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Clouds and hazes play a major role in (exo)planetary atmospheres. They can absorb and reflect light from UV to thermal infrared wavelengths, changing the atmospheric emission, reflection, and transmission spectra dramatically. The organic aerosols forming the haze can act as cloud condensation nuclei. Then can also settle down onto the surface, hence participating in its composition. Dedicated laboratory experiments have been developed to produce solid materials that are analogs of haze and cloud particles, under different experimental conditions (molecular precursors, temperature, pressure, energy source…). These experimental studies are key to investigating the physical and chemical processes that drive the formation of solid particles from gas and solid phase molecular precursors in planetary environments. These experiments also allow the characterization of the physical, optical and chemical properties of the laboratory-generated haze and cloud particle analogs, hence providing critical information that can be used as input parameters in models for the analysis and interpretation of observational data (e.g. optical constants, vapor pressures, spectral features, grain morphology, etc).

Here, as examples of these laboratory efforts, we will present various studies that combine (1) experiments performed to produce analogs of Titan and Pluto atmospheric aerosols from gas phase molecular precursors, (2) experiments conducted to simulate the formation of benzene ice cloud particles in Titan’s stratosphere, and (3) experiments carried out to characterize the haze and cloud particle analogs to provide, in particular, optical constants and vapor pressures. We will show how important these studies are for the interpretation of observational data from past, current and future (exo)planetary missions. We will also introduce the newly funded NASA Center for Optical Constants whose overarching goal is to support a stable, long-term, synergistic laboratory effort to address a critical need throughout the broader planetary science community for the development of a comprehensive database containing complex refractive indices (optical constants) of laboratory-generated analogs of organic refractory materials, and ices present in planetary atmospheres and surfaces.

How to cite: Sciamma-O'Brien, E., Barth, E., Bertrand, T., Cook, J., Cruikshank, D., Dalle Ore, C., Dubois, D., Grundy, W., Iraci, L., Mastrapa, R., Roush, T., Salama, F., and Vinatier, S. and the NASA Center for Optical Constants (NCOC) Team: On the Importance of Producing and Characterizing Laboratory Analogs of Planetary Atmospheric Aerosols and Clouds and Their Use to Interpret Observations, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-469, https://doi.org/10.5194/epsc2021-469, 2021.

• Introduction

Titan, Saturn's largest moon, has a thick nitrogen-based atmosphere, where during its flyby, the IRIS infrared spectrometer aboard the Voyager 1 spacecraft detected nitriles, such as HCN, formed by EUV photochemistry based on methane CH₄ and molecular nitrogen N₂. Titan is also surrounded by an organic photochemical haze, according to the observations of the Cassini-Huygens and Voyager 1 missions.

In-situ measurements by the Huygens spacecraft [1], and laboratory experiments synthesizing Titan analog aerosols (called tholins), have revealed that HCN is one of the major chemical signatures extracted from the aerosols, and their laboratory analogues.

Laboratory experiments were conducted using a powder plasma reactor mimicking Titan's ionosphere, replicating the formation of Titan-like organic aerosols, and the associated chemistry. In this work, we simultaneously study the temporal evolution of HCN present in the gas phase, and the formation and growth of Titan tholins.

• 1 - Aerosol production

Analogues of Titan aerosols are formed using the experiment PAMPRE [4]. PAMPRE is a reaction chamber, where a cold radiofrequency plasma capacitively coupled is ignited at low pressure (~0.9 mbar). A gas mixture of 95% N₂ and 5% CH₄ is introduced into the reactor. Subsequently, a 12W discharge is generated between two electrodes, which ionizes the N₂ and CH₄ present.

In this plasma, the radicals and products formed by ionization of methane and nitrogen, will combine by several reaction pathways, to form more complex organic particles and nitriles such as HCN, in the same way as in the ionosphere of Titan.

In this study, the gas flow rate was optimized, to increase the residence time of the gas mixture in the reactor as much as possible. The chemical growth of the solid particles is thus favored, allowing to follow simultaneously the formation and the evolution of the particles, as well as the co-evolution of the gas mixture composition.

• 2 - Morphology analysis of tholins by scanning electron microscopy

The formed samples were observed by scanning electron microscopy (SEM field emission gun).

Figure 1 - SEM picture of Titan tholins obtained after 160 s in the discharge

The images show primary nanometer monomers coagulated and formed aggregates (Figure 1). These aggregates continue to evolve to form single spherical particles of ~1.5 µm in diameter (Figure 1). These experimental results are in agreement with the evolution predicted by the model [2] on aerosols residing in the atmosphere of Titan.

• 3 - Chemical analysis of tholins by infrared spectroscopy

Absorbance measurement is performed using an FTIR, on pellets composed of ~ 99.6% KBr and ~ 0.4% tholins. Three absorption bands stand out on the mid-infrared absorption spectrum. In particular, the intense bands at 1560 cm-1 and 1630 cm-1 (Figure 2) which correspond to different functional groups, such as aromatic or aliphatic -NH₂, C=N, C=C, aromatic or heteroaromatic groups, difficult to distinguish from each other. The presence of these nitrogenous aromatic compounds can be promoted by HCN [3] [5].

Figure 2 - Normalized IR spectra showing the aromatic or aliphatic groups bands

• 4 - Conclusions and Perspectives

The growth of the particles could be observed by the SEM analysis. Moreover, the IR analysis shows the presence of nitrogenous chemical bonds, corresponding to aromatic compounds or nitrile. This growth may be due to the incorporation of HCN chemistry on the surface of the tholins. Indeed, MS analysis of the gas phase shows a progressive consumption of HCN during the formation and growth of  Titan tholins. Our experimental results are consistent with numerical models based on Cassini-Huygens observations: A formation of atmospheric HCN on Titan, and its efficient incorporation in Titan’s haze to produce large organic polymers.

References

[1] Israël G. et al., Nature 438 : 796-99 (2005).

[2] Lavvas P. et al., The Astrophysical Journal (2011).

[3] Imanaka H. et al., Icarus, 168: 344-66 (2004).

[4] Szopa C. et al., Planetary and Space Science 54 (2006).

[5] Gautier T. et al., Icarus, 221, 320-327 (2012).

How to cite: Perrin, Z., Carrasco, N., Chatain, A., Jovanovic, L., Ruscassier, N., Vettier, L., and Cernogora, G.: An atmospheric origin for HCN-derived polymers on Titan, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-491, https://doi.org/10.5194/epsc2021-491, 2021.

Abstract

 Aerosols are ubiquitous in planetary atmospheres in solar and extrasolar systems. Since aerosols greatly impact atmospheric thermal structures and observations, it is vital to understand how they form in a variety of environments. Here, we investigate photochemical haze formation on Triton---the largest moon of Neptune---using a detailed microphysical model. We found that simulated haze profiles without condensation of C2H4 ices hardly explain both UV and visible observations of Voyager 2 simultaneously, while icy haze models can reasonably explain the observations. Thus, condensation of hydrocarbon ice likely plays vital roles in the haze formation on Triton. Our study highlights the importance of condensation physics in haze formation, which has often been neglected in modeling studies, especially for exoplanetary hazes.

Introduction

Aerosols are universally present in atmospheres on solar system planets. Recent observations also suggest that photochemical hazes are ubiquitous even in the extrasolar planets (e.g., Sing et al. 2016, Crossfield & Kreidberg 2017). Because the hazes significantly impact on atmospheric energy balance (e.g., Zhang et al. 2017) as well as the observability of atmospheres (e.g., Lavvas & Koskinen 2017, Kawashima & Ikoma 2018, Gao & Zhang 2020, Ohno & Kawashima 2020), it is crucial to figure out how they form in a variety of environments. To this end, solar system objects provide a great opportunity to study the haze formation process thanks to abundantly available observations. Titan, Pluto, and Triton are typical solar system objects that possess hazy atmospheres. For Titan and Pluto, haze formation processes have been studied by microphysical models (e.g., Lavvas et al. 2010, 2020, Gao et al. 2017). For Triton, however, the haze formation process has not been investigated in detail since the discovery of near-surface haze layer during the Voyager 2 flyby 1989. 

Methods

We have developed a new microphysical model to study the haze formation on Triton (Ohno et al. 2021). Our model simulates the evolution of both size and porosity distributions of haze particles in a self-consistent manner without assuming a specific fractal dimension (Figure 1). We compare the simulated haze profiles with those constrained by Voyager’s observations of solar occultation in UV and scattered lights in visible (Herbert & Sandel 1991, Rages & Pollack 1992). We investigate several possible nature of Triton’s haze, namely spherical and non-spherical particles with and without condensation of C2H4 ice.