The Optical Properties of Titan’s Haze Analogs: A Cross-Laboratory Study

Introduction: In Titan’s nitrogen–methane atmosphere, photochemistry leads to the production of complex organic particles. These photochemical hazes play a crucial role in radiative transfer processes, absorbing solar radiation and influencing atmospheric chemistry. While photochemical models can reproduce the formation of simple organic molecules up to a few carbons and nitrogens [Lavvas et al., 2008], they cannot simulate the rich organic chemistry and haze formation observed in Titan. This limitation, combined with the lack of returned samples for direct analysis, has made laboratory simulation experiments especially important for investigating the properties of Titan’s hazes. These experiments typically simulate Titan’s atmospheric composition and use energy sources such as UV light or plasma to drive chemical reactions. The resulting products, referred to as “tholins” [Sagan et al., 1979] serve as analogs for Titan’s atmospheric aerosols and are widely used to study their optical behavior and chemical complexity.

Over the last few decades, multiple laboratories with different experimental setups have synthesized tholins that broadly captured the general characteristics of Titan’s hazes. However, substantial variations still exist in the data reported across different research groups [Brassé et al., 2015 and references therein].  These variations can be attributed to several factors during sample production – such as the different gas mixtures, energy sources, pressures, and temperatures employed. The technique and methodology used to characterize tholins can further complicate comparisons [Drant et al., 2024]. Other factors, such as substrate employed and atmospheric exposure can impact both production and measurement conditions.

To address this gap, the goal of this work is to conduct a comprehensive and systematic comparative study of the optical properties that cover a broad wavelength range (0.19-30 µm) of multiple tholin samples produced in four independent laboratory facilities and characterized at a single facility using standardized protocols.

Sample Preparation: Tholin samples were produced across four independent laboratory facilities: the COSmIC facility at NASA Ames Research Center (ARC) employing cold plasma discharge; the PHAZER chamber at John Hopkins University (JHU) utilizing both cold plasma discharge and UV lamp irradiation; the Tholinator at Southwest Research Institute (SwRI) employing cold plasma discharge; and the PAC chamber at the University of Northern Iowa (UNI) employing UV lamp irradiation (see Figure 1). In all facilities a common set of gas mixtures was used, ranging from 0.01-10% CH₄ in N₂.

To minimize variability introduced during handling and analysis, all tholin samples were deposited on identical substrates cleaned using the same protocol, transported in specialized vacuum sealed vessels and stored under N₂ atmosphere to preserve sample integrity.

Methodology: Sample characterization was performed at a dedicated facility: the Planetary Characterization Facility (PMCHEF) at the University of Texas at San Antonio (UTSA). The optical constants of haze samples were measured across a broad spectral range (0.19-30 micron) using two J.A. Woollam ellipsometer systems (M-2000 and IRVASE). To avoid degradation from air exposure and atmospheric interference, both instruments are housed in inter-connected dry-nitrogen-supplied MBraun glove boxes (see Figure 2).

Results: Spectroscopic Ellipsometry (SE) measures the change in polarization of light upon reflection on the sample to determine the complex reflectance ratio: the ratio of amplitude change and the relative phase difference. To then determine the properties of interest, a model-based analysis is used, and regression analysis is required. As seen in Figure 3, the optical constants of tholin samples exhibit broad absorption features from electronic interband transitions at the short wavelength range, as well as wide transparent regions. On the other hand, the numerous sharp and overlapping phonon features in the IR data arising from amines, nitriles, and hetero-aromatics vibrational modes [Gavilan et al., 2018], poses additional challenges when modeling SE data, often resulting in inadequate models that are unable to resolve sharp features and weak absorptions [Brassé et al., 2015]. We addressed this issue by using Kramer-Kronig-consistent B-spline parametrization [Mohrmann et al., 2020].

Despite being fabricated under nominally identical conditions (5% CH₄ in N₂, cold plasma discharge) and characterized identically, the two samples in Figure 3 show noticeable differences in their dispersion curves. The observed differences may be attributed to factors such as variations in plasma discharge uniformity, which can lead to non-uniformities in either the optical constants or thickness. These variations may occur both between nominally identical samples and within different regions of a single sample, contributing to discrepancies in optical properties. To address these uncertainties, mapping capabilities will be utilized. A custom-designed sample holder was developed, and the data was processed and visualized using tailored analysis scripts. An example of this mapping feature is shown in Figure 4.

The analysis of the complete set of samples, including those synthesized with varying precursor ratios, will provide further insight into how these variations influence the optical constants across the spectrum. Expanding the dataset will help determine whether the observed differences in optical constants are intrinsic to the materials themselves or arise from extrinsic factors during fabrication. This comprehensive analysis will enhance our understanding of the optical properties of tholins and their potential applications as planetary analogs to better understand Titan’s atmosphere and climate.

Figure 1: Schematics of the four independent tholin production laboratory facilities used in this work: (a) PHAZER/JHU; (b) PAC/UNI; (c) COSmIC/ARC; (d) Tholinator/SwRI.

Figure 2: The ellipsometry system at PMCHEF/UTSA for optical constant characterization.

Figure 3: Derived optical constant comparison between two Titan haze analog samples generated using a 5% CH₄ in N₂ gas mixture. The Spectroscopic Ellipsometry (SE) measured data are compared with existing spectrophotometry data measured by FTIR [10].

Figure 4: Tholin thickness (nm) vs position (cm) for JHU/PLA 5% CH₄ in N₂ sample.