Refractive indices of photochemical haze analogs for Solar System and exoplanet applications : a cross-laboratory comparative study between the PAMPRE and COSmIC experimental set-ups

TL;DR

This cross-laboratory study delivers UV–FIR refractive indices ($n$ and $k$) for Titan, Pluto, and exoplanet haze analogs produced under varied gas compositions and two distinct experimental setups (PAMPRE and COSmIC). It demonstrates that the extraction of optical constants is sensitive to measurement method and to gas-phase composition, particularly nitrogen incorporation, with COSmIC hazes generally more absorbing than PAMPRE hazes. The work introduces a unified modeling framework (thin-film theory with KK-consistent constraints) and compares multiple dispersion descriptions (Cauchy vs. Tauc-Lorentz), finding Cauchy-based approaches more reliable for weakly absorbing UV–Vis–NIR data. It further shows that haze refractive indices strongly modulate radiative properties and thus influence albedo and heating, with important implications for Titan, Pluto, Solar System giant planets, and exoplanet atmospheres observed by JWST. The data set up to 200 μm provides essential inputs for climate models and spectral retrievals, and the authors advocate using both lab setups to capture the range of possible haze behaviors across atmospheric environments.

Abstract

Previous observations of Titan, Pluto and Solar System gas giants along with recent observations of exoplanet atmospheres with the James Webb Space Telescope taught us that photochemical hazes are ubiquitous and form in a variety of temperature, gas composition and irradiation environments. Despite being crucial to understand their impact on observations and on the planetary radiative budget, the refractive indices of these haze particles are unknown and strongly influenced by changes in the gas phase chemistry. In this study, we perform a cross-laboratory investigation to assess the effect of the experimental set-up and gas composition on the refractive indices of Titan, Pluto and exoplanet haze analogs. We report new data in a broad spectral range from UV to far-IR (up to 200 microns) for future use in climate models and retrieval frameworks.

Figures (21)

• Figure 1: Images of the Titan haze analogs produced with the PAMPRE (top) and COSmIC (bottom) set-ups from a mixture of 95% N$_2$ and 5% CH$_4$. The analog organic material is deposited onto MgF$_2$ windows and silicon wafers.
• Figure 2: Transmission spectra obtained on the Titan 1 and Exoplanet 2 PAMPRE haze analogs (see Table \ref{['tab:samples']}) from 0.3 to 2.5 $\mu$m. (top) Data and analysis using the Swanepoel method for the Titan 1 sample produced from a gas mixture of 90% N$_2$ and 10% CH$_4$. (bottom) Data and best fit using Cauchy and Tauc-Lorentz dispersion laws for the Exoplanet 2 analog produced from 95% Ar and 5% CH$_4$ in the initial gas mixture.
• Figure 3: Reflection spectra obtained on the Titan 1 and Exoplanet 2 COSmIC haze analogs (see Table \ref{['tab:samples']}) from $\approx$ 0.4 to 1.67 $\mu$m. The data is fitted with both Tauc-Lorentz and Cauchy functions to determine the refractive indices.
• Figure 4: Ellipsometric data obtained in reflection configuration for the Titan 1 COSmIC haze analog (top panel) and Exoplanet 2 PAMPRE haze analog (bottom panel). The data are fitted to the theoretical model considering both Cauchy and Tauc-Lorentz parametrized functions for the dispersion of the refractive indices.
• Figure 5: Spectroscopic transmission spectra from 1.5 to 200 $\mu$m for the Titan 1 COSmIC haze analog (top) and the Exoplanet 2 PAMPRE haze analog (bottom). The simulated absorption-free transmission assuming a constant real refractive index n is shown (black curve). The film thickness is fitted to match the observed interference fringes in the NIR. The simulated absorption-free transmission spectrum now considering spectral variations of n in the IR (with SSKK model) is also shown (red curve).