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

Previous observations of Titan, Pluto, and Solar System gas giants, along with recent observations of exoplanet atmospheres with the James Webb Space Telescope, have taught us that photochemical hazes are ubiquitous and form in a variety of temperature, gas composition, and irradiation environments. Despite their crucial role in understanding their impact on observations and on the planetary radiative budget, the refractive indices of these haze particles remain unknown and are strongly influenced by changes in gas-phase chemistry. In this study, we performed a cross-laboratory investigation to assess the effect of the experimental setup 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 μm) for future use in climate models and retrieval frameworks. We compare the refractive indices of laboratory haze analogs produced from six different gas compositions, in which we varied the relative abundances N2/CH4 and CH4/CO in the initial gas mixture, using the PAMPRE (LATMOS, France) and COSmIC (NASA Ames Research Center, USA) experimental setups. We observed strong variations in the k values in the spectral range from UV to near-IR between the different analogs, which are caused by both the experimental setup and changes in the gas N2/CH4 ratio. We find that the gas N2/CH4 ratio has a stronger influence on the haze refractive indices in the entire spectral range compared to the gas CH4/CO ratio. The experimental setup is the primary factor affecting the refractive indices, confirming that the gas residence time, irradiation, pressure, and gas temperature are important parameters influencing the composition of the solid analog. The higher n and k values in the UV-visible range, along with the stronger amine, alkene, aromatic, and/or hetero-aromatic signatures in the mid-IR for the COSmIC analogs, are consistent with a greater incorporation of nitrogen into the COSmIC solid analogs compared to the PAMPRE analogs, even at similar nitrogen abundances in the gas phase. Haze analogs produced in gas mixtures without nitrogen, similar to the stratospheres of Solar System gas giants and the H2-dominated atmospheres of sub-Neptunes, are generally more transparent with lower n values across the entire spectral range from UV to mid-IR and should therefore be carefully considered in climate and observational applications. The variations in IR absorption features between hazes produced with and without nitrogen could help constrain the presence of N2 in exoplanet atmospheres.