Refractive indices of Titan, Pluto and Exoplanet photochemical haze analogs from UV to far-IR : a comparative study between the PAMPRE and COSmIC experimental setups

The observations of planetary atmospheres and surfaces strongly rely on the use of experimental data to understand the interaction between light and particles. The intrinsic optical properties of these particles, also known as the refractive indices, describing light dispersion (i.e., n) and absorption (i.e., k), are required to consider the influence of their chemical composition. Only with these data can we interpret observations and avoid large degeneracy in the retrieval data analyses. Climate modeling is also strongly sensitive to these experimental data as atmospheric particles absorb and scatter radiations, and thus modify the temperature profile.

Photochemical hazes are observed in the atmospheres of the different objects in the outer Solar System (Titan, Pluto, Triton, Jupiter, Saturn) as well as in exoplanet atmospheres (e.g., GJ1214 b). The observations of these different objects as well as the modeling suggest that the composition of photochemical hazes varies from one object to the next following changes in the irradiation efficiency, temperature, pressure and gas composition. These differences in composition suggest that the refractive indices of photochemical hazes are also function of pressure, temperature, irradiation efficiency and gas composition. In the present work, we produced laboratory analogs of these hazes from various gas mixtures with controlled abundances. We used 6 different gas compositions to mimic the atmospheric compositions of Titan, Pluto and exoplanets. Among the 6 conditions, we modified the N2/CH4 and CO/CH4 abundance ratios in the gas mixture to assess the influence of N2 and CO on the refractive indices of the haze material. In addition, we compared the influence of the experimental setup by using haze analogs produced with the PAMPRE (LATMOS, France) and COSmIC (NASA Ames, USA) experimental setups. This cross-laboratory comparison allows us to assess the influence of temperature, pressure, gas residence time and irradiation which are changing between PAMPRE and COSmIC. Using several optical measurements, we covered a broad spectral range from UV to far-IR (up to 200 microns) which is essential for climate calculations and to interpret the various remote-sensing observations of these planetary bodies.

Our results revealed a significant difference between the refractive indices obtained on PAMPRE and COSmIC analogs, even for similar gas compositions. Based on previous elemental analyses of the haze analogs, we know that the COSmIC analogs are richer in nitrogen relative to carbon compared to the PAMPRE analogs.  Here, we show that this difference in composition leads to higher n and k values for the COSmlC analogs in the entire spectral range. We found that changes in the CO/CH4 gas ratio have a rather poor influence on the refractive indices compared to the effect of the N2/CH4 ratio. We also found that hazes produced without nitrogen are more transparent in the entire spectral range from UV to IR with very different mid-IR absorption features that could help distinguish between N-rich and N-poor exoplanet atmospheres. These data should be used for future observational analyses and modeling simulations of sub-Neptune atmospheres and Solar System gas giants.