Refractive indices of photochemical haze analogs relevant for Solar System and exoplanetary objects

Introduction

Photochemical hazes are solid particles produced in the upper layers of planetary atmospheres. They are observed in Solar System objects (Titan, Pluto, Triton, Gas Giant planets) as well as in exoplanet atmospheres. After their formation, these small spherical particles agglomerate forming fractal aerosols which later sediment at the surface. They strongly impact observations of planetary atmospheres and surfaces, their intrinsic optical properties are essential data used to interpret observations as well as predict their influence on the climate.

The composition of these photochemical aerosols is unknown, it however dictates the intrinsic optical properties, also known as refractive indices or optical constants, which are the main data needed to compute their overall radiative effect in different types of models. Laboratory setups were developed since the 1980s to produce analogs of these aerorols and directly measure their refractive indices [1,2,3,4].

Aim

In the era of the James Webb Space Telescope (JWST) and in preparation of future missions dedicated to atmospheres of exoplanets (ARIEL) and Solar System objects (e.g. Dragonfly), refractive indices of laboratory haze analogs are needed for a variety of gas compositions, in a broad spectral range. Previous data were usually limited to a narrow spectral range making their use in climate calculations difficult. Previous work also revealed that the various experimental conditions changing from one setup to another affect the composition of haze analogs and thus their refractive indices [5].

In this work, we aim to improve our understanding on the link between gas composition and refractive indices along with the influence of the experimental setup. As part of a new collaboration between LATMOS (France) and NASA Ames (US), we produced analog samples from similar gas compositions but using different setups. We produced analogs from gas compositions mimicking the atmosphere of Titan (N2-CH4) and Pluto (N2-CH4-CO) along with N-poor atmospheres relevant for Solar System gas giants and exoplanet atmospheres. Using these different gas compositions and both experimental setups, we aim to better understand the influence of nitrogen and carbon monoxide in the optical properties of these solid organic materials.

Methods

We use different measurements to retrieve the refractive indices in a broad spectral ranging from UV to far-IR. We use reflection ellipsometry along with transmission and reflection spectroscopy to derive the refractive indices from UV to near-IR. Fourier-Transform (FT) transmission spectroscopy measurements were performed at Synchrotron Soleil (France) to derive the refractive indices from near-IR to far-IR (up to 200 microns). Different models were developed and validated to assess the accuracy of the retrieved refractive indices.

Results

For an analog produced with similar gas composition but with different setups, we found significant variations in the k values in the UV-visible spectral range. This suggests an important change in composition between the different samples caused by the experimental conditions (residence time of the gas, irradiation efficiency, temperature). Given that the upper layers of planetary atmospheres are heated from the absorption of stellar radiation by these photochemical hazes, the differences revealed in these k values should be considered. In the mid-IR, the signature strength of amine groups (-NH, -NH2) relative to aliphatics (-CH2, -CH3) is affected by the experimental setup and the methane concentration (relative to N2). The observations of these signatures in photochemical hazes of Solar System and exoplanetary objects might help us understand their composition and formation mechanisms.

During the presentation, we will further discuss the refractive indices of Titan, Pluto and exoplanet analogs. We will also highlight the impact of these refractive indices in radiative parameters controlling the observations and the climate of planetary bodies (e.g. absorption coefficient and single-scattering albedo). We will discuss implications for the re-analysis of the Titan VIMS observations acquired during the Cassini-Huygens mission. We will discuss implications for observations of hazy exoplanet atmospheres with JWST.

References

[1] Khare, B.N., Sagan, C., Arakawa, E.T., et al. 1984, Icarus, 60, 127-137

[2] Imanaka, H., Cruikshank, D.P., Khare, B.N., McKay, C.P. 2012, Icarus, 218, 247–261

[3] Sciamma-O’Brien, E., Dahoo, P.-R., et al. 2012, Icarus, 218, 356–363

[4] He, C., Hörst, S.M., Radke, M., Yant, M. 2022, Planetary Science Journal, 3

[5] Brassé, C., Muñoz, O., Coll, P., Raulin, F. 2015, Planetary and Space Science, 109-110, 159–174