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Light Scattering and Polarization of Warm Sub-Neptune Cloud Analogs in the Laboratory

Presentation #114.09 in the session Laboratory Investigations (Oral Presentation)

Published onOct 23, 2023
Light Scattering and Polarization of Warm Sub-Neptune Cloud Analogs in the Laboratory

Clouds and hazes appear to be ubiquitous features of exoplanetary atmospheres, and their presence has stark consequences on the light scattering properties and radiative balance of these atmospheres. Exoplanet clouds are typically inferred from muted transmission spectra, as optically thick clouds at the terminator keep photons from penetrating the atmosphere, leaving the exact cloud composition and structure of these unknown with current spectral observations. A laboratory-based understanding of how these cloud particles scatter light across particle size distributions, chemical compositions, and shapes is needed to make the most of current and future observations.

Here we present a distinct system for the study of exoplanet cloud scattering and polarization properties - The Exoplanet Cloud Ensemble Scattering System (ExCESS), an apparatus for the study of an ensemble of particles at cloud-relevant sizes and number densities. ExCESS illuminates particles with a polarized laser beam (currently 405 or 532 nm, with extension to new wavelengths) and uses a high temperature photomultiplier tube to sweep the plane of illumination from viewing angles of 20° - 169°. We will discuss the scattering phase functions and DOLP measurements for potassium chloride (KCl) particles at three distinct size distributions (̅ ~ 0.6−1.2 um) representative of modeled clouds for the warm (T ~ 500 K) exoplanet GJ 1214b. KCl particles are aerosolized with either a wet or dry generation method to produce cubic and cuboid/irregular shaped particles, respectively. We compare our laboratory findings to Lorenz-Mie calculations, widely used in current day exoplanet atmospheric models. While we conclude that Mie scattering offers an inaccurate depiction of irregular particle scattering, and that more work is needed to accurately simulate the scattering properties of irregular cloud particles, our measurements indicate that low backscattering and preferentially vertically polarized observations may be signs of non-spherical KCl cloud particles in warm exoplanets.

Finally, we will briefly discuss the extension of our measuring capabilities to the red and near infrared to provide data comparable to current and upcoming space telescopes, the future of this system, data availability plans, and how our findings may be used to help characterize exoplanet observations and synthetic spectra models.

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