Exoplanetology: Exoplanets & Exomoons

Light Scattering Measurements of KCl Particles as an Exoplanet Cloud Analog

By Keith Cowing
Status Report
astro-ph.EP
November 5, 2024
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Light Scattering Measurements of KCl Particles as an Exoplanet Cloud Analog
Size distributions and shapes of the three KCl cloud analogs used in this study. (a) Size distribution of the cloud analogs used here. The mean particle radii and total concentrations are provided in the legend. Our small KCl distribution is produced through wet generation (red), which creates a narrow size distribution. The medium (solid blue) and large (dashed blue) KCl distributions are produced through dry generation, which creates broader size distributions with larger particle number densities. (b) and (c) Scanning electron microscope images of small cubic KCl particles produced using wet generation and large cuboid and irregularly shaped KCl particles produced using dry generation, respectively. — astro-ph.EP

Salt clouds are predicted to be common on warm exoplanets, but their optical properties are uncertain. The Exoplanet Cloud Ensemble Scattering System (ExCESS), a new apparatus to measure the scattering intensity and degree of linear polarization (DOLP) for an ensemble of particles, is introduced here and used to study the light scattering properties of KCl cloud analogs.

ExCESS illuminates particles with a polarized laser beam (532 nm) and uses a photomultiplier tube detector to sweep the plane of illumination. Scattering measurements for KCl particles were collected for three size distributions representative of modeled clouds for the warm exoplanet GJ 1214b.

Our measurements show that Lorenz-Mie calculations, commonly used to estimate the light scattering properties of assumedly spherical cloud particles, offer an inaccurate depiction of cubic and cuboid shaped KCl particles.

All of our measurements indicate that Lorenz-Mie scattering overestimates the backscattering intensity of our cloud analogs and incorrectly predicts the scattering at mid-phase angles (~90 degrees) and preferential polarization state of KCl scattered light.

Our results align with the general scattering properties of non-spherical particles and underscore the importance of further understanding the effects that such particles will have on radiative transfer models of exoplanet atmospheres and reflected light observations of exoplanets by the upcoming Nancy Grace Roman Space Telescope and Habitable Worlds Observatory.

A top-down diagram and corresponding picture of ExCESS. Laser light is passed through an EOM that changes, and rapidly alternates, the polarization state of light before it is directed to the scattering region, where it impinges upon our cloud analog sample. An ND filter mitigates the intensity of scattered light entering the PMT detector. The PMT is mounted onto a stage that rotates around the scattering region where particles are flown in from above (into the page). ExCESS is a compact scattering system, contained within an 18’ × 24’ dark box, that collects measurements from 20° to 169°. — astro-ph.EP

Colin D. Hamill, Alexandria V. Johnson, Peter Gao

Comments: Published in the Planetary Science Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2411.00952 [astro-ph.EP] (or arXiv:2411.00952v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2411.00952
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Journal reference: 2024, Planet Sci J, 5, 186
Related DOI:
https://doi.org/10.3847/PSJ/ad6569
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Submission history
From: Colin Hamill
[v1] Fri, 1 Nov 2024 18:05:39 UTC (800 KB)
https://arxiv.org/abs/2411.00952
Astrobiology

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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