Scattering Matrices of Single Levitated Particles for Earth and Beyond

By observing the planets and moons in our solar system it is clear that clouds are a ubiquitous atmospheric feature. Furthermore, from Jupiter’s raging red spot to Venus’ persistent sulfuric acid clouds, Terrestrial based cloud microphysics has helped expand our knowledge of extreme and exotic atmospheres while simultaneously advancing our understanding of Earth’s own atmosphere.

The work presented here builds on this theme of extrospective insight by investigating how particulates representative of exoplanet clouds, or clouds in the atmospheres of planets orbiting other stars, interact with radiation. Although a main goal is to aid in the detection of exoplanet spectroscopic signatures it is a means by which to tackle major gaps that remain in our understanding of how non-spherical particles scatter and polarize incident radiation. Modeling the highly complex scattering phase functions produced by non-spherical particles is challenging, and broad stroke simplification of particulate properties, such as with an oblate spheroid approximation, can lead to large uncertainties in numerical model outputs and satellite retrieval corrections for any planetary body.

For this study we developed a single particle scatterometer to directly measure the scattering phase functions of non-spherical particles. Our novel instrument consists of a vapor diffusion electrodynamic balance (EDB), used to levitate single particles in a precisely controlled atmospheric environment, coupled with a photomultiplier for the collection of scattered light as a function of angle along the plane of illumination. We will discuss the development of our EDB scatterometer, validation experiments with spherical particles, and preliminary results on non-spherical particles. For spherical validation experiments we will compare our results to Mie theory and use this to address any biases in our measurements. Although single particle studies allow for the precise control of a sample and its environment, their relevance to atmospheres and the complexity therein can be questioned. To address this we aim to discuss the relationship between single particle and bulk sample (or particle ensemble) non-spherical scattering phase functions through comparison with data from the Amsterdam-Granada Light Scattering Database.

Future work will be directed towards expanding the system to include the collection of polarized light, with the ultimate goal of retrieving Muller matrix elements for a complete description of scattering on the single particle basis. We also aim to expand the temperature regime of the EDB to mimic other solar system bodies and ‘cooler’ exoplanets with estimated upper atmospheric temperatures in excess of 400K.