Exploring Enceladus’ Plume in Polarization

From Cassini’s in situ and remote sensing characterizations, we know that Enceladus’ plume is made of primarily water vapor and ice particles (e.g. Waite et al. 2006, 2009, 2017; Postberg et al. 2011). Salts and organics make up the rest of the ice composition (Postberg et al. 2011, 2018, 2023; Khawaja et al. 2019), with salt-rich grains tending to be smaller than pure ice or organic-rich grains (Ershova et al 2024). Particles of different sizes are ejected at different velocities (Hedman et al. 2009; Sharma et al. 2023), and ejections vary in time and between the four tiger stripes (Hedman et al. 2013; Nimmo et al. 2014; Porco et al. 2014; Helfenstein & Porco 2015; Ingersoll & Ewald 2017; Ingersoll et al. 2020; Spitale et al. 2025). Variations in infrared spectra of ejections from different tiger stripes (e.g. Dhingra et al. 2017) suggest a difference in the grain composition and/or grain size. Enceladus’ plume is thus known to be a dynamic phenomenon, ultimately sourced from a subsurface ocean.

However, the nature of the connection between the ocean and the plume remains unknown. Whether the plume is fed directly or through some intermediary reservoir (e.g. Matson et al. 2012; Kite & Rubin 2016; Ingersoll & Nakajima 2016; Mitchell et al. 2024; Meyer et al. 2025) and whether the physical properties of conduits promote or inhibit fractionation (e.g. Neveu et al. 2024), for example, have critical implications for interpreting the more sensitive investigations into the organic content that would be made by return missions to Enceladus (e.g. Choblet et al. 2022; MacKenzie et al. 2023).

The plume particle size distribution is therefore an important parameter to understand as it is a function of how particles are formed and accelerated. To provide additional fodder for modeling plume ejection in the Cassini era, we investigate observations captured with the Imaging Science Subsystem (ISS) in visible color and polarization filters. We will compare these high phase observations to Mie scattering models for published plume particle size distributions (e.g. Gao et al. 2016, Porco 2018, Ershova et al. 2024), using three wavelengths corresponding to the effective central wavelengths of the UV3, GRN, and MT2 ISS filters.