A Photometric Study of the South Polar Terrain of Enceladus

• Main Author: Mamo, Bereket Daniel
• Format: Text
• Language: English
• Published: Cornerstone: A Collection of Scholarly and Creative Works for Minnesota State University, Mankato 2020
• Subjects: Bayesian inversion; Enceladus; Hapke model; photometry; Remote Sensing; The Sun and the Solar System; South Pole; South pole
• Online Access: https://cornerstone.lib.mnsu.edu/etds/1058; https://cornerstone.lib.mnsu.edu/cgi/viewcontent.cgi?article=2056&context=etds

Saturn's moon, Enceladus, is a unique body in the Solar System. It is an icy world with ongoing geologic activity that spews out jets of ice particles, water vapor, and other gases from active tectonic fractures (called sulci) near its south pole. Emission from these jets is the dominant source of particles for Saturn's diffuse E ring, which envelops other icy satellites. The south polar fractures are also sources of tidally generated high heat flow. Accurate determination of this endogenic thermal output requires knowledge of the thermophysical properties of the surface, which are reliably estimated by modeling the photometric behavior across a range of wavelengths. This study attempts to estimate fundamental photometric parameters of three geologic units within Enceladus' South Polar Terrain: funiscular plains, south polar reticulated plains (SPRP), and Baghdad sulcus flanks. Images acquired in the clear filter (λ=651 nm) by the Imaging Science Subsystem (ISS) onboard the Cassini spacecraft are used to solve for Hapke photometric model parameters under a Bayesian inversion framework. The angular scattering pattern of particles in geologic units ranged from strongly backscattering for Baghdad sulcus flanks, to moderately backscattering for funiscular plains, to strongly forward scattering for SPRP. The sulcus flank region has the lowest brightness with a single-scattering albedo w=0.667 ± 0.001 while the SPRP has the highest albedo value of one; the funiscular plains have intermediate albedo with w=0.795 ± 0.001 although this value is lower than previously reported albedo values for Enceladus. In addition, the macroscopic roughness parameter revealed the SPRP has the least roughness (θ =4.0°± 0.1°) as opposed to Baghdad sulcus (θ =21.1° ± 0.4°) and funiscular plains (θ =37.46° ± 0.09°). However, based on particle size distributions, the sulcus region would be expected have the largest macroscopic roughness. The data set used lacked observations near zero phase angle, so the opposition effect parameters could not be constrained. In addition, observations at phase angles above 90° are needed to better constrain the angular scattering and roughness parameters. The above uncertainty values are only statistical and do not include systematic errors, which are larger. Nonetheless, the results show that particles in the different geologic units are photometrically distinct.