Investigating sub-resolution surface properties of comet 67p/churyumov- gerasimenko from optical photometry and hapke modeling

Introduction: Observations from the Rosetta spacecraft’s Optical, Spectroscopic, and Infrared Imaging System (OSIRIS) Narrow Angle Camera (NAC) of Comet 67P/Churyumov-Gerasimenko (67P) have provided the high resolution images of a cometary surface at multiple observation geometries. The dataset has revealed that the surface is dominated by smooth plains, cauliflower plains, pitted plain, and consolidated terrains [1-3]. In this study, we present multispectral phase curves between 0° and 160° in 10 filters for type examples of each primarily morphologic unit. We fit these phase curves to Hapke, (2012) photometric functions [4] to derive photometric properties including single scattering albedo and macroscopic roughness. Results are compared to previously published best-fit Hapke coefficients of disk-integrated reflectance [5,7]. Our results are consistent with the qualitative interpretation that the smooth plains are the photogeologically smoothest of the four regions with the highest albedo and the consolidated terrains are the photogeologically roughest terrains with the lowest albedo (hereafter “smooth” and “rough” terrains respectively). Macroscopic roughness tends to be correlated with other photometric properties in each terrain. Aspects including parameter trends across wavelengths, typeexamples within single morphologies, between different terrain types, and against disk-integrated models are indicative of variable levels of erosional and active processes. We discuss these results in the context of their importance to the surface properties of 67P. Method: Rosetta’s OSIRIS NAC images, taken in wavelengths ranging from ~200-1000 nm, revealed a diverse and evolving landscape across the surface of the comet, showing two broad terrain categories: smooth terrains and rough terrains [1]. Smooth terrains consist of small, granular material that is hypothesized to have been previously consolidated bedrock. These terrains include smooth, pitted, and cauliflower plains. In contrast, consolidated terrains (low reflectance [5,7]) consist of the exposed comet nuclei, or bedrock, including cliff terrains [1]. We have chosen four morphologies, as defined by [1], for this study and aim to quantitatively distinguish between them. Three of the four representative sites that were chosen are examples of smooth terrain: smooth plains in the Imhotep region, cauliflower plains in the Ash region and pitted plains in the Ma’at region. The fourth site is an example of consolidated terrain, located in the Agilkia (or Abydos) region. We use the SPICE toolkit from NAIF (Navigation and Ancillary Information Facility, [6]) to generate navigation information for each pixel and determine the relevant incidence, emission, and phase angles. For each of the selected sites, we chose a representative image and manually selected a region of interest. The navigation information for all of the pixels within each region of interest (<1 m pixel scale were used to create the phase curves (I/F radiance factors vs. phase angle). Resulting phase functions were also compared with disk integrated models derived by [7, 8]. We utilized the reflectance function from [4] that includes macroscopic roughness to fit our phase curves and in doing so, determine our best fit “Hapke parameters”: single scattering albedo ( ), an asymmetry pa-