Revealing regolith properties of the nucleus of comet 67P and other small bodies using computer modeling of their brightness and polarization with the Plane Wave Plane Parallel (PWPP) technique

Our study is based on a new technique that allows revealing physical properties of regolithic surfaces of cosmic bodies, e.g., asteroids and cometary nuclei, based on their photopolarimetric characteristics. The technique is called Plane Wave Plane Parallel (PWPP) technique and enables computer modeling of light scattering by layers of randomly distributed arbitrary particles (Mackowski, JQSRT, 213, 2018). Similar to the Discrete Dipole Approximation, DDA (Draine and Flatau, JOSA, A11, 1994), the PWPP represents a volume-discretized numerical approximation to the solution of Maxwell’s wave equations but is applied to an infinite layer of particulate material of a given thickness. By using Fourier expansion, the mathematical formulation in PWPP is considerably simpler and more compact than that derived using the DDA, and the method obviates any need to define or calculate dipole polarizability. This results in significantly accelerated solution times when applied to densely packed systems of particles. We show examples of some PWPP applications, specifically modeling phase curves of brightness and polarization typical for small planets and moons. One case, the opposition effect observed by Rosetta OSIRIS instrument for the nucleus of comet 67P/Churyumov-Gerasimenko, will be considered in more detail. Fitting the observational data with the computations allowed estimations of the cometary regolith particle size and composition as well as the porosity of the surface layer. We found that the best fit to the characteristics of the 67P opposition peak, such as the peak amplitude and width (expressed though HWHM) as well as the ratio of the brightness at the phase angles 0 and 5 deg., is provided by a mixture of 30% of silicates (forsterite) with 70% of IOM (Insolvable Organic Matter); the size of particles is between 0.1 and 0.4 micron, and the porosity of the layer is about 85%.

This work has been supported by the NASA SSW grant #80NSSC17K0731.