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Description
Sunlight that is scattered by a planetary surface is altered, meaning that asteroid regolith surfaces can be studied via light scattering.
The regolith surface properties that contribute to light scattering are its material, particle shape, particle size, and size distribution. As an inverse problem, the same properties can be derived for the surface from scattered light. If one of the properties is to be derived, the others need to be known. To tackle the inverse problem, the surface can be simulated in light scattering simulations, and parameters are adjusted until a match between the simulations and the observations is found.
The ultimate goal of the study is to create methods and a pipeline to invert the surface properties of atmosphereless planetary bodies. The study focuses on the near-Earth asteroid (3200) Phaethon and the derivation of its surface properties. Phaethon is shown to have regolith heterogeneity between its northern and southern hemispheres. If an accurate model for Phaethons surface can be constructed, a parameter space of surface properties can be found that explains whether the heterogeneity is caused by either surface material or size, thus shedding light on the processes that have caused it.
To construct a model for Phaethon, new research results are utilized, in which rare CY-meteorites have been proposed to have originated from Phaethon. CY-meteorites are borrowed from the Natural History Museum of London and measured at University of Helsinki light scattering laboratory. The idea is to create a model for the meteorite samples that replicates both spectral and polarization measurements. A presumption is then made that Phaethons surface material matches the CY-meteorites. Phaethons surface is then modeled using the meteorites as input for material, and from the models, the size of the regolith is derived.
The main minerals contributing to CY-meteorites light scattering are olivine and troilite. Olivine being abundant in the Earth's mantle it is well characterized, but troilite is commonly found only in meteorites. To simulate the meteorite samples, the optical properties of troilite need to be derived first. The spectra and polarization of crushed and sieved synthetic troilite samples are measured. Simulations are then done with varying refractive indices, and the correct values are interpreted from the best fit results between measurements and simulations.