Modeling the surface properties of asteroid (3200) Phaethon using CY-chondrite meteorites

The near-Earth asteroid (3200) Phaethon displays activity when it passes its perihelion at 0.14 au from the Sun. It is also thought to be linked to the Geminid meteor shower (e.g. [1], [2]). Phaethon has been hypothesized to be connected to, for example, the main-belt asteroid (2) Pallas. The two asteroids could be remnants of a common parent body or Phaethon could be an ejected piece of Pallas. Thermal models of Phaethon show heterogeneity between its northern and southern hemispheres either in the surface grain size or porosity, or both [3]. Recently, Yamato-type (CY) carbonaceous chondrite meteorites have been connected to Phaethon and the CY meteorites are now believed to be originated from the asteroid [4]. Using CY meteorites, a light-scattering model is created for the surface of Phaethon to study its surface properties and heterogeneity.

Samples of 6 different CY meteorites are studied at the University of Helsinki Astrophysical Scattering Laboratory. The ground samples reflectance spectra is measured using an integrating sphere spectrometer and linear polarization using a polarizing goniometer. The mineralogy of the samples has been studied with XRD by King et al. [5]. The samples contain mostly olivine and iron sulphide, and small amounts of pyroxene and metals. The iron sulphide is in the form of troilite. For modeling, the samples are simplified as olivine particles with troilite inclusions. A multiparticle media is then constructed of these particles. Light-scattering simulations with SIRIS (geometric optics with diffuse scatterers framework) [6], RT-CB (radiative transfer and coherent backscattering code) [7], and exact calculations based on the Maxwell Equations are combined to recreate the measured spectral and polarization properties for the samples. A simulation model of the meteorite sample should be able to replicate both the spectral and polarization measurements, and the simulation parameters are tuned until these requirements are met.

Simulating the multiparticle media follows a method described in Martikainen et al. [8]. First the light-scattering properties of troilite inclusions are obtained by simulating troilite as different sized single particles using SIRIS and exact calculations. The light-scattering properties of these particles are then averaged over a size distribution to produce values that describe an ensemble of different sized particles. These values are then used as an internal medium in an olivine single particle, modeled using SIRIS. Light-scattering properties of different sized olivine particles are then averaged over their respective size distribution. The values of the size averaged olivine particles are then used to construct a multiparticle regolith media. The regolith media simulations are run using both SIRIS and RT-CB. RT-CB takes coherent backscattering into account in the modeling, and is thus suited for polarization modeling. Spectra are modeled using SIRIS, which is computationally lighter.

Initially the troilite was replaced by iron in the models. Troilite is a rare mineral in Earth’s crust and mostly found in meteorites. Troilite’s light-scattering properties have thus not been properly characterized, and its refractive index not accurately derived. In Moreau et al. [9], troilite has been synthesized, and from these samples the refractive index of troilite will be derived in the 400 nm – 2500 nm region. To derive the refractive index, the same method from Martikainen et al. [8] will be used. Ground and sieved troilite’s reflectance spectra will be measured. The spectra will then be modeled using multiparticle media, with varying refractive index values to find a best match between measurements and simulations.

A model is created that explains the scattering properties of the CY meteorite samples. The model for the meteorites is then compared to the observations of Phaethon. Model values representing particle size and roughness, composition, and porosity are optimized against Phaethon’s spectra and polarization data, especially focusing on explaining the surface heterogeneity between the hemispheres. The model can also be used to study the connection between Phaethon and Pallas by fitting it to spectral and polarization data from Pallas.

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