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Kernel Density Estimation of the Distribution of Asteroid Radar Polarization Ratios as a Function of Taxonomy

Presentation #506.06 in the session Asteroids: Near-Earth Objects (Oral Presentation)

Published onOct 23, 2023
Kernel Density Estimation of the Distribution of Asteroid Radar Polarization Ratios as a Function of Taxonomy

Disk integrated S-band (12.6 cm, 2380 MHz) circular polarization ratios (CPRs) of near-Earth asteroids (NEAs) may be associated with visible-infrared taxonomic class [1]. Because CPR is a zeroth-order approximation of near-surface, wavelength-scale complexity, variations in CPR between taxonomic classes may indicate differences in regolith properties. Here, we revisited earlier work [1] by nearly doubling the number of asteroids used for analysis with Arecibo radar observations over the past decade [2]. Additionally, we furthered previous studies by approximating the CPR frequency distribution as a function of taxonomic class using Kernel Density Estimation (KDE).

In [1], 110 NEAs with published taxonomic classes and measured CPR values were used. Here, we included an additional 91 asteroids. Visible-infrared taxonomic class was obtained from the MIT-Hawaii Near-Earth Object Spectroscopic Survey [3]. CPR measurements were obtained at the Arecibo Observatory via continuous wave experiments. Probability Density Functions (PDFs) were developed by representing each CPR measurement by a Gaussian distribution with a bandwidth given by its uncertainty and summing these by taxonomy. Sensitivity studies for features of the PDF were conducted by varying the bandwidth of each Gaussian up to 4-σ.

We found that, with the exception of E-type asteroids, the PDFs are non-Gaussian. In particular, V and X types are well characterized by bimodal distributions that are preserved at the 4-σ level. The V types have two peaks centered at CPR = 0.19 and 0.56 and the X types at 0.21 and 1.0. The bimodal nature of the V types likely results from the confusion between V and S types, while for X types, it is likely because the complex is a grouping of M, P, and E types.

Additionally, we found that, given only radar measurements, NEAs with CPR > 0.7 are more likely E types and CPR > 1 are confidently E types. Additionally, NEAs with CPR < 0.1 are more likely S types. Although at the 1-σ level, NEAs with 0.53 < CPR < 0.7 are more likely V types, at the 2-σ level the range falls to 0.63 < CPR < 0.7, and at the 3-σ level V-types are not distinguishable based on their CPR. Overall, for 0.1 < CPR < 0.53 radar measurements alone are not diagnostic of taxonomy. Our work supports [1] in that near-surface, wavelength-scale physical properties may vary with some taxonomic class. Further analysis, including laboratory work into radar scattering, is needed to help resolve the reason for this potential association.

[1] Benner et al. (2008) Icarus 198, 294-304.

[2] Virkki et al. (2022) PSJ 3:222.

[3] Binzel et al. (2019) Icarus 324, 41-76.

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