Uncertainty in Grain Size Estimations of Volatiles on Trans-Neptunian Objects (TNOs) and Kuiper Belt Objects (KBOs)

Abstract: We analyze the uncertainty in grain size estimation of pure methane (CH4) and nitrogen saturated with methane (N2:CH4) ices, the most abundant volatile materials on trans-Neptunian objects (TNOs) and Kuiper belt objects (KBOs). We compare the single scattering albedo, which determines the grain size estimation of outer solar system regolith (Hansen, 2009), of these ices using the Mie scattering model and two other Hapke approximations (Hapke, 1993) in radiative transfer scattering models (RTM) at near-infrared (NIR) wavelengths (1-5 $\mu$m). The equivalent slab (Hapke Slab) approximation model predicts results much closer to Mie scattering over the NIR wavelengths at a wide range of grain sizes. In contrast, even though the internal scattering model (ISM) predicts an approximate particle diameter close to the Mie model for particles with a 10 $\mu$m radii, it exhibits higher discrepancies in the predicted estimation for larger grain sizes (e.g., 100 and 1000 $\mu$m radii). Owing to the Rayleigh effect on single-scattering properties, neither Hapke approximate models could predict an accurate grain size estimation for the small particles (radii $\leq$ 5 $\mu$m). We recommend that future studies should favor the equivalent slab approximation when employing RTMs for estimating grain sizes of the vast number of TNOs and KBOs in the outer solar system.