Discrepancy in Grain Size Estimation of H${}_2$O Ice in the Outer Solar System and the Interstellar Medium

Abstract

Widespread detection of amorphous and crystalline water (H${}_2$O) ice in the outer solar system bodies and the interstellar medium has been confirmed over the past decades. Radiative transfer models (RTMs) are used to estimate the grain sizes of H${}_2$O ice from near-infrared (NIR) wavelengths. Wide discrepancies in the estimation of H${}_2$O ice grain size on the Saturnian moons (Hansen, 2009), as well as nitrogen (N${}_2$) and methane (CH${}_4$) ices on Kuiper belt objects have been reported owing to different scattering models used (Emran and Chevrier, 2022). We assess the discrepancy in the grain size estimation of H${}_2$O ice at a temperature of 15, 40, 60, and 80 K (amorphous) and 20, 40, 60, and 80 K (crystalline) - relevant to the outer solar system and beyond. We compare the single scattering albedos of H${}_2$O ice phases using the Mie theory (Mie, 1908) and Hapke approximation models (Hapke, 1993) from the optical constant at NIR wavelengths (1 - 5 $\mu$m). This study reveals that the Hapke approximation models - Hapke slab and internal scattering model (ISM) - predict grain size of the crystalline phase, overall, much better compared to the amorphous phase at temperatures of 15 - 80 K. However, the Hapke slab model estimates much approximate grain sizes, in general, to that of the Mie model's prediction while ISM exhibits a higher uncertainty. We recommend using the Mie model for unknown spectra of outer solar system bodies and beyond in estimating H${}_2$O ice grain sizes. While choosing the approximation model for employing RTMs, we recommend using a Hapke slab approximation model over the internal scattering model.

arXiv comment15 pages, 4 figures, 2 tables

References in corpus (9)

• SciPy 1.0--Fundamental Algorithms for Scientific Computing in Python
• Array Programming with NumPy
• A new method to determine the grain size of planetary regolith
• Pluto's global surface composition through pixel-by-pixel Hapke modeling of New Horizons Ralph/LEISA data
• Saturn's icy satellites and rings investigated by Cassini - VIMS. III. Radial compositional variability
• Methane and Nitrogen Abundances On Pluto and Eris
• The temporal evolution of exposed water ice-rich areas on the surface of 67P/Churyumov-Gerasimenko: spectral analysis
• Compositional Constraints for Lucy Mission Trojan Asteroids via Near-Infrared Spectroscopy
• Uncertainty in Grain Size Estimations of Volatiles on Trans-Neptunian Objects (TNOs) and Kuiper Belt Objects (KBOs)

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