Abstract
Visual-to-infrared (VIS-IR) remote sensing observations of different classes of outer solar system objects indicate the presence of water ice and organics. Here, we present laboratory reflectance spectra in the 0.5–4.2 μm spectral range of binary particulate mixtures of water ice, organics analogue (kerite), and an opaque iron sulphide phase (pyrrhotite) to investigate the spectral effects of varying mixing ratios, endmember grain size, and mixing modality. The laboratory spectra are also compared to different implementations of the Hapke reflectance model (Hapke, 2012). We find that minor amounts (≲1 wt%) of kerite (investigated grain sizes of 45–63 μm and <25 μm) can remain undetected when mixed in coarse-grained (67 ± 31 μm) water ice, suggesting that organics similar to meteoritic insoluble organic matter (IOM) might be characterized by larger detectability thresholds. Additionally, our measurements indicate that the VIS absolute reflectance of water ice-containing mixtures is not necessarily monotonically linked to water ice abundance. The latter is better constrained by spectral indicators such as the band depths of water ice VIS-IR diagnostic absorptions and spectral slopes. Simulation of laboratory spectra of intimate mixtures with a semi-empirical formulation of the Hapke model suggests that simplistic assumptions on the endmember grain size distribution and shape may lead to estimated mixing ratios considerably offset from the nominal values. Finally, laboratory spectra of water ice grains with fine-grained pyrrhotite inclusions (intraparticle mixture) have been positively compared with a modified version of the Hapke model from Lucey and Riner (2011).
Highlights
IntroductionWidespread water ice and organic material have been identified on the surface of the asteroids (24) Themis and (65) Cybele from ground-based IR observations [17,18,19]
It can be noted that for the kerite45–63 intimate mixtures, the modelled spectra reproduce the measured ones fairly well, with normalized root mean square deviation (NRMSD) ≤10%, and a reasonable accuracy is achieved for the kerite
The results of our measurements on binary mixtures including kerite and/or water ice powders provide certain insights into the possibility to characterize the abundance and physical properties of water ice and organics from remote sensing observations of planetary surfaces and spectral modelling, which we enumerate and discuss below: 1. We find that minor amounts ( 1 wt%) of aliphatic organics can remain undetected in mixtures with coarse-grained water ice if searched by means of the diagnostic absorption features around 3.4 μm
Summary
Widespread water ice and organic material have been identified on the surface of the asteroids (24) Themis and (65) Cybele from ground-based IR observations [17,18,19]. Mixed with water ice dominating the surfaces of the icy moons and rings of Saturn [23], organic compounds have been identified by means of VIS-IR observations on Iapetus, Phoebe, and Hyperion [24] and references therein, possibly in the C ring [25,26] and Cassini Division [26], and suggested as UV-absorbers for the major moons [23,27] and rings [28,29,30].
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