Laboratory infrared reflection spectrum of carbon dioxide clathrate hydrates for astrophysical remote sensing applications

We present 1–5 μm IR reflectance spectra of CO2 clathrate hydrates acquired under temperature and pressure conditions representative of the icy moons’ surfaces. The IR reflectance spectrum of the CO2 clathrate hydrates is similar to the water ice IR reflectance spectrum except for two main absorption bands corresponding to the CO2 guest molecule at 2.71 and 4.28 μm (3693 and 2334 cm−1). The specific configuration of the clathrate hydrate structure is identified through the ν 3 absorption band splitting which produces a band at 4.26 μm (2347 cm−1) for molecules trapped in small cages and a band at 4.28 μm (2334 cm−1) for molecules trapped in large cages. In general, the reflection spectra are similar to spectra obtained in transmission spectroscopy. But, it appears that the aspect of the ν 3 absorption band is strongly influenced by physical (roughness, thickness, mixing properties) and optical (n and k) characteristics of the sample. A qualitative discussion of the effects of these sample properties on near-IR signatures of clathrate hydrates is proposed. Finally, a comparison between the absorption bands of CO2 clathrate hydrates obtained in this work and CO2 absorption bands as detected by VIMS on the icy satellites of Saturn is achieved. The experimental near-IR reflection spectra, made in pressure–temperature (P–T) conditions close to those of the icy surfaces, confirm that VIMS data are not consistent with the presence of structure I CO2 clathrate hydrates on the surface of the icy moons.