Letter to the Editor

Methane clathrate hydrate FTIR spectrum

Implications for its cometary and planetary detection

Institut d'Astrophysique Spatiale, UMR-8617, Université Paris-Sud, Bât. 121, 91405 Orsay, France e-mail: emmanuel.dartois@ias.u-psud.fr

Received: 6 September 2008
Accepted: 16 September 2008

Abstract

Context. The physical behaviour of methane clathrate hydrate, a crystallographic ice crystal is of major importance for both the earth and the stability of gases in many astrophysical bodies (planets, comets, etc.).

Aims. We provide an infrared spectroscopic identification for astrophysical methane clathrate hydrates and investigate the crystal field experienced by the trapped molecule.

Methods. A methane clathrate crystal was produced in a moderate-pressure optical cell. Using FTIR spectroscopy, the asymmetric CH-stretching mode of the entrapped methane molecule is recorded from 7 K to 80 K, then back to 7 K.

Results. It is shown that the trapped methane molecules in the clathrate hydrate is a quasi rotor, displaying gaseous behaviour at low temperatures. A series of ro-vibrational specific lines is observed, shifted in frequency by the water-ice cage interactions with the trapped methane molecules. Because these transitions are unique to methane clathrate hydrate, they represent a crucial identification pattern for astrophysical icy bodies at low temperatures, such as comets and/or interstellar grains.