Search

Clathrate hydrate early entered the field of space astrophysics as they were suggested to explain the anomalous desorption behaviour of gaseous parent molecules in comets by providing a dissociation pressure curve intermediate between the pure compound sublimation equilibrium and the water ice phase. It is nowadays the source for many astrophysical investigations with application ranging from explaining giant planets (and/or their satellites) observed molecular abundances, the possible occurrence of Mars carbon dioxide/methane clathrates or ethane sequestration on the Saturn's moon Titan. We experimentally investigate near to mid-infrared spectroscopic signature of clathrates in the low temperature range adapted to these icy bodies of our solar system. We discuss the implications for space-based astrophysical satellites or probes remote sensing in order to constrain their possible presence.

Clathrate hydrates, ice inclusion compounds, are of major importance for the Earth’s permafrost regions and may control the stability of gases in many astrophysical bodies such as the planets, comets and possibly interstellar grains. Their physical behavior may provide a trapping mechanism to modify the absolute and relative composition of icy bodies that could be the source of late-time injection of gaseous species in planetary atmospheres or hot cores. In this study, we provide and discuss laboratory-recorded infrared signatures of clathrate hydrates in the near to mid-infrared and the implications for space-based astrophysical tele-detection in order to constrain their possible presence.

Context. Recently, we recorded the infrared spectrum of the methane clathrate hydrate stretching mode at low temperature, a caged compound of possible interest for solar system studies as well as interstellar ice mantles.Aims. We provide a practical infrared spectroscopic identification for methane clathrate hydrate to examine its astrophysical presence or absence. We investigate the crystal field induced shifts, and assign the different transitions to the different encaged molecules environments in this clathrate hydrate.Methods. A methane clathrate crystal is produced in an infrared transmitting moderate-pressure closed cell. Using Fourier transform infrared (FTIR) spectroscopy, the overtones (3ν 4 , 2ν 3 ) and combination modes (ν 2 +2ν 4 , ν 1 +ν 4 , ν 3 +ν 4 , ν 2 +ν 3 , ν 3 +2ν 4 , 2ν 2 +2ν 4 , ν 2 +ν 3 +ν 4 ) falling in the 6000–3000 cm-1 (~1.65–3.4 μ m) and their temperature behaviour are investigated. In addition, non-astrophysical CH4 /CF4 gas mixtures are used to build clathrates with different methane large and small cage occupancies to help in assignments.Results. Combination modes show the two distinct cages and the quasi-free rotor low temperature ro-vibrational structure expected for methane clathrate hydrates. A comparison with the pure phase I is performed. Implications for methane clathrate hydrate detection are clearly identified.Conclusions. Solid methane actual remote observations of solar system objects surfaces do not display the clathrate hydrates' specific shift and occupancy signatures. Observationnally, a search for their infrared spectroscopic specific signatures should be performed, focusing on thermodynamically favourable objects like trans- neptunian objects (TNOs) or recently exposed (e.g. fresh impact) planet (or their satellites) surfaces. On the modeling side, efforts must be undertaken to progressively implement clathrate formation kinetics.