Clathrate hydrates in interstellar environment

Abstract

Clathrate hydrates (CHs) are ubiquitous in earth under high-pressure conditions, but their existence in the interstellar medium (ISM) remains unknown. Here, we report experimental observations of the formation of methane and carbon dioxide hydrates in an environment analogous to ISM. Thermal treatment of solid methane and carbon dioxide-water mixture in ultrahigh vacuum of the order of 10^-10 mbar for extended periods led to the formation of CHs at 30 and 10 K, respectively. High molecular mobility and H bonding play important roles in the entrapment of gases in the in situ formed 5¹² CH cages. This finding implies that CHs can exist in extreme low-pressure environments present in the ISM. These hydrates in ISM, subjected to various chemical processes, may act as sources for relevant prebiotic molecules.

Keywords: ISM; amorphous solid water; clathrate hydrate; interstellar medium; ultra-high vacuum.

Fig. 1.

CH₄ hydrate formation as studied by RAIR spectroscopy and quantum chemical calculations. (A) Normalized time-dependent RAIR spectra of 300 MLs CH₄+H₂O (1:1) mixed ice at 10, 20, and 30 K at the C–H antisymmetric stretching region. (B) Time-dependent RAIR spectra of the same system at 30 K. Here, the blue trace was divided by a factor of 7 to match the intensity of orange trace. Difference in intensity is due to desorption of CH₄ at 30 K, near its desorption temerature. Deconvoluted IR peaks are shown by cyan (3,009 cm⁻¹) and pink shade (3,017 cm⁻¹). (C) DFT-optimized structure of CH₄ trapped within CH (5¹² cage). Here, water cage and guest molecule (CH₄) are shown. Color code used: gray, C; red, O; cyan, H.

Fig. 2.

CO₂ hydrate formation as studied by RAIR spectroscopy and quantum mechanical calculations. (A) Normalized temperature dependent RAIR spectra of 300 MLs CO₂+H₂O (1:5) mixed ice at C = O antisymmetric stretching region. A new peak at 2,346 cm⁻¹ arises due to the formation of CO₂ hydrate. (B) Ratio-dependent RAIR spectra of 300 MLs CO₂+H₂O at 10 K (normalized). (C) DFT-optimized structure of CO₂ trapped inside CH (5¹² cage). Here, water cage and guest molecule (CO₂) are shown. Color code used: gray, C; red, O; cyan, H.

Fig. 3.

TPD mass spectra of 300 MLs of codeposited ice systems at different ratio (heating rate = 30 K ⋅ min⁻¹). Here, the intensities of CH₃⁺ (m/z = 15), and CO₂⁺ (m/z = 44) are plotted. (A) Desorption of CH₄ after hydrate formation (magenta line) and before hydrate formation (blue line). MV peaks are shown in the Insets. *, peaks are attributed to desorption due to structural transitions of ASW upon annealing. (B) Desorption of CO₂ after hydrate formation at different ratios, as indicated. #, the peak is due to the predissociation of CO₂ hydrate cage. (C) Schematic representation of MV upon crystallization of ice.