Ammonia clathrate hydrates as new solid phases for Titan, Enceladus, and other planetary systems

Abstract

There is interest in the role of ammonia on Saturn's moons Titan and Enceladus as the presence of water, methane, and ammonia under temperature and pressure conditions of the surface and interior make these moons rich environments for the study of phases formed by these materials. Ammonia is known to form solid hemi-, mono-, and dihydrate crystal phases under conditions consistent with the surface of Titan and Enceladus, but has also been assigned a role as water-ice antifreeze and methane hydrate inhibitor which is thought to contribute to the outgassing of methane clathrate hydrates into these moons' atmospheres. Here we show, through direct synthesis from solution and vapor deposition experiments under conditions consistent with extraterrestrial planetary atmospheres, that ammonia forms clathrate hydrates and participates synergistically in clathrate hydrate formation in the presence of methane gas at low temperatures. The binary structure II tetrahydrofuran + ammonia, structure I ammonia, and binary structure I ammonia + methane clathrate hydrate phases synthesized have been characterized by X-ray diffraction, molecular dynamics simulation, and Raman spectroscopy methods.

Fig. 1.

The large and small cages of the cubic structure II THF-ammonia binary clathrate hydrate from single crystal X-ray diffraction. The ammonia guest has moved a water molecule out of its normal position by pulling it into the small cavity by forming a H₂N-H⋯OH₂ or H₃N⋯HOH hydrogen bond.

Fig. 2.

PXRD patterns of NH₃ and H₂O co-deposition sample (A) recorded at 100, 140, and 150 K; (B) expansion of the pattern at 150 K. Lattice parameters (150 K) are: a = 0.4527(1) nm, b = 0.5586(1) nm, c = 0.9767(3) nm for ammonia monohydrate (space group: P2₁2₁2₁); a = 0.4496(1) nm and c = 0.7339(1) nm for ice Ih (space group P6₃/mmc); a = 1.1818(2) nm for sI ammonia clathrate hydrate (space group Pm-3n); a = 0.8404(2) nm, b = 0.8441(3) nm, and c = 5.345(1) nm for ammonia hemihydrate (space group Pbnm); and a = 0.7125(1) nm for ammonia dihydrate (space group P2₁3).

Fig. 3.

PXRD patterns of NH₃, CH₄, and H₂O co-deposition sample (A) recorded at 112, 143, 160, and 180 K; (B) expansion of the pattern at 160 K. Vertical thin red lines and arrows indicate the peaks from sII hydrate and blue lines and arrows indicate the peaks from sI hydrate. Lattice parameter (160 K): a = 0.4491(3) nm and c = 0.7333(5) nm for Ih (space group P6₃/mmc), a = 0.6361(3) nm for Ic (space group Fd-3m), a = 1.1847(8) nm for sI hydrate (space group Pm-3n), and a = 1.7161(9) nm for sII hydrate (space group: Fd-3m).

Fig. 4.

Sample configurations of the NH₃ guests hydrogen bonding with the water molecules of the large (L) and small (S) cages of the sI and sII clathrate hydrate (Top: sI, Bottom: sII cages). The snapshots of the cages were extracted from the periodic simulation cell. In cases where NH₃ is incorporated into the water lattice, the displaced water molecule is also shown.