Stable methane hydrate above 2 GPa and the source of Titan's atmospheric methane

Abstract

Methane hydrate is thought to have been the dominant methane-containing phase in the nebula from which Saturn, Uranus, Neptune and their major moons formed. It accordingly plays an important role in formation models of Titan, Saturn's largest moon. Current understanding assumes that methane hydrate dissociates into ice and free methane in the pressure range 1-2 GPa (10-20 kbar), consistent with some theoretical and experimental studies. But such pressure-induced dissociation would have led to the early loss of methane from Titan's interior to its atmosphere, where it would rapidly have been destroyed by photochemical processes. This is difficult to reconcile with the observed presence of significant amounts of methane in Titan's present atmosphere. Here we report neutron and synchrotron X-ray diffraction studies that determine the thermodynamic behaviour of methane hydrate at pressures up to 10 GPa. We find structural transitions at about 1 and 2 GPa to new hydrate phases which remain stable to at least 10 GPa. This implies that the methane in the primordial core of Titan remained in stable hydrate phases throughout differentiation, eventually forming a layer of methane clathrate approximately 100 km thick within the ice mantle. This layer is a plausible source for the continuing replenishment of Titan's atmospheric methane.

Pressure-induced phase transformations in theBa8Si46clathrate

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Very low compressibility in a Na-doped Si clathrate with the type-II structure was first reported, 12 which transforms into an amorphous phase at ϳ12 GPa. In contrast, the type-I Si 46 clathrates of K ͑Ref.…”

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

“…ice | single crystal X-ray diffraction | hydrogen bonding | hydrate inhibitors | ethane A mmonia has long been seen as a key species in extraterrestrial space, both interstellar and on outer planets, moons, and comets and the interplay of ammonia, methane, and water has been the subject of a considerable number of studies and speculation (1)(2)(3)(4)(5)(6)(7)(8). The main role assigned to ammonia has been that of an antifreeze for ice and clathrate hydrate formation, modifying the stability region of the solid ice and methane clathrate hydrate phases as a thermodynamic inhibitor (2,5,9).…”

Astrophysics in 2001

“…No clearly starred papers here, but we read about lavas made of water ice on Ganymede (Schenk et al 2001) and hydrated rocks as well (McCord et al 2001), ocean survival on Callisto , and rain on Titan (Griffith et al 2000). Admittedly, the rain drops are liquid methane, but the methane in turn comes from methane hydrate (Loveday et al 2001). So there!…”