The inner mantles of icy giant planets such as Uranus and Neptune are mainly composed by water, ammonia and methane, while their atmospheres are made of hydrogen and helium. Under high pressures inside giant planets, it is unclear whether the helium can diffuse into the depths and react with the mantle materials. Moreover, exotic phenomena such as superionicity (a partially molten phase) and plasticity (spinning molecules in a regular crystal) might occur in these compounds under the environments of giant planets. Therefore, it is interesting to explore the formation of helium-methane compounds and their dynamical properties under planetary condition.
Recently, the group of Prof. Jian Sun in Nanjing University collaborated with researchers from the University of Cambridge and the University of Edinburgh and predicted a novel helium-methane compound He3CH4, using first-principles calculations and crystal structure prediction methods. He3CH4 is a typical molecular crystal composed by helium atoms and methane molecules. It is predicted to be stable from 55 to 155 GPa, which is a much wider range than that for pure methane. The insertion of helium atoms not only changes the original packing of pure methane molecules but also suppresses the polymerization of methane into longer hydrocarbons at higher pressures.
The authors further investigated dynamical behaviors of the helium-methane compound at different temperatures using ab initio molecular dynamics. They found a series of phase transitions upon heating. Firstly, the compound has a phase transition from a solid state (Fig. 1(a)(c)) to a plastic phase (Fig. 1(b)(e)), where the methane molecules rotate freely. At higher temperatures, helium atoms start to diffuse while methane molecules keep rotating (Fig. 1(d)(f)). Such an exotic phase has never been discovered in previous works and is expected to have intriguing properties such as efficient heat transport that will affect a planet's interior and surface temperature. Several analysis methods based on charge density were used to determine the nature of the interactions in He3CH4. The results indicated that van der Waals interactions exist between methane molecules, which is weaker than hydrogen bonds in the helium-water and helium-ammonia compounds. It results in a relatively fragile framework in the He3CH4 compound and an easier transition to the fluid state.
Compared with helium-water and helium-ammonia compounds predicted by the same groups previously, the helium-methane compound has a narrower range of the diffusive behavior within the P-T phase diagram. These recent works suggest that, as inert as helium is, it can still react with methane, water, and ammonia inside planets, almost every main component in the icy planet mantles. These theoretical predictions should stimulate further experimental investigations and makes a contribution to the understanding and building new models of icy giant planets, as well as the physics and chemistry of helium.