Diamond formation from methane hydrate under the internal conditions of giant icy planets

Abstract

Hydrocarbon chemistry in the C–O–H system at high pressure and high temperature is important for modelling the internal structure and evolution of giant icy planets, such as Uranus and Neptune, as their interiors are thought to be mainly composed of water and methane. In particular, the formation of diamond from the simplest hydrocarbon, i.e., methane, under the internal conditions of these planets has been discussed for nearly 40 years. Here, we demonstrate the formation of diamond from methane hydrate up to 3800 K and 45 GPa using a CO2 laser-heated diamond anvil cell combined with synchrotron X-ray diffraction, Raman spectroscopy, and scanning electron microscopy observations. The results show that the process of dissociation and polymerisation of methane molecules to produce heavier hydrocarbons while releasing hydrogen to ultimately form diamond proceeds at milder temperatures (~ 1600 K) and pressures (13–45 GPa) in the C–O–H system than in the C–H system due to the influence of water. Our findings suggest that diamond formation can also occur in the upper parts of the icy mantles of giant icy planets.

Significance of the high-pressure properties and structural evolution of gas hydrates for inferring the interior of icy bodies

“…As shown in Fig. 6, the diamond formation observed in our experiments (Kadobayashi et al 2021) occurred under milder conditions than those previously reported. Wentorf (1965) reported that the conditions of diamond formation depend significantly on the composition of the initial hydrocarbon material.…”

“…Moreover, octahedral-shaped diamond particles were observed in the recovered sample quenched under ambient temperature and pressure conditions, and Raman spectroscopy revealed a characteristic Raman peak for the diamonds at 1331 cm −1 . The presence of hydrogen molecules was also observed in the sample chamber (Kadobayashi et al 2021). Although several experimental reports on diamond formation have been published (e.g., Zerr et al 2006;Kraus et al 2017), our in situ XRD, Raman spectroscopy, and SEM observations and recovered sample data provide unambiguous evidence for diamond formation based on theoretical prediction made 40 years ago (Ross 1981).…”

“…Neptune is believed to have a hydrogen and helium atmosphere; an ice mantle containing water, methane, and ammonia; and a rocky core (Redmer et al 2011;Nettelmann et al 2013). The findings of our experiments (Kadobayashi et al 2021;Kadobayashi et al 2020a, b, c) indicate that diamonds are formed by the molecular dissociation and polymerization of methane molecules in the presence of water at significantly lower temperatures and pressures (T > 1600 K, P > 13 GPa) than previously reported. The temperature and pressure conditions for diamond formation overlap with the predicted internal conditions of icy planets (Nettelmann et al 2016), demonstrating that diamond formation can occur in the upper regions of the ice mantle of an icy planet.…”

“…5, and solid methane and ice VII melt according to their own melting curves at elevated temperatures (Lin et al 2005;Lobanov et al 2013). To determine the effect of higher temperatures, our group has conducted high-temperature and high-pressure experiments using a laser-heated DAC (Kadobayashi et al 2021). Solid methane has been subjected to numerous experiments and theoretical calculations by our group and others, and the molecular dissociation of methane molecules and subsequent diamond formation have been reported (e.g., Gao et al 2010;Hirai et al 2008bHirai et al , 2009Benedetti et al 1999;Ancilotto et al 1997).…”

“…To investigate the intrinsic behavior of MH, we conducted experiments on the methane-water system using a CO 2 laser for heating (Kadobayashi et al 2021) to avoid the need for metallic absorbers. These experiments were designed to analyze the MH sample alone under static compression conditions closer to those estimated for Neptune's upper mantle (Nettelmann et al 2016), i.e., 45 GPa and up to 3800 K. Consequently, both solid methane and ice melted, methane molecules underwent molecular dissociation, and diamonds were formed from carbon, as detected by in situ XRD.…”