Better explicating the strengths and shortcomings of these models will help refine projections and improve transparency in the years ahead.
Image credit: Witsawat.S.
Edited by Russell J. Hemley, Carnegie Institution of Washington, Washington, DC, and approved March 18, 2011 (received for review October 4, 2010)
Article Figures & SI
Figures
- Fig. 1.
Results from ab initio MD simulations. (A) Methane and hydrocarbon products found in our ab initio MD simulations of pure fluids (no metals or carbon surfaces present), as a function of pressure and temperature. Black squares indicate pressure-temperature conditions where pure methane does not dissociate in direct ab initio MD simulations. Blue filled triangles represent conditions where formation of hydrocarbons is observed in our simulations. The solid blue curve indicates the Earth mantle geotherm. The dashed green circle identifies the experimental P-T condition achieved in ref. 6 where formation of hydrocarbons from methane was detected in DAC experiments with absorbers. (B) Chart diagrams of carbon products found in our simulations at 4, 7.5, and 12 GPa, at 4,000 K. Cn (n = 1,5) indicates the number of carbon atoms in the final products. (C) Percentage of hydrogen in molecules found at 4, 7.5, and 12 GPa, at 4,000 K.
- Fig. 2.
Vibrational spectra. Computed carbon vibrational density of state for pure methane (black line), ethane (orange line), and hydrocarbons mixtures at 7.5 GPa (red line), 12 GPa (green line), and 30 GPa (blue line) at T = 2,000 K. Alkane mixtures were prepared by annealing configurations generated with MD simulations at 4,000 K.
- Fig. 3.
Proposed boundaries for methane stability. Temperature boundaries for methane stability are indicated by dashed lines. Green square and red dots indicate the results of our free energy calculations (see Table 1). At 4 GPa and T < 2,000 K, the entropies of liquid methane and of the mixtures were estimated by using atomic velocities rescaled from those computed at 2,000 K. Experimental points denote the lowest pressure and temperature points at which hydrocarbon formation was observed. The solid red line represents P-T conditions along the Earth geotherm. Conditions in the lithospheric mantle correspond approximately to the region highlighted in orange.
- Fig. 4.
Snapshots from ab initio simulations. (A) Methane in contact with an Ir4 cluster at 2,500 K and 4 GPa: The Ir4 cluster acquires a butterfly shape and several fragments—CH3 and CH2—are observed during the simulation together with molecular hydrogen and ethane. (B) Liquid methane in contact with a partially H-terminated diamond surface [100] at 2,500 K and P ≈ 4 GPa. A CH3 group attached to the surface and a newly formed ethane molecule are highlighted.
Tables
- Table 1.
Enthalpy (ΔH) and entropy (ΔS) differences per carbon atom (eV) of C-H systems with respect to methane, at different P-T conditions
ΔH -TΔS ΔG T = 1,000 K; P = 2 GPa CH4 → 1/2 C2H6 + 1/2 H2 0.46(3) −0.15(9) 0.31(0.12) CH4 → 1/3 C3H8 + 4/3 H2 0.57(3) −0.32(9) 0.25(0.12) T = 2,000 K; P = 4 GPa CH4 → 1/2 C2H6 + 1/2 H2 0.41(5) −0.53(5) −0.11(0.1) T = 2,000 K; P = 8 GPa CH4 → 1/2 C2H6 + 1/2 H2 0.35(5) −0.52(6) −0.17(0.1) ΔH = Hmixture - Hmethane and -TΔS = -T(Smixture - Smethane)
- Table 2.
Rotational, vibrational, and translational contributions to the entropic term -TΔS for liquid methane and a mixture of ethane and hydrogen, at T = 2,000 K and P = 4 GPa
-TS (eV) Rotation Vibration Translation Total Methane −26.5(2) −78.5(1) −94.0(7) −199(1) Ethane −20.3(1) −99.5(1) −50.5(4) −170.3(6) Hydrogen −7.6(5) −5.7(1) −41.1(3) −54.4(9) -TΔS/CH4 −0.02(2) −0.55(3) 0.04(2) −0.53(5) T is the temperature and S is the entropy
Stability of hydrocarbons at deep Earth pressures and temperatures
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- Chemistry
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