http://arxiv.org/abs/1808.07925
We investigate the thermal equation of state, bulk modulus, thermal expansion coefficient, and heat capacity of MH-III (CH$_4$ filled-ice Ih), needed for the study of CH$_4$ transport and outgassing for the case of Titan and super-Titans. We employ density functional theory and ab initio molecular dynamics simulations in the generalized-gradient approximation with a van der Waals functional. We examine the finite temperature range of $300$K-$500$K and pressures between $2$GPa-$7$GPa. We find that in this P-T range MH-III is less dense than liquid water, and may migrate outward during episodic melting events. There is uncertainty in the normalized moment of inertia (MOI) of Titan; it is estimated to be in the range of $0.33-0.34$. If Titan’s MOI is close to $0.34$, MH-III may not be stable at present, yielding an easier path for the outgassing of CH$_4$. However, if its MOI is closer to $0.33$, MH-III is stable at the bottom of a hypothesized ice-rock internal layer, and outgassing would become dependent on the evolution of melt on the mixed ice-rock to core boundary. We further find that the heat capacity of MH-III is higher than measured values for pure water-ice, larger than heat capacity often adopted for ice-rock mixtures, with implications for internal heating of icy moons and ice-rich exoplanets.
A. Levi and R. Cohen
Mon, 27 Aug 18
28/46
Comments: 14 pages, 13 figures