A possible deep CH4 reservoir in Titan, Super-Titan exoplanets and moons

We investigate the thermal equation of state, bulk modulus, thermal expansion coefficient, and heat capacity of MH-III (CH4 filled-ice Ih), needed for the study of CH4 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 300K-500K and pressures between 2GPa-7GPa. 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 CH4. 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.

The Equation of State of MH-III: a possible deep CH4 reservoir in Titan, Super-Titan exoplanets and moons

Amit Levi, Ronald E. Cohen
(Submitted on 23 Aug 2018)

Comments: 14 pages, 13 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Materials Science (cond-mat.mtrl-sci); Geophysics (physics.geo-ph)
Cite as: arXiv:1808.07925 [astro-ph.EP] (or arXiv:1808.07925v1 [astro-ph.EP] for this version)
Submission history
From: Amit Levi
[v1] Thu, 23 Aug 2018 20:05:03 GMT (369kb,D)
https://arxiv.org/abs/1808.07925
Astrobiology