Thermal Conductivity and Compressional Velocity of Methane at High Pressure: Insights Into Thermal Transport Properties of Icy Planet Interiors

• Published in: Journal of geophysical research. Planets 2022-03, Vol.127 (3), p.n/a
• Main Authors: Meyer, Dylan W., Hsieh, Wen‐Pin, Hsu, Han, Kuo, Ching‐Yi, Lin, Jung‐Fu
• Format: Article
• Language: English
• Subjects: Ammonia; Atomic structure; compressional velocity; Density; Energy transport; Equations of state; Heat conductivity; Heat transfer; High pressure; Ice; icy planets; Icy satellites; Lower bounds; Magnetic fields; Methane; Oceans; Phase boundaries; Physical properties; Physiochemistry; Planetary interiors; Pluto; Pluto (dwarf planet); Remote sensing; Room temperature; Solar system; solid methane; Thermal conductivity; Transport properties; Velocity
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1029/2021JE007059

Methane is a primary component of the “ice” layers in icy bodies whose thermal transport properties and velocity‐density profiles are essential to understanding their unique geodynamic and physiochemical phenomena. We present experimental measurements of methane's thermal conductivity and compressional velocity to 25.1 and 45.1 GPa, respectively, at room temperature, and theoretical calculations of its equation of state, velocity, and heat capacity up to 100 GPa and 1200 K. Overall, these properties change smoothly with pressure and are generally unaffected by the imposed atomic structure; though we observe a discrete spike in conductivity near the I‐A phase boundary. We cross‐plot the thermal conductivity and compressional velocity with density for the primary “ice” constituents (methane, water, and ammonia) and find that methane and water are the upper and lower bounds, respectively, of conductivity and velocity in these systems. These physical properties provide critical insights that advance the modeling of thermo‐chemical structures and dynamics within icy bodies. Plain Language Summary Neptune, Pluto, and Uranus all belong to an important subclass of objects in our solar system, and beyond, known as icy planetary bodies. These systems commonly have an internal “ice” layer, consisting of water, methane, and ammonia, that controls the outward flow of heat from the core. The dynamics within this layer contribute to the intriguing behaviors, such as supercritical subsurface oceans and odd magnetic fields, present in these systems. To understand these dynamics, we collected unique measurements under extreme pressure on methane's thermal conductivity, the fundamental property defining the rate of energy transport through a material. We quantified the rate of conductivity increase with pressure up to 25 gigapascals (GPa; 1 GPa ≈ 104 atm) and captured an anomalous spike at a phase boundary that has never been previously observed. We combined this information with similar measurements on the other major “ice” components to constrain the composition and bulk physical properties of these layers. As our understanding of icy planetary bodies improves, primarily through remote‐sensing, our research provides a framework to link surficial measurements to empirically‐derived interior properties. This insight is critical for comprehending both the external and internal phenomena exhibited by icy bodies. Key Points Direct measurements of methane's thermal conductivity