Planetary Interiors

Thermal and Velocity/Density Structure of Planetary Interiors

Thermal Conductivity of Iron Alloys at High Pressure

The efficiency of heat transfer by conduction in a planetary core controls the dynamics of convection and limits the power available from the heat source. It also affects the thermal evolution of the planet. Understanding the energy sources and thermal evolution depends critically on knowledge of the thermal conductivity of the core because this physical property is directly proportional to the maximum amount of energy that can be conductively lost from the core. To constrain the energy balance and thermal structure of the core, We have initiated a new research program that is aimed at measuring the electrical resistivity (inversely proportional to thermal conductivity through the Wiedemann-Franz law) of iron alloys using both multi-anvil and the diamond anvil cell techniques. This is complementary to our on-going research on planetary cores, providing constraints on the heat budget and thermal structure of the interior.

Fig. 1. a) Multi-anvil assembly for electrical resistivity measurements with four Pt leads. b) Polished cross section of the recovered sample.

Sound Velocity Measurements of Iron at High Pressure and Temperature

Seismic data are crucial for our understanding of the internal structure the Earth. We have only limited lunar seismic data and no data on other terrestrial planets. Re-analysis of the Apollo-era lunar seismic data yielded some insight into the structure and physical state of the lunar core. Any seismic data on Mars from planned or future missions will greatly improve our knowledge of Martian interior. However, planetary seismic data will always be limited because of the limitation of resource and technology and their interpretation will heavily rely on laboratory measurements, particularly sound velocities and densities of mantle and core materials. Phase relations and physical properties of iron play an important role in understanding planetary cores. While many studies have been focused on hcp-Fe because of its application to the Earth’s core, the stable iron phase in small planetary bodies such as the Moon and smaller terrestrial planets such as Mars and Mercury is fcc-Fe. We have determined the stability field and the thermal equation of state of fcc-Fe [Komabayashi et al., 2009; Komabayashi and Fei, 2010]. These data are essential for modeling the density profiles of Mercury and Mars and assessing the contracting Mercury in conjunction with the observations from MESSENGER. In this proposal, we will focus on measurements of the sound velocity of fcc-Fe using inelastic x-ray scattering (IXS) technique. We have established experimental technique to measure the sound velocity of iron under simultaneous high pressure and temperature by combining inelastic x-ray scattering with the externally heated diamond-anvil cell [Antonangeli et al., 2015].

Fig. 2. (left) Experimental configuration for inelastic x-ray scattering measurements at ESRF. (right) Representative aggregate phonon dispersion curves at various temperatures and density [Antonangeli et al., 2012]. Inset. Example of collected IXS spectra. The arrows indicate the longitudinal acoustic (LA) phonon of iron and, the transverse acoustic (TA) and longitudinal acoustic (LA) phonon of diamond.