Session O3: Geophysics and Planetary Science I: Planetary Interiors and Impacts

X-ray diffraction of MgO along the shock Hugoniot

The structure of MgO upon shock compression was interrogated at the Omega Laser at the Laboratory for Laser Energetics, University of Rochester. Laser drives of up to 2 kJ over 7 ns focused onto a polyimide ablator were used to shock compress 50-$\mu{}m$ thick polycrystalline or single-crystal MgO. Scattered He-$\alpha$ X-rays from an Fe backlighter timed with maximum compression were collected using the PXRDiP diagnostic, in which image plates line the inner walls of a box attached to the target package. Other diagnostics utilized were VISAR (velocity history) and Streaked Optical Pyrometry (temperature). We will present experiments probing the $B1$$-$$B2$ phase transition of MgO and discuss the implications for detection of melting.\\[4pt] In collaboration with Thomas Duffy, Ray Smith, Rick Kraus, Federica Coppari, Dayne Fratanduono, Marius Millot, Amy Jenei, Jon Eggert, and Gilbert Collins, Lawrence Livermore National Laboratory.   

Recreating planetary interiors in the laboratory by laser-driven ramp-compression

Recent advances in laser-driven compression now allow to reproduce conditions existing deep inside large planets in the laboratory. Ramp-compression allows to compress matter along a thermodynamic path not accessible through standard shock compression techniques, and opens the way to the exploration of new pressure, density and temperature conditions. By carefully tuning the laser pulse shape we can compress the material to extremely high pressure and keep the temperature relatively low (i.e. below the melting temperature). In this way, we can probe solid states of matter at unprecedented high pressures. This loading technique has been combined with diagnostics generally used in condensed matter physics, such as x-ray diffraction and x-ray absorption spectroscopy (EXAFS, Extended X-ray Absorption Fine Structure, in particular), to provide a complete picture of the behavior of matter in-situ during compression. X-ray diffraction provides a snapshot of the structure and density of the material, while EXAFS has been used to infer the temperature. Simultaneous optical velocimetry measurements using VISAR (Velocity Interferometer for Any Reflector) yield an accurate determination of the pressure history during compression. In this talk I will present some of the results obtained in ramp-compression experiments performed at the Omega Laser Facility (University of Rochester) where the phase maps of planetary relevant materials, such as Fe, FeO and MgO, have been studied to unprecedented high pressures. Our data provide experimental constraints on the equations of state, strength and structure of these materials expected to dominate the interiors of massive rocky extra-solar planets and a benchmark for theoretical simulations. Combination of these new experimental data with models for planetary formation and evolutions is expected to improve our understanding of complex dynamics occurring in the Universe.   

The Role of Vaporization in High Angular Momentum Moon-forming Giant Impacts

In the giant impact hypothesis, the Moon accretes from a disk around the proto-Earth. In the canonical model, the impact also sets the present-day angular momentum (AM). Recently, an alternative model was proposed where the Moon forms via a high-AM giant impact and the present-day AM was established by a subsequent lunar orbital resonance. The physical state of the Earth after a high angular momentum impact is fundamentally different than in the canonical case. The impact energies are significantly higher, leading to vaporization of several wt\% of the Earth. Thus, impact-induced vaporization is a critical component of the new high-AM moon formation models. The post-impact planet possess a continuous pressure- and rotationally-supported fluid-to-vapor structure from the mantle to the disk. The surface of the structure cools radiatively and forms droplets; the droplets settle to the mid plane beyond the Roche radius and form moonlets. If mixing between the outer layers of the structure is efficient, then a wide range of high-AM giant impact geometries may produce the intriguing isotopic similarity between the Earth and Moon.   

Material properties for asteroid deflection

Impulsive strategies to prevent asteroid impacts depend upon knowledge of asteroidal material state and response at extreme conditions. Numerical modeling of kinetic impactor and nuclear ablation scenarios to deflect or disrupt asteroids reveals sensitivities to equation of state, strength, and porosity. We report advances in material models for asteroid mitigation simulations. Equation of state development focuses on asteroidal materials, such as hydrated silicates. Shock experiments are being performed to measure properties of meteoritic material; initial sample temperature can be controlled from 100-1000 K, important for different intercept scenarios. New constitutive models allow improved thermomechanical response predictions for porous asteroids.