Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p21a1581n&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P21A-1581
Physics
[3914] Mineral Physics / Electrical Properties, [3924] Mineral Physics / High-Pressure Behavior, [5440] Planetary Sciences: Solid Surface Planets / Magnetic Fields And Magnetism, [5460] Planetary Sciences: Solid Surface Planets / Physical Properties Of Materials
Scientific paper
Planetary magnetic fields are generated by convective motion of conducting fluids. The highest pressure on oxides in Earth is about 130 GPa (1.3 Mbar) at about 3000 K at the core-mantle boundary. At these conditions electrical conductivities and viscosities of solid oxides are too small and large, respectively, to produce a significant contribution to Earth’s magnetic field. However, oxides in super-Earth exoplanets reach interior pressures and temperatures much larger than those in Earth. Recent work has shown that solid Al2O3 is highly disordered up to ~400 GPa [1] and probably becomes a metallic glass with minimum metallic conductivity (MMC) at ~300 GPa under both shock and static compression [2]. This insulator-metal transition is probably entropy-driven; i. e., substantial compressive energy is absorbed by breaking chemical bonds, which leads to metallic energy bands. Since Al2O3 is estimated to melt on the Hugoniot at ~400 GPa, viscosity is expected to decrease near this pressure. Since interior pressures of 300 GPa are achieved at greater depths than in Earth, magnitudes of fields from oxides in super Earths are expected to be relatively small compared to the surface field of Earth. Nevertheless, the possibility exists that many extrasolar rocky planets have finite magnetic fields whether or not they have fluid Fe cores. Because of disorder, this conclusion based on experiments on Al2O3 is not expected to depend sensitively on which oxide is present. It is worth noting that fluid Fe in Earth and fluid metallic H in Jupiter [3] are also expected to have MMC, ~2000/(ohm-cm). [1] W. J. Nellis, G. I. Kanel, S. V. Razorenov, A. S. Savinykh, A. M. Rajenderan, J. Phys.: Conf. Ser. 215, 012148 (2010). [2] W. J. Nellis, Phys. Rev. B (in press). [3] W. J. Nellis, S. T. Weir, and A. C. Mitchell, Science 273, 936 (1996).
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