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The melting curve of iron, the primary constituent of Earth's core, has been measured to pressures of 250 gigapascals with a combination of static and dynamic techniques. The melting temperature of iron at the pressure of the core-mantle boundary (136 gigapascals) is 4800 +/- 200 K. whereas at the inner core-outer core boundary (330 gigapascals), it is 7600 +/- 500 K. Corrected for melting point depression resulting from the presence of impurities, a melting temperature for iron-rich alloy of 6600 K at the inner core-outer core boundary and a maximum temperature of 6900 K at Earth's center are inferred. This latter value is the first experimental upper bound on the temperature at Earth's center, and these results imply that the temperature of the lower mantle is significantly less than that of the outer core.

[1] We present new equation-of-state (EoS) data acquired by shock loading to pressures up to 245 GPa on both low-density samples (MgSiO 3 glass) and high-density, polycrystalline aggregates (MgSiO 3 perovskite + majorite). The latter samples were synthesized using a large-volume press. Modeling indicates that these materials transform to perovskite, postperovskite, and/or melt with increasing pressure on their Hugoniots. We fit our results together with existing P-V-T data from dynamic and static compression experiments to constrain the thermal EoS for the three phases, all of which are of fundamental importance to the dynamics of the lower mantle. The EoS for perovskite and postperovskite are well described with third-order Birch-Murnaghan isentropes, offset with a Mie-Grüneisen-Debye formulation for thermal pressure. The addition of shock data helps to distinguish among discrepant static studies of perovskite, and for postperovskite, constrain a value of K 0 significantly larger than 4. For the melt, we define for the first time a single EoS that fits experimental data from ambient pressure to 230 GPa; the best fit requires a fourth-order isentrope. We also provide a new EoS for Mg 2 SiO 4 liquid, calculated in a similar manner. The Grüneisen parameters of the solid phases decrease with pressure, whereas those of the melts increase, consistent with previous shock wave experiments as well as molecular dynamics simulations. We discuss implications of our modeling for thermal expansion in the lower mantle, stabilization of ultra-low-velocity zones associated with melting at the core-mantle boundary, and crystallization of a terrestrial magma ocean.

The maximum superheating and undercooling achievable at various heating ͑or cooling͒ rates were investigated based on classical nucleation theory and undercooling experiments, molecular dynamics ͑MD͒ simulations, and dynamic experiments. The highest ͑or lowest͒ temperature T c achievable in a superheated solid ͑or an undercooled liquid͒ depends on a dimensionless nucleation barrier parameter ␤ and the heating ͑or cooling͒ rate Q. ␤ depends on the material: ␤ϵ16␥ sl 3 /(3kT m ⌬H m 2 ) where ␥ sl is the solid-liquid interfacial energy, ⌬H m the heat of fusion, T m the melting temperature, and k Boltzmann's constant. The systematics of maximum superheating and undercooling were established phenomenologically as ␤ϭ(A 0 Ϫb log 10 Q) c (1Ϫ c ) 2 where c ϭT c /T m , A 0 ϭ59.4, bϭ2.33, and Q is normalized by 1 K/s. For a number of elements and compounds, ␤ varies in the range 0.2-8.2, corresponding to maximum superheating c of 1.06 -1.35 and 1.08 -1.43 at Q ϳ1 and 10 12 K/s, respectively. Such systematics predict that a liquid with certain ␤ cannot crystallize at cooling rates higher than a critical value and that the smallest c achievable is 1/3. MD simulations (Q ϳ10 12 K/s) at ambient and high pressures were conducted on close-packed bulk metals with Sutton-Chen many-body potentials. The maximum superheating and undercooling resolved from single-and two-phase simulations are consistent with the c -␤-Q systematics for the maximum superheating and undercooling. The systematics are also in accord with previous MD melting simulations on other materials ͑e.g., silica, Ta and ⑀-Fe͒ described by different force fields such as Morse-stretch charge equilibrium and embedded-atom-method potentials. Thus, the c -␤-Q systematics are supported by simulations at the level of interatomic interactions. The heating rate is crucial to achieving significant superheating experimentally. We demonstrate that the amount of superheating achieved in dynamic experiments (Qϳ10 12 K/s), such as planar shock-wave loading and intense laser irradiation, agrees with the superheating systematics.

The Cassini-Huygens Cosmic Dust Analyzer (CDA) is intended to provide direct observations of dust grains with masses between 10 −19 and 10 −9 kg in interplanetary space and in the jovian and saturnian systems, to investigate their physical, chemical and dynamical properties as functions of the distances to the Sun, to Jupiter and to Saturn and its satellites and rings, to study their interaction with the saturnian rings, satellites and magnetosphere. Chemical composition of interplanetary meteoroids will be compared with asteroidal and cometary dust, as well as with Saturn dust, ejecta