Localized temporal change of the Earth's inner core boundary

Compressional waves of an earthquake doublet (two events occurring in the South Sandwich Islands on 1 December 1993 and 6 September 2003), recorded at three seismic stations in Russia and Kyrgyzstan and reflected off Earth's inner core boundary, arrived at least from 39 to 70 milliseconds earlier in the 2003 event than in the 1993 event. Such changes indicate that Earth's inner core radius enlarged locally beneath middle Africa by 0.98 to 1.75 kilometers between the times of these two events. Changes of the inner core radius may be explained by either a differential motion of the inner core, assuming that irregularities are present at the inner core boundary and fixed to the inner core, or a rapid growth of the inner core by this amount.     

Elastic anisotropy of Earth's inner core

Earth's solid-iron inner core is elastically anisotropic. Sound waves propagate faster along Earth's spin axis than in the equatorial plane. This anisotropy has previously been explained by a preferred orientation of the iron alloy hexagonal crystals. However, hexagonal iron becomes increasingly isotropic on increasing temperature at pressures of the inner core and is therefore unlikely to cause the anisotropy. An alternative explanation, supported by diamond anvil cell experiments, is that iron adopts a body-centered cubic form in the inner core. We show, by molecular dynamics simulations, that the body-centered cubic iron phase is extremely anisotropic to sound waves despite its high symmetry. Direct simulations of seismic wave propagation reveal an anisotropy of 12%, a value adequate to explain the anisotropy of the inner core.     

The Melting Curve of Iron to 250 Gigapascals: A Constraint on the Temperature at Earth's Center

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.     

Seismic evidence for an inner core transition zone

Seismic waves that traverse Earth's inner core along north-south paths produce unusually broad pulse shapes at long periods (compared with waves along east-west paths) and reflections from below the inner core boundary at short periods. The observations provide compelling evidence for a seismic velocity discontinuity along north-south paths about 200 kilometers below the inner core boundary separating an isotropic upper inner core from an anisotropic lower inner core. The triplication associated with such a structure might be responsible for reported waveform complexity of short-period inner core arrivals along north-south paths and, if the depth of the boundary is laterally variable, their large travel-time variation.     

The structure of iron in Earth's inner core

Earth's solid inner core is mainly composed of iron (Fe). Because the relevant ultrahigh pressure and temperature conditions are difficult to produce experimentally, the preferred crystal structure of Fe at the inner core remains uncertain. Static compression experiments showed that the hexagonal close-packed (hcp) structure of Fe is stable up to 377 gigapascals and 5700 kelvin, corresponding to inner core conditions. The observed weak temperature dependence of the c/a axial ratio suggests that hcp Fe is elastically anisotropic at core temperatures. Preferred orientation of the hcp phase may explain previously observed inner core seismic anisotropy.     

Equatorially dominated magnetic field change at the surface of Earth's core

Slow temporal variations in Earth's magnetic field originate in the liquid outer core. We analyzed the evolution of nonaxisymmetric magnetic flux at the core surface over the past 400 years. We found that the most robust feature is westward motion at 17 kilometers per year, in a belt concentrated around the equator beneath the Atlantic hemisphere. Surprisingly, this motion is dominated by a single wavenumber and persists throughout the observation period. This phenomenon could be produced by an equatorial jet of core fluid, by hydromagnetic wave propagation, or by a combination of both. Discrimination between these mechanisms would provide useful constraints on the dynamics of Earth's core.     

Viscosity near Earth's solid inner core

Anomalous splitting of the two equatorial translational modes of oscillation of Earth's solid inner core is used to estimate the effective viscosity just outside its boundary. Superconducting gravimeter observations give periods of 3.5822 +/- 0.0012 (retrograde) and 4.0150 +/- 0.0010 (prograde) hours. With the use of Ekman layer theory to estimate viscous drag forces, an inferred single viscosity of 1.22 x 10(11) Pascal seconds gives calculated periods of 3.5839 and 4.0167 hours for the two modes, close to the observed values. The large effective viscosity is consistent with a fluid, solid-liquid mixture surrounding the inner core associated with the "compositional convection" that drives Earth's geodynamo.     

Mars: a new core-crystallization regime

The evolution of the martian core is widely assumed to mirror the characteristics observed for Earth's core. Data from experiments performed on iron-sulfur and iron-nickel-sulfur systems at pressures corresponding to the center of Mars indicate that its core is presently completely liquid and that it will not form an outwardly crystallizing iron-rich inner core, as does Earth. Instead, planetary cooling will lead to core crystallization following either a "snowing-core" model, whereby iron-rich solids nucleate in the outer portions of the core and sink toward the center, or a "sulfide inner-core" model, where an iron-sulfide phase crystallizes to form a solid inner core.     

Anisotropy in the Inner Core: Could It Be Due To Low-Order Convection?

A recently assembled data set of inner core-sensitive free oscillation splitting measurements and body wave differential travel times provides constraints on the patterns of anisotropy in the Earth's inner core. Applying a formalism that allows departures from radial symmetry and cylindrical anisotropy results in models with P-wave velocity distributions whose strength and pattern are incompatible with frozen-in anisotropy, but rather suggest a simple large-scale convection regime in the inner core.     

Planet Within a Planet: Rotation of the Inner Core of Earth

The time dependence of the orientation of Earth's inner core relative to the mantle was determined using a recently discovered 10-degree tilt in the axis of symmetry of the inner core's seismic-velocity anisotropy. Two methods of analyzing travel-time variations for rays traversing the inner core, on the basis of 29 years of data from the International Seismological Centre (1964-1992), reveal that the inner core appears to rotate about 3 degrees per year faster than the mantle. An anomalous variation in inner-core orientation from 1969 to 1973 coincides in time with a sudden change ("jerk") in the geomagnetic field.     

Physics of iron at Earth's core conditions

The bulk properties of iron at the pressure and temperature conditions of Earth's core were determined by a method that combines first-principles and classical molecular dynamic simulations. The theory indicates that (i) the iron melting temperature at inner-core boundary (ICB) pressure (330 gigapascals) is 5400 (+/-400) kelvin; (ii) liquid iron at ICB conditions is about 6% denser than Earth's outer core; and (iii) the shear modulus of solid iron close to its melting line is 140 gigapascals, consistent with the seismic value for the inner core. These results reconcile melting temperature estimates based on sound velocity shock wave data with those based on diamond anvil cell experiments.     

Origin of the low rigidity of the Earth's inner core

Earth's solid-iron inner core has a low rigidity that manifests itself in the anomalously low velocities of shear waves as compared to shear wave velocities measured in iron alloys. Normally, when estimating the elastic properties of a polycrystal, one calculates an average over different orientations of a single crystal. This approach does not take into account the grain boundaries and defects that are likely to be abundant at high temperatures relevant for the inner core conditions. By using molecular dynamics simulations, we show that, if defects are considered, the calculated shear modulus and shear wave velocity decrease dramatically as compared to those estimates obtained from the averaged single-crystal values. Thus, the low shear wave velocity in the inner core is explained.     

Earth's core and the geodynamo

Earth's magnetic field is generated by fluid motion in the liquid iron core. Details of how this occurs are now emerging from numerical simulations that achieve a self-sustaining magnetic field. Early results predict a dominant dipole field outside the core, and some models even reproduce magnetic reversals. The simulations also show how different patterns of flow can produce similar external fields. Efforts to distinguish between the various possibilities appeal to observations of the time-dependent behavior of the field. Important constraints will come from geological records of the magnetic field in the past.     

Magnetic Collapse in Transition Metal Oxides at High Pressure: Implications for the Earth

Magnetic collapse in transition metal ions is predicted from first-principles computations at pressures reached in the Earth's lower mantle and core. Magnetic collapse would lead to marked changes in geophysically important properties, such as elasticity and conductivity, and also to different geochemical behavior, such as element partitioning, than estimated by extrapolating low-pressure data, and thus change the understanding of Earth's structure and evolution. Magnetic collapse results from band widening rather than from changes in crystal field splitting under pressure. Seismic anomalies in the outer core and the lowermost mantle may be due to magnetic collapse of ferrous iron, dissolved in iron liquid in the outer core, and in solution in magnesiowustite in the lowermost mantle.     

Magnetic properties of hexagonal closed-packed iron deduced from direct observations in a diamond anvil cell

The attraction of hexagonal closed packed (hcp) iron to a magnet at 16.9 gigapascals and 261 degrees centigrade suggests that hcp iron is either paramagnetic or ferromagnetic with susceptibilities from 0. 15 to 0.001 and magnetizations from 1800 to 15 amperes per meter. If dominant in Earth's inner core, paramagnetic hcp iron could stabilize the geodynamo.     

Inner core differential motion confirmed by earthquake waveform doublets

We analyzed 18 high-quality waveform doublets with time separations of up to 35 years in the South Sandwich Islands region, for which the seismic signals have traversed the inner core as PKP(DF). The doublets show a consistent temporal change of travel times at up to 58 stations in and near Alaska, and they show a dissimilarity of PKP(DF) coda. Using waveform doublets avoids artifacts of earthquake mislocations and contamination from small-scale heterogeneities. Our results confirm that Earth's inner core is rotating faster than the mantle and crust at about 0.3 degrees to 0.5 degrees per year.     

A Laboratory Model for Convection in Earth's Core Driven by a Thermally Heterogeneous Mantle

Thermal convection experiments in a rapidly rotating hemispherical shell suggest a model in which the convection in Earth's liquid outer core is controlled by a thermally heterogeneous mantle. Experiments show that heterogeneous boundary heating induces an eastward flow in the core, which, at a sufficiently large magnitude, develops into a large-scale spiral with a sharp front. The front separates the warm and cold regions in the core and includes a narrow jet flowing from the core-mantle boundary to the inner-core boundary. The existence of this front in the core may explain the Pacific quiet zone in the secular variation of the geomagnetic field and the longitudinally heterogeneous structure of the solid inner core.     

Body-centered cubic iron-nickel alloy in Earth's core

Cosmochemical, geochemical, and geophysical studies provide evidence that Earth's core contains iron with substantial (5 to 15%) amounts of nickel. The iron-nickel alloy Fe(0.9)Ni(0.1) has been studied in situ by means of angle-dispersive x-ray diffraction in internally heated diamond anvil cells (DACs), and its resistance has been measured as a function of pressure and temperature. At pressures above 225 gigapascals and temperatures over 3400 kelvin, Fe(0.9)Ni(0.1) adopts a body-centered cubic structure. Our experimental and theoretical results not only support the interpretation of shockwave data on pure iron as showing a solid-solid phase transition above about 200 gigapascals, but also suggest that iron alloys with geochemically reasonable compositions (that is, with substantial nickel, sulfur, or silicon content) adopt the bcc structure in Earth's inner core.     

Neural correlates of a magnetic sense

Many animals rely on Earth's magnetic field for spatial orientation and navigation. However, how the brain receives and interprets magnetic field information is unknown. Support for the existence of magnetic receptors in the vertebrate retina, beak, nose, and inner ear has been proposed, and immediate gene expression markers have identified several brain regions activated by magnetic stimulation, but the central neural mechanisms underlying magnetoreception remain unknown. Here we describe neuronal responses in the pigeon's brainstem that show how single cells encode magnetic field direction, intensity, and polarity; qualities that are necessary to derive an internal model representing directional heading and geosurface location. Our findings demonstrate that there is a neural substrate for a vertebrate magnetic sense.     

Earth's Variable Rotation

Recent improvements in geodetic data and practical meteorology have advanced research on fluctuations in the Earth's rotation. The interpretation of these fluctuations is inextricably linked with studies of the dynamics of the Earth-moon system and dynamical processes in the liquid metallic core of the Earth (where the geomagnetic field originates), other parts of the Earth's interior, and the hydrosphere and atmosphere. Fluctuations in the length of the day occurring on decadal time scales have implications for the topography of the core-mantle boundary and the electrical, magnetic, and other properties of the core and lower mantle. Investigations of more rapid fluctuations bear on meteorological studies of interannual, seasonal, and intraseasonal variations in the general circulation of the atmosphere and the response of the oceans to such variations.