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Getting to the problem of the core
[T]he core cannot be made from iron alone because its density, determined by measuring the speed of seismic waves - is too low. This means there must be lighter elements there that are lowering its density. What is more, they have to be elements that are common in our solar system... "When you're talking about possible impurities in the Earth's core, the most likely things are the most common things like silicon, sulphur and oxygen".
Earth’s core chemistry is silicon enhanced
A team of scientists led by Jung-Fu Lin, a doctoral student in geophysical sciences at the University of Chicago, has found experimental evidence suggesting that the Earth's inner core largely consists of two exotic forms of iron instead of only one. These exotic forms of iron now appear to be alloyed with silicon. A previous study had once practically eliminated silicon as a candidate lighter element of the inner core... Seismologists have made further deductions about the characteristics of Earth's core from the way that seismic waves travel through Earth from earthquakes and explosives. "They noticed that there has to be about 10 weight percent of a lighter element in the outer core and anywhere from zero to 4 weight percent of a lighter element in the inner core" Heinz explained.
Hydrogen In The Core:
An Evidence For The State Of Lost Protoatmosphere
T. Okuchi, Y. Abe and H. Iwamori
Once a magma ocean was formed due to the blanketing effect of an impact-induced steam atmosphere of hundreds of bars, it absorbs most of H2O in the accreting planetesimals. Then the hydrogen is partitioned between the silicate melt and the molten iron that is sinking through the magma ocean to form the core. Thus, hydrogen is partitioned between the atmosphere, magma ocean and core... Two types of the protoatmospheres were proposed to produce the magma ocean: the primary atmosphere consists of the solar nebula gas, and the secondary atmosphere consists of the impact-degassed volatile... atmospheric hydrogen was oxidized by FeO in the magma ocean at its surface, transported through the magma ocean as H2O, and reduced by metallic iron in the deeper part of the magma ocean to form the iron-hydrogen alloy... On the other hand, if the magma ocean was formed through the blanketing effect of the impact-induced steam atmosphere, the hydrogen incorporation into molten iron may have decreased the mass and optical thickness of the atmosphere, and weakened the blanketing effect, because there was no nebula gas that supplies hydrogen to the atmosphere.
Wobbles within wobbles
probe planet's core
A new theory proposes that iron-rich sediments are floating to the top of the Earth's core and sticking like gum to the bottom of the mantle, creating drag that throws the Earth's wobble off by a millimetre or two over a period of about 18.6 years... As the Earth spins on its axis the moon and sun tug on its bulging equator and create a large wobble or “precession”, producing the precession of the equinoxes with a period of 25,800 years. Other periodic processes in the solar system nudge the Earth, too, creating small wobbles - called “nutations” - in the wobble. The principal components of the nutation are caused by the Earth's annual circuit of the sun and the 18.6-year precession of the moon's orbit... An annual deviation that lagged behind the tidal pull of the sun first suggested to Buffett 10 years ago that strange processes may be going on at the boundary between the mantle, made up of viscous rock that extends 1,800 miles below the crust, and the outer core, which is thought to be liquid iron with the consistency of water. The inner core, made of very pure, solid iron, rotates along with the outer core, dragging the Earth's magnetic field with them... Because the Earth's core rotates about a slightly different axis than the mantle (due to the tug of the Sun and Moon), the core's magnetic field is dragged through the mantle, passing unhindered because the mantle does not conduct electricity. The porous, iron-containing sediment stuck to the mantle, however, would resist the rotation of the magnetic field, creating just enough tug to perturb the Earth's rotation.