Inner core age, anisotropy and rotation

6 problems

• What produces the inner core's seismic anisotropy and hemispherical asymmetry?

  Open

  Seismic waves cross the inner core a few percent faster along Earth's axis than along the equator, and its eastern and western halves look different. The crystal alignment and process behind this are not known.

• Thermal conductivity of liquid iron alloy at core-mantle boundary conditions

  Contested

  How well molten iron conducts heat under core pressures controls how much heat is left to stir the core. Calculations and lab experiments disagree by a large factor.

• When did Earth's solid inner core begin to grow?

  Contested

  Estimates of the inner core's age range from about half a billion to several billion years. The answer depends on how fast the core loses heat.

• Which light elements make Earth's core less dense than iron?

  Open

  Earth's liquid core is about 10 percent less dense than pure iron-nickel would be at core pressure, so lighter elements must be dissolved in it. Which ones (silicon, oxygen, sulfur, carbon or hydrogen) and how much of each is unknown.

• How did the inner core nucleate without implausible supercooling?

  Open

  For the first crystal of solid iron to form at Earth's center, the liquid must cool well below its freezing point. The cooling required appears far larger than the core ever experienced.

• How does the inner core rotate relative to the mantle?

  Contested

  Earthquake waves passing through the inner core change slightly over decades, suggesting it spins a little faster or slower than the rest of Earth. Whether it speeds ahead, oscillates back and forth, or changes shape instead is disputed.