Inge Lehmann found Earth's solid heart by reading distant earthquake echoes
In 1936 the Danish seismologist Inge Lehmann studied records of New Zealand earthquakes and noticed waves that seemed to bounce off something deep inside the planet. She concluded that Earth has a solid inner core within its molten outer core, and estimated its radius at 1,400 kilometres, remarkably close to today's figure of about 1,221.
Nobody can sample the core, so nearly everything known about it comes from seismic waves. Pressure waves pass through both solids and liquids, while shear waves travel only through rigid material. Some pressure waves hitting the inner core at a slant convert into shear waves, cross it, and convert back, which proves the centre is solid. Shear waves passing through were first reported in 2005, a claim that was controversial at first but is gaining acceptance. Big quakes also set the whole planet ringing like a bell, and those vibrations reveal the density and size of the inner layers.
The numbers are extreme. The core's surface is roughly 5,700 kelvin, about as hot as the surface of the Sun, yet it stays solid because crushing pressure, some 330 to 360 gigapascals, drives iron's melting point sharply upward. Density reaches about 13 kilograms per litre at the centre, nearly four times that of rocks near the surface. The ball holds under one percent of Earth's volume but about 1.7 percent of its mass, and gravity at its surface is less than half what we feel.
It is probably mostly iron with up to 10 percent nickel, arranged in a hexagonal crystal form. Pure iron would be about 3 percent too dense, so a sprinkling of lighter elements such as silicon, oxygen or sulfur is likely present. Though made of iron, it is far too hot to be magnetic itself, but it shapes the currents in the liquid outer core that generate the planet's field.
Scientists once expected a uniform ball, even a single giant crystal. In 1983, though, researchers found that waves cross it about 1 percent faster from pole to pole than across the equator. Seismic waves cannot tell a true solid from an extraordinarily thick fluid, and one 2009 estimate put its viscosity at over a billion times that of pitch; recent work hints the ball may be slightly deformable.
Source: Earth's inner core
