Wed Jun 29 2011 11:44
Seismologists discover that the inner core rotates
Researchers are now probing what may turn out to be the most curious small body the solar system has yet presented for scrutiny: a globe the size of the moon that appears to be a well ordered crystalline entity. This body is poised little more than 5,000 kilometers away, yet it is completely invisible. Located at the center of the earth, it is known simply as the inner core. Two seismologists have just shown that this strange crystal sphere is turning slowly within the rocky and liquid metal enclosure that keeps it all but hidden from scientific investigation.
Geophysicists realised decades ago that a solid inner core exists, but they knew precious little else about it. They believed the inner core and the liquid shell surrounding it were made largely of iron, yet other features of the heart of the planet remained enigmatic.
But during the 1980s, seismologists examining earthquake waves that pierce the inner core made a startling find. Rather than being "isotropic" (the same in all directions) in its physical properties, the inner core proved to be somewhat like a piece of wood, with a definite grain running through it. Waves traveling along the planet’s north-south axis go 3 to 4 percent faster through the inner core than those that follow paths close to the equatorial plane.
Geophysicists have struggled to explain why this grain (or "seismic anisotropy") should exist. The leading theory is that at the immense pressures of the inner core, iron takes on a hexagonal crystal form that has inherently directional physical properties. Some force apparently keeps the hexagonal iron crystals all in close alignment….