Waves pass through

Science News, Dec 6, 2008 by Tom Turner, Sid Perkins

Regarding "Solid evidence about Earth's core" (SN: 9/13/08, p. 14): If Earth has a solid inner core but a liquid outer core, then any direction you look at it, the shear waves have to go through some liquid outer core before they get to the solid inner core. So how do they get through the molten outer core?

Tom Turner, Newport, R.I.

When shear waves travel down through the mantle and reach the liquid outer core, some of their energy is converted into pressure waves (which can travel through liquid, unlike shear waves). When those pressure waves reach the solid inner core, some of their energy is converted back into shear waves. Once these vibrations reach the other side of the solid inner core, some of their energy is reconverted into pressure waves that rise through the liquid outer core. Finally, when these waves reach the mantle, some of their energy is again converted to shear waves that then spread and can be detected by seismometers at Earth's surface.

Each of these conversions--four, count "em!--is relatively inefficient, leaving little energy in the particular vibrations that the scientists needed to identify in order to bolster their theory.--Sid Perkins