Science Opportunities - Basic Research Opportunities in Earth Science - page 47

elements are released by soil, biologically cycled, and moved through the
food chain is important to ensuring food security and safety while maintaining or
improving environmental quality.
It is now possible to reproduce experimentally the high pressures and
temperatures existing throughout the Earth and to perform accurate
measurements of material properties at these conditions. This is significant
because seismology is revealing the structure of the Earth's deep interior with
rapidly improving resolution. By comparing seismological observations with
laboratory measurements and theoretical analyses ( Figure 2.9 ), it is possible to
determine the state and composition of the planetary interior as well as the
processes by which it evolves.
FIGURE 2.9 Quantum mechanics at the Earth's center. Theoretical calculations
(curve) of the elastic properties of hexagonal close-packed (HCP) iron at
conditions of the inner core reproduce seismologically observed variations of
travel times at depth (points). The observations represent the time required for
seismic waves to travel through the inner core, showing that travel times (hence
wave velocities) vary systematically with propagation direction, from polar
(along the Earth's rotation axis: polar angle = 0°) to equatorial (polar angle =
90°). The theory is from “first principles,” meaning that the quantum
mechanical calculations are free of any experimental input. SOURCE: Modified
from Microscopic to Macroscopic: Opportunities in Mineral and Rock Physics
and Chemistry, results of a workshop held in Scottsdale, Arizona, May 28-30,
1999. Reprinted with permission from L. Stixrude and R. E. Cohen, High-
pressure elasticity of iron and anisotropy of earth's inner core, Science, v. 267,
p. 1972-1975, 1995. Copyright 1995 American Association for the
Advancement of Science.