Man has walked on the moon, sent probes to or near the surfaces of Mars and Venus, and dispatched vehicles to fly near comets, asteroids, and the outer planets and their moons. In contrast, a journey to the center of the Earth remains as unattainable and fictional as in Jules Verne's day. We know the interior of our planet from several types of evidence: direct sampling of rock brought to the surface by geologic process from depths no greater than 200 km, remote sensing especially by seismology (the study of natural or anthropogenic sound waves passing through the Earth), inferences from the chemistry of meteorites and from other geochemical arguments, and laboratory and computational simulations of the conditions at depth. The laboratory study of Earth materials at high temperatures and pressures is the subject of this review. In a sense, the basic question of deep earth geophysics is a peculiar sort of inverse problem in materials science; rather than determining the properties of a given material, one seeks to find materials, under constraints of natural elemental abundances, which have properties consistent with seismological and other geophysical observations.
What are the “hard facts” about planet Earth as a material system? Its radius and mass are well constrained, as are pressure and density as a function of depth (see Figure 1). Its vertical temperature distribution is less well known, but it is clear that the interior is hot, with temperatures in the mantle reaching perhaps 3000 K and in the core perhaps 6000–7000 K.