The remote setting of the Earth's core tests our ability to assess its physical and chemical characteristics. Extending out to half an Earth radii, the metallic core constitutes a sixth of the planet's volume and a third of its mass (see Table 1 for physical properties of the Earth's core). The boundary between the silicate mantle and the core (CMB) is remarkable in that it is a zone of greatest contrast in Earth properties. The density increase across this boundary represents a greater contrast than across the crust-ocean surface. The Earth's gravitational acceleration reaches a maximum (10.7 m s-2) at the CMB and this boundary is also the site of the greatest temperature gradient in the Earth. (The temperature at the base of the mantle (̃2,900 °C) is not well established, and that at the top of the inner core is even less securely known (̃3,500-4,500 °C).) The pressure range throughout the core (i.e., 136 GPa to >360 GPa) makes recreating environmental conditions in most experimental labs impossible, excepting a few diamond anvil facilities or those with high-powered, shock-melting guns (see Chapter 2.14). Thus, our understanding of the core is based on very few pieces of direct evidence and many fragments of indirect observations. Direct evidence comes from seismology, geodesy, geo- and paleomagnetism, and, relatively recently isotope geochemistry (see Section 2.15.6). Indirect evidence comes from geochemistry, cosmochemistry, and meteoritics; further constraints on the core system are gained from studies in experimental petrology, mineral physics, ab initio calculations, and evaluations of the Earth's energy budget (e.g., geodynamo calculations, core crystallization, heat flow across the core-mantle boundary). Figure 1 provides a synopsis of research on the Earth's core, and the relative relationship between disciplines. Feedback loops between all of these disciplines refine other's understanding of the Earth's core. Table 1. Physical properties of the Earth's core UnitsRefs. Mass Earth5.9736E+24kg1 Inner core9.675E+22kg1 Outer core1.835E+24kg1 Core1.932E+24kg1 Mantle4.043E+24kg1 Inner core to core (%)5.0% Core to Earth (%)32.3% Depth Core-mantle boundary3,483±5km2 Inner-outer core boundary1,220±10km2 Mean radius of the Earth6,371.01±0.02km1 Volume relative to planet Inner core7.606E+09(0.7%)km3 Inner core relative to the bulk core4.3% Outer core1.694E+11(15.6%)km3 Bulk core1.770E+11(16.3%)km3 Silicate earth9.138E+11(84%)km3 Earth1.083E+12km3 Moment of inertia constants Earth mean moment of inertia (I)0.3299765Ma21 Earth mean moment of inertia (I)0.3307144MR021 Mantle: Im/Ma20.29215Ma21 Fluid core: If/Ma20.03757Ma21 Inner core: Iic/Ma22.35E-4Ma21 Core: If+ic/Mf+icaf20.392Ma21 1 - Yoder (1995), 2 - Masters and Shearer (1995). M is the Earth's mass, a is the Earth's equatorial radius, R0 is the radius for an oblate spheroidal Earth, Im is the moment of inertia for the mantle, If is the moment of inertia for the outer (fluid) core, Iic is the moment of inertia for the inner core, and If+ic/Mf+icaf2 is the mean moment of inertia for the core. (11K)Figure 1. The relative relationship between disciplines involved in research on the Earth's core and the nature of data and information that come from these various investigations. Studies listed in the upper row yield direct evidence on properties of the core. Those in the middle row yield indirect evidence on the composition of the Earth's core, whereas findings from disciplines listed on the bottom row provide descriptions of the state conditions for the core and its formation.