Introduction by C. McCammon, Bayerisches Geoinstitut (Germany)

Processes that take place on the surface of the Earth and in its interior are intimately linked to the properties and structure of the Earth's materials down to the atomic scale. Probing the Earth's interior is challenging, since most measurements are constrained to be made from the Earth's surface. Much of what we know for certain about the Earth's interior is derived from seismology, which requires information on the structure and elasticity of Earth-forming minerals. Experimental studies therefore play a crucial role in constraining the properties of the deep Earth, since they allow measurements of the relevant properties at conditions appropriate to the Earth's interior. Pressure is a variable common to all studies of the deep Earth. Advances in technology have allowed experimental access to the pressures at the centre of the Earth, but at the cost of reduction in sample size. Synchrotron radiation is ideally suited to the study of these small samples, and allows the measurement of properties at the atomic level that can be related to properties and processes on a global scale.

The selected highlights showcase results on materials that have advanced our understanding of processes within the Earth. Gas hydrates such as methane hydrate have been found to occur in large quantities within the Earth's crust, and may represent the largest source of hydrocarbon on Earth. The release of methane from this reservoir may have contributed to variation in past atmospheric methane levels, with obvious consequences for past and future climate change. Seismic methods have been used to identify the existence of gas hydrate zones, for which elastic properties are an important parameter.

Iron is the most abundant element in the Earth's crust and mantle with a variable oxidation state. The electronic structure of iron, particularly its spin state, affects the degree to which different wavelengths of light are absorbed and hence the radiative thermal conductivity of the mantle. High-spin to low-spin transitions at high pressure can therefore influence the transport of heat in the mantle, and hence the dynamics of mantle convection. Iron is also the most abundant element in the core. The discovery of seismic anisotropy in the Earth's solid inner core, where seismic velocities are faster in N-S directions than along equatorial paths, has important implications for dynamic processes such as those that give rise to the Earth's magnetic field. On the horizon, the application of novel methods such as resonant X-ray emission spectroscopy (RXES) to samples under high pressure provides the chance for improved characterisation of the electronic structure of iron in relevant minerals at mantle conditions, and hence a better understanding of the properties and processes that occur within the Earth's interior.