Earth's deep mantle: Structure, composition, and evolution

Understanding the inner workings of our planet and its relationship to processes closer to the surface remains a frontier in the geosciences. Manmade probes barely reach 10 km depth and volcanism rarely brings up samples from deeper than 150 km. These distances are dwarfed by Earth's dimensions, and our knowledge of the deeper realms is pieced together from a range of surface observables, meteorite and solar atmosphere analyses, experimental and theoretical mineral physics and rock mechanics, and computer simulations. A major unresolved issue concerns the nature of mantle convection, the slow (1-5 cm/year) solid-state stirring that helps cool the planet by transporting radiogenic and primordial heat from Earth's interior to its surface. Expanding our knowledge here requires input from a range of geoscience disciplines, including seismology, geodynamics, mineral physics, and mantle petrology and chemistry. At the same time, with better data sets and faster computers, seismologists are producing more detailed models of 3-D variations in the propagation speed of different types of seismic waves; new instrumentation and access to state-of-the-art community facilities such as synchrotrons have enabled mineral physicists to measure rock and mineral properties at ever larger pressures and temperatures; new generations of mass spectrometers are allowing geo-chemists to quantify minute concentrations of diagnostic isotopes; and with supercomputers geodynamicists are making increasingly realistic simulations of dynamic processes at conditions not attainable in analogue experiments. But many questions persist.