HIGH-PRESSURE RESEARCH: APPLICATIONS TO EARTH, PLANETS AND BEYOND

High-pressure research has revolutionized the ability to understand the properties and processes determining the geological evolution of planetary interiors. Collapse of atoms, changes in chemical bonding character (e.g., from lithophile to siderophile) and rearrangements of atomic packing – both in crystals and liquids – have all been documented at the million-atmosphere (Mbar = 10^11 Pa) pressures of planetary interiors. This metamorphism of atomic orbitals and bonds characterizes the bulk of the mineral and fluid properties inside our planet, including intense chemical reactions in the D'' region: the contact-metasomatic zone between the iron alloy of the liquid outer core and the silicate minerals of the lowermost mantle. Dynamic experiments, emulating the shock waves characteristic of hypervelocity impact – the most important process during the origin, growth and early evolution of planets – continue to yield key measurements of density and elasticity at the high pressures and temperatures occurring deep inside planets. Static experiments, notably with laser-heated diamond cells at synchrotron facilities, reveal major changes in crystal and melt structures as well as in atomic orbitals, for example confirming that Fe2+ undergoes a volume collapse at mantle pressures due to a high-spin —> low-spin transition, as predicted 40 years ago in the geochemical literature. A new class of experiments combine static and dynamic approaches, with samples that are compressed inside diamond cells being subjected to laser-driven shock waves. These experiments can achieve Gbar pressures, relevant to the interiors of giant and super-giant bodies that now represent the vast majority of known planets, those found outside our Solar System. Such conditions also open a window to “kilovolt chemistry,” a new regime of chemical bonding involving core atomic orbitals rather than being limited to the valence electrons that control bonding at Earth-surface conditions.