Origin and evolution of Earth's water inventory

Abstract

Due to its formation well inside the planetary ‘snow line’, nascent Earth may have been deficient in water during its initial formation, with water being delivered late from the outer solar system by planetesimals and/or by pebble accretion. The terrestrial D/H seemingly rules out sourcing water directly from the solar nebular gas, and perhaps long period comets as well, but is in the range of chondrite meteorites, short period comets, and some Kuiper belt objects. However, a variety of processes could have modified the D/H of the ultimate source(s). It was long assumed that any acquired terrestrial water would have dissolved initially in a magma ocean that would later degas to form a secondary atmosphere (and early ocean) as the mantle crystallized. Alternatively, exoplanet observations suggest that rocky planets commonly form with hydrogen-rich envelopes that were subsequently lost and that reaction of magma with that enveloping gas could potentially produce large amounts of primary water. The maximum water storage capacity of the mantle may be as much as 4 ocean masses today or higher but decreases earlier in Earth history with higher mantle potential temperatures. Estimates of the hydrogen storage capacity of the core range up to 100 present ocean masses but the actual amount is largely unconstrained. Constant continental freeboard provides little constraint on the Archean to present ocean volume but simple parametrized convection models suggests that early global ocean mass cannot have exceeded about 1.5 times the present value given what we know about post-Archean subaerial exposure. Hadean zircons, particularly those obtained from the Jack Hills metaquartzites, yield multiple lines of geochemical evidence that imply the existence of liquid water at or near Earth's surface by ca. 4.3Ga. Some evidence exists that suggest that a global-scale reservoir may have then existed. It is now questionable that a Late Heavy Bombardment occurred in the inner solar system at ca. 3.9Ga and thus estimates of volatile species such an event could have contributed to Earth's inventory are largely speculative.