impacts; magma oceans; segregation of the core; early forms of continents, oceans, and the atmosphere; the onset of plate tectonics; and, of course, the origin of life. Because Earth grew and differentiated rapidly, the energy available to the Earth system during its early history was far higher than today, permitting whole sets of physical and chemical processes without counterparts in the modern Earth. The overarching challenge here is to understand how Earth transitioned from its formative state into the hospitable planet of today (see Box 2.1). Lessons learned from the early Earth will help us interpret the processes occurring in the hundreds of extrasolar planetary systems now being discovered by astronomers.
Accretion of Earth
The birthplace of Earth was a protoplanetary accretion disk, a cloud of gas and dust surrounding the early Sun. Modern astronomy provides a glimpse of what this environment may have been like, in the form of debris disks that surround young stars, some of which have been imaged by the Hubble Space Telescope (see Figure 2.1). Accretion disks are subject to instabilities driven by powerful gravitational and electromagnetic forces that collect dust particles into planetesimals, typically 1-kilometer-sized objects that were the fundamental building blocks of Earth and the other terrestrial planets. Once a sufficient density of planetesimals developed in the nebular cloud, increasingly violent collisions began to dominate the accretion process, forming an ever-smaller number of growing planetary embryos that swept up most of the remaining nebular debris.
Although much effort has been directed toward understanding accretion from the perspective of solar system dynamics, many related processes that were important for early Earth have not received the same attention. Accretion models, for example, often assume that colliding planetesimals simply adhere, ignoring effects like fragmentation, spin and precession, melting, vaporization, condensation, and differentiation (Chambers, 2004). There is mounting evidence for these processes, many of which bear directly on the final chemistry and structure of the accreting body (Halliday, 2004).
Geochemical and cosmochemical observations provide important constraints on the timing and the mechanisms of accretion and segregation of the core, although several interpretations are possible. For example, in the Hafnium-Tungsten (Hf-W) system, the excess radiogenic 180W in the silicate Earth relative to chondritic meteorites has been interpreted as rapid accretion or alternatively as incomplete mixing of the impactor with the growing Earth (Halliday, 2008; Rudge et al., 2010). Similar interpretations have come from other short-lived isotope systems, such as146Sm-142Nd (O’Neil et al., 2008), which also have implications for the earliest crust. The fusion of geochemistry and geophysics offers many promising avenues for better understanding formative processes that governed the early history of the Earth.
Based on isotopic evidence from meteorites, what originated as occasional planetesimal collisions soon began to run away, leaving a small number of rapidly growing planetary embryos. Improved chronological methods reveal that melting and differentiation occurred within a few million years of the formation of the first solids, probably driven by collisions and assisted by now-extinct radioactive heat producers such as26Al and60Fe. Accordingly, the assumption that Earth formed by a continuous influx of small particles made of pristine solar system condensates has given way to a much more dramatic model, in which Earth was assembled by a relatively small number of traumatic collisions involving larger objects, some of these already having differentiated interiors and well as their own internal dynamics (Canup and Asphaug, 2001). Future progress on the processes and timing of Earth’s growth in the coming decade will rely on a diversity of approaches, including:
• Application of new isotope techniques for dating methods
• Closer integration of isotope geochemistry with astrophysical approaches to planetary formation
• More comprehensive and more realistic dynamical models of the accretion process
• Evolutionary studies of the chemistry and physics of planetesimal-sized objects and planetary embryos