A recent AGU Geophysical Monograph, The Early Earth: Accretion and Differentiation, provides a multidisciplinary overview of the state of the art in understanding the formation and primordial evolution of the Earth. The evolution of the Earth from a molten ball of metal and magma to the tectonically active, dynamic, habitable planet that we know today is unique among the terrestrial planets. AGU asked the editors of the book to highlight some of the important results that have emerged from a wide range of disciplines and some of the important questions that remain.
Q1: What was the motivation for this book?
For several consecutive years we organized topical sessions at the AGU Fall Meeting aimed at bringing together scientists across a range of disciplines investigating aspects of the origin and earliest evolution of Earth. At some point we realized that there has not been a collection of papers on the subject for decades, and that the field has progressed in so many areas that the time seemed right to put together a book that would synthesize the state-of-the-art.
Q2: The book brings together authors and work using a variety of recent techniques and methods on the formation of the solar system and Earth and follows on recent sample returns and other new data. Is there a particular new view that is emerging from this synthesis and combination?
Yes, the book is wide in scope and we aimed for a series of papers that would bring into focus the main questions around how our Earth formed and evolved in the first few hundred million years, starting from condensation and accretion of primitive objects in the solar nebula, right through to core formation and magma ocean evolution and crystallization.
We think there are lots of new views since the last monograph on this subject in the 90’s, although many unknowns and healthy debates remain. There has been tremendous progress in better understanding the timing of events through the advent of short-lived isotopic tracers like 182W and 142Nd, and these have revolutionized our understanding of core formation and early magmatic differentiation in terrestrial objects, sparked fantastic debates about early differentiation, and challenged the long-held ‘chondrite’ model for bulk planetary composition.
For instance, while the old school of thought was that Earth accreted from chondrites, we now know thanks to planetary dynamical modeling that the Earth and most probably the other terrestrial planets accreted a range of larger already differentiated objects, such as planetesimals or even proto-planets. While the chondritic model is still by and large accepted as a starting point for planetary building blocks, we all have come to terms with the fact that no terrestrial planet was built from chondrites per se, and that there are a series of steps leading from chondrites to planets that are still being elucidated.
Core formation has greatly benefited from experimental and theoretical advances, resulting in coordinated efforts for comprehensive models that explain all aspects of the problem, rather than just focusing on one part and disregarding the others. In that sense, the most recent models attempt to explain simultaneously core formation in terms of (1) the depletion of siderophile elements in the mantle, (2) the isotopic fractionation of elements impacted by core formation, and (3) the density-deficit related to the light element content of the core.
Great strides have also occurred in geodynamic modeling of deep magma ocean processes, with entirely new models for how the magma ocean crystallized and evolved, the fact that the early molten Earth may not have frozen from the bottom up, but maybe from the top down, or from the center outwards, and these may explain some of the large scale features we see today at the base of the mantle.
Lastly, our better understanding of the Moon and the Earth-Moon system is allowing us to make strong statements about Moon-forming scenarios and those obviously allow understanding the formation of the Earth in a planetary perspective.
Q3: What would you say are the two or three major outstanding questions (among many) and how might we go about addressing them?
We need to understand how the Earth became a solid planet, what geochemical reservoirs formed early on, and whether they survived or not to this day. We need better experiments in the laser-heated diamond anvil cell on the detailed phase relations of crystallization of the magma ocean in the deep mantle and trace element partitioning at extreme conditions. We also need a new class of geodynamical models that allow us to simulate magma ocean crystallization, by bridging the tremendous gap between solid and liquid convection models.
Another area of active research is in accretion models that account for erosion and loss of material – accretionary differentiation – as well as those that link volatilization and core formation. How much material was eroded and lost from the Earth or Earth-Moon system by early impacts? How much was scavenged by the core? How (and how much) was the “Late Veneer” delivered to Earth, and what was it made of?
We also really need to nail down the composition of the core, especially in terms of light elements. Is the core exsolving mantle components or dissolving them at present, and what exactly is occurring at the core-mantle boundary? To what extent are the core and mantle in equilibrium and what exactly are they exchanging?
Earth is a habitable planet because, among other reasons, it has an active dynamo generating a magnetic field. Geodynamicists know how to produce a field up to 1-1.5 billion years ago, but not earlier than that. So what made the early dynamo go, and how can we produce a magnetic field as early as 4.2 billion years ago?
The structure of the deep mantle is becoming more complicated by the day, as seismologists user ever-evolving tools to discover new patches, piles, plumes, and slabs in the deep mantle. How can we interpret those in terms of composition or temperature? How can we interpret complex anisotropy them in terms of dynamics? What are LLSVPs? Or ULVZs? What exactly is the D’’ layer; is it compositionally distinct? Or dynamically/thermally different from the rest of the mantle? With all these new results, the debate about whole mantle vs. layered mantle convection is long forgotten: but what new view is emerging in terms of mixing scales, times, and model for the mantle?