Final Report Summary - ACCRETE (Accretion and Early Differentiation of the Earth and Terrestrial Planets)
The ERC Advanced Grant “ACCRETE” has resulted in major advances in our understanding of the chemistry of the early Solar System and how the terrestrial planets (Earth, Venus and Mars) formed and evolved chemically and physically. The advances were made by combining, for the first time, the physics and chemistry involved in the formation of the metallic cores of the planets with computer simulations of planetary growth by accretion (that occurred by collisions with other bodies). Constraints on modeling this complex process were provided by the chemical composition of Earth’s silicate rocky mantle and its core-mantle mass ratio. By fitting the output of the combined model to these parameters has enabled a wealth of new information to be obtained about the early Solar System and Earth’s formation. For example, the chemical compositions of all early-formed bodies that accreted to form the planets have been determined and show that there must have been a strong oxidation gradient in the early Solar system: bodies that formed close to the Sun were highly reduced (oxygen poor) such that all iron was present as metal whereas in bodies from beyond the current orbit of Jupiter all iron was present in an oxidized form as FeO. The chemical evolution of the mantles and cores of the planets during accretion is predicted and results show that chemical equilibration between metal and silicate during core formation occurred at great depth in the Earth, close to the core-mantle boundary. This means that the most energetic collisions between the early Earth and smaller (e.g. Mars-size) bodies caused much, or all, of the Earth to melt because temperatures increased to 4000 °C or more during such events. Additional results include: a new and novel determination of the age of the Moon, new information on when the volatile components water, sulfur and carbon were added to the Earth and other planets, and a new mechanism to explain how the highly-siderophile elements (HSEs) platinum, palladium, ruthenium and iridium were largely extracted to the Earth’s core and are therefore extremely rare in the crust and mantle. Concentrations of HSEs in the lunar mantle are strongly depleted compared with the Earth’s mantle and we developed a new model that explains the differences; this model also has important implications for the lunar bombardment – which is shown to result from the tail end of planetary accretion and not from a pronounced spike in the impact history as previously believed.