How was the Earth-Moon system formed? New insights from the geodynamo.

The most widely accepted scenario for the formation of the Earth-Moon system involves a dramatic impact between the proto-Earth and some other cosmic body. Many features of the present-day Earth-Moon system provide constraints on the nature of this impact. Any model of the history of the Earth must account for the physical, geochemical, petrological, and dynamical evidence. These constraints notwithstanding, there are several radically different impact models that could in principle account for all the evidence. Thus, in the absence of further constraints, we may never know for sure how the Earth-Moon system was formed. Here, we put forward the idea that additional constraints are indeed provided by the fact that the Earth is strongly magnetized. It is universally accepted that the Earth's magnetic field is maintained by a dynamo operating in the outer liquid core. However, because of the rapid rotation of the Earth, this dynamo has the peculiar property that it can maintain a strong field but cannot amplify a weak one. Therefore, the Earth must have been magnetized at a very early epoch, either preimpact or as a result of the impact itself. Either way, any realistic model of the formation of the Earth-Moon system must include magnetic field evolution. This requirement may ultimately constrain the models sufficiently to discriminate between the various candidates.

Milankovitch cycles in banded iron formations constrain the Earth-Moon system 2.46 billion years ago.

The long-term history of the Earth-Moon system as reconstructed from the geological record remains unclear when based on fossil growth bands and tidal laminations. A possibly more robust method is provided by the sedimentary record of Milankovitch cycles (climatic precession, obliquity, and orbital eccentricity), whose relative ratios in periodicity change over time as a function of a decreasing Earth spin rate and increasing lunar distance. However, for the critical older portion of Earth's history where information on Earth-Moon dynamics is sparse, suitable sedimentary successions in which these cycles are recorded remain largely unknown, leaving this method unexplored. Here we present results of cyclostratigraphic analysis and high-precision U-Pb zircon dating of the lower Paleoproterozoic Joffre Member of the Brockman Iron Formation, NW Australia, providing evidence for Milankovitch forcing of regular lithological alternations related to Earth's climatic precession and orbital eccentricity cycles. Combining visual and statistical tools to determine their hierarchical relation, we estimate an astronomical precession frequency of 108.6 ± 8.5 arcsec/y, corresponding to an Earth-Moon distance of 321,800 ± 6,500 km and a daylength of 16.9 ± 0.2 h at 2.46 Ga. With this robust cyclostratigraphic approach, we extend the oldest reliable datum for the lunar recession history by more than 1 billion years and provide a critical reference point for future modeling and geological investigation of Precambrian Earth-Moon system evolution.

Emergence of Life on Earth: A Physicochemical Jigsaw Puzzle.

We review physicochemical factors and processes that describe how cellular life can emerge from prebiotic chemical matter; they are: (1) prebiotic Earth is a multicomponent and multiphase reservoir of chemical compounds, to which (2) Earth-Moon rotations deliver two kinds of regular cycling energies: diurnal electromagnetic radiation and seawater tides. (3) Emerging colloidal phases cyclically nucleate and agglomerate in seawater and consolidate as geochemical sediments in tidal zones, creating a matrix of microspaces. (4) Some microspaces persist and retain memory from past cycles, and others re-dissolve and re-disperse back into the Earth's chemical reservoir. (5) Proto-metabolites and proto-biopolymers coevolve with and within persisting microspaces, where (6) Macromolecular crowding and other non-covalent molecular forces govern the evolution of hydrophilic, hydrophobic, and charged molecular surfaces. (7) The matrices of microspaces evolve into proto-biofilms of progenotes with rudimentary but evolving replication, transcription, and translation, enclosed in unstable cell envelopes. (8) Stabilization of cell envelopes 'crystallizes' bacteria-like genetics and metabolism with low horizontal gene transfer-life 'as we know it.' These factors and processes constitute the 'working pieces' of the jigsaw puzzle of life's emergence. They extend the concept of progenotes as the first proto-cellular life, connected backward in time to the cycling chemistries of the Earth-Moon planetary system, and forward to the ancient cell cycle of first bacteria-like organisms. Supra-macromolecular models of 'compartments first' are preferred: they facilitate macromolecular crowding-a key abiotic/biotic transition toward living states. Evolutionary models of metabolism or genetics 'first' could not have evolved in unconfined and uncrowded environments because of the diffusional drift to disorder mandated by the second law of thermodynamics.

On the evolution of the protolunar disc.

The structure and viscous evolution of a post-impact, protolunar disc is examined. The equations for a silicate disc in two-phase (vapour-liquid) equilibrium are employed to derive an analytical solution to vertical structure. Both a vertically mixed phase disc and a stratified disc, where a magma layer exists in the mid-plane surrounded by a vapour reservoir, are considered. The former largely reproduces the low gas mass fraction, x≪1, profiles of the disc described in earlier literature that proposed that the disc would hover on the brink of gravitational instability. In the latter, the vapour layer has x∼1 and is generally gravitationally stable, while the magma layer is vigorously unstable. The viscous evolution of the stratified model is then explored. Initially, the disc quickly settles to a quasi-steady state with a vapour reservoir containing the majority of the disc mass. The magma layer viscously spreads on a time scale of approximately 3-4 years, during which vapour continuously condenses into droplets that settle to the mid-plane, maintaining the magma surface density in spite of disc spreading. Material flowing inwards is accreted by the Earth; material flowing outwards past the Roche boundary can become incorporated into accreting moonlets. This evolution persists until the vapour reservoir is depleted in approximately 50-100 years, depending on its initial mass.

Lunar exploration: opening a window into the history and evolution of the inner Solar System.

The lunar geological record contains a rich archive of the history of the inner Solar System, including information relevant to understanding the origin and evolution of the Earth-Moon system, the geological evolution of rocky planets, and our local cosmic environment. This paper provides a brief review of lunar exploration to-date and describes how future exploration initiatives will further advance our understanding of the origin and evolution of the Moon, the Earth-Moon system and of the Solar System more generally. It is concluded that further advances will require the placing of new scientific instruments on, and the return of additional samples from, the lunar surface. Some of these scientific objectives can be achieved robotically, for example by in situ geochemical and geophysical measurements and through carefully targeted sample return missions. However, in the longer term, we argue that lunar science would greatly benefit from renewed human operations on the surface of the Moon, such as would be facilitated by implementing the recently proposed Global Exploration Roadmap.