#	Pagina
attuale pagina	/open-h2020/projects/201179/results.html

Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - FETA (Fluid impacts in EarTh Accretion)

Teaser

Summary

Thanks to extinct radioactivity we know that the Earth formed 4.6 billion years ago by successive collisions between planetary objects. After each collision, the colliding objects merge and form a larger and larger planet. It took about 100 million years to form the fully-grown Earth with a metallic core overlaid by a rocky mantle.

The large impacts that formed the Earth set the initial temperature and composition for the core and the mantle. They therefore determined the long-term evolution of our planet, including the initiation of plate tectonics, the generation of the geomagnetic field, the formation of oceans and atmospheres, and the development of life. Remote in space and time, Earth formation by large impacts is poorly understood and fundamental questions remain: How did the core and the mantle of the Earth form? What were their initial composition and temperature? Answering these questions was the main objective of this project.

Much of what we know about Earth formation comes from geochemical observations. Isotopic data tell us about the timing of core and mantle differentiation. The composition of Earth\'s mantle constrains pressure and temperature. Following an impact, chemical species partition into the core and mantle. Therefore, in order to interpret the geochemical observations, we must know the efficiency of chemical transfers between the core and the mantle, which depends on physical processes. Unfortunately the physics of Earth differentiation by impacts had been little investigated.

After each impact, prodigious amounts of energy were released, melting the impactor and the proto-Earth (figure 1). The metallic core of the impactor was then released into the Earthâ€™s liquid mantle, called magma ocean. This process is highly turbulent: inertia is large compared to viscous forces. Turbulence generates small-scale mixing between the impactor core and the mantle silicates. Such mixing allows for chemical transfers between the two liquids. The specific objective of this project was to predict the efficiency of mixing and chemical transfers. This was key to deciphering the geochemical observations and understanding the origin of the Earth, planets, and exoplanets.

Work performed

Planetary impacts have been extensively studied using numerical simulations. However, simulations resolve length scales larger than 100 km while chemical transfers are efficient on length scales smaller than 1 cm. Thus, existing simulations cannot constrain the efficiency of chemical transfers during an impact.

The novelty of our approach was to develop analog experiments of planetary impacts, using the principles of dimensional analysis to scale down the geophysical system. Our experiments reproduce the large-scale flow observed in impact simulations but also produce turbulence and small-scale mixing, allowing estimation of chemical transfers. We developed two experimental setups to investigate deformation and mixing following planetary impacts.

First, we investigated the impact of a liquid volume (analog for the impactor) into a water pool (analog for the magma ocean) in order to estimate mixing by the impact itself (figure 2). We developed the first release mechanism able to produce large and spherical liquid impactors. Second, we investigated mixing in the aftermath of an impact, when the impactor core falls into the magma ocean (figure 3).

MAIN RESULTS:

Recent fluid dynamical models estimate mixing following an impact, but they entirely neglect the inertia of the impactor. Using our impact experiments (figure 2), we investigated the effect of the impact velocity on mixing between the impactor and the mantle silicates. We obtain scalings for mixing as a function of the impactor velocity and size. Applied to Earth formation, we predict full chemical equilibration for impactors much smaller than the proto-Earth but partial equilibration for giant impacts. We found that the degree of chemical equilibration is up to five times larger than in previous models. Our new scaling therefore affects the interpretation of geochemical data.

With our second experimental setup, we developed the first release mechanism able to inject a dense liquid volume into another immiscible liquid at high speeds (up to 3 m/s) (figure 3). This technical achievement allowed us to mimic the fall of the impactor core into the Earthâ€™s mantle after an impact. We observed fragmentation of the released liquid into drops (figure 3). We obtained scaling laws for the size of the resulting fragments as a function of injection velocity and surface tension. Applied to Earth formation, our scalings predict that the impactor core fragments into sub-millimeter fragments in Earthâ€™s mantle. Such fragments are small enough to allow for chemical equilibration of the entire impactor core.

Our impact experiments were then extended to investigate smaller planetary impacts, as these that formed the craters at the surface of the Earth, the Moon and other planets. In contrast with the giant impacts of Earth formation, a smaller impact does not melt entirely the target material. To model such impacts, we investigated the impact between yield-stress fluids. These are good analogs for solid rocks: they behave as solids under low stress but as liquid above a critical stress. We showed that our experiments reproduce the morphology of impact craters at the surface of planets. We obtained regime diagrams for the crater morphology as a function of the impact velocity, the yield-stress and the impactor size. Then, we characterized the final dispersion of the impactor into the target material. Applied to planetary impacts, these results will help to estimate the impactor size and the impact speed from geological observations.

Final results

SIGNIFICANCE:

Planet formation is at the interaction between geochemistry, geophysics, planetary sciences, fluid dynamics and astrophysics. This project has been among the first attempts to connect these fields, incorporating geophysical constraints into astrophysical and geochemical models of Earth formation. Our results have brought quantitative constraints on chemical transfers between metal and silicates during and following the large impacts of Earth formation. With these constraints the community is now able to fully interpret geochemical and isotopic data in terms of the timing, temperature and pressure of Earth formation.

WIDER SOCIETAL IMPLICATIONS:

In addition to its importance for Earthâ€™s formation, the dynamics of impacts and fragmentation in immiscible liquids are key to a number of environmental and industrial systems. For instance, the sudden release and fragmentation of oil in the deep ocean that occurred during the Deepwater Horizon disaster (Gulf of Mexico, 2010) resulted in the largest offshore oil spill in history. The Deepwater Horizon oil emission came from two turbulent plumes in which oil fragmented into submillimeter droplets. Our experiments in figure 3 bring key constraints on the drop size and therefore on the rate of chemical transfers between the drops and the ocean. This is key to predicting and mitigating the impact of such events on our environment.