Widespread mixing and burial of Earth’s Hadean crust by asteroid impacts

Abstract

The history of the Hadean Earth (4.0–4.5 billion years ago) is poorly understood because few known rocks are older than 3.8 billion years old1. The main constraints from this era come from ancient submillimetre zircon grains2,3. Some of these zircons date back to 4.4 billion years ago when the Moon, and presumably the Earth, was being pummelled by an enormous flux of extraterrestrial bodies4. The magnitude and exact timing of these early terrestrial impacts, and their effects on crustal growth and evolution, are unknown. Here we provide a new bombardment model of the Hadean Earth that has been calibrated using existing lunar4 and terrestrial data5. We find that the surface of the Hadean Earth was widely reprocessed by impacts through mixing and burial by impact-generated melt. This model may explain the age distribution of Hadean zircons and the absence of early terrestrial rocks. Existing oceans would have repeatedly boiled away into steam atmospheres as a result of large collisions as late as about 4 billion years ago.

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Figure 1: Mass accreted by the Earth during the late accretion phase.
Figure 2: Spatial distribution and sizes of craters formed on the early Earth.
Figure 3: Melt production by large impacts on the Earth.
Figure 4: Detrital Hadean zircon ages compared with the computed distribution of impact-generated ages.

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Acknowledgements

We thank H. J. Melosh, S. J. Mojzsis, T. M. Harrison, A. J. Cavosie, A. I. S. Kemp, W. L. Griffin, M. M. Wielicki, R. J. Walker, E. B. Watson, P. Holden, O. Abramov, R. M. Canup, H. F. Levison, D. Nesvorny, O. Nebel, N. H. Sleep and N. Arndt for comments and criticisms that helped to shape the current paper. We gratefully acknowledge the developers of iSALE-2D/3D (http://www.isale-code.de/). S.M., W.F.B. and D.A.K. received support from the NASA Solar System Exploration Research Virtual Institute grant no. NNA14AB03A and NNA14AB07A; K.W. and M.B. were supported by the Helmholtz-Gemeinschaft Deutscher Forschungszentren e.V. Alliance ‘Planetary Evolution and Life’. A.M. was supported by the European Research Council Advanced Grant ‘ACCRETE’ (contract number 290568).

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Authors

Contributions

S.M. conceived the paper, built the Monte Carlo code and executed the simulations. W.F.B. and A.M. contributed to calibration and testing of the code, and to performing the fit of zircon age distributions. M.B. and K.W. executed iSALE impact simulations and processed the results. L.T.E.T. performed the geophysical interpretation of the output of the simulations. D.A.K. interpreted the melt volume. All authors contributed to the discussion of the results and their implications and to the crafting of the manuscript.

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Correspondence to S. Marchi.

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Extended data figures and tables

Extended Data Figure 1 Early Earth’s impactor size–frequency distributions.

The red curve corresponds to current main-belt asteroids larger than 10 km. The largest object is Ceres, whose diameter is 913 km. The black curve (vertically shifted for clarity) is a replicate of the main-belt curve, extrapolated to 4,000 km by using the slope in the size range 500–913 km.

Extended Data Figure 2 Lunar impact fluxes.

The differential number of lunar craters >20 km (N20) as a function of time and per unit surface for several scenarios discussed in the text.

Extended Data Figure 3 Solidus and geotherm used in iSALE simulations.

Note the temperature increase in the lithosphere that results in an increase in the temperature of buried surface material. Other processes resulting in an increase of the temperature of the buried crust are discussed in the text. The assumed thermal gradient is a lower limit (see Extended Data Table 1), implying that the increase in the temperature of the buried material can be significantly higher than is shown here.

Extended Data Figure 4 Impact-generated melt volume.

Left, comparison of melt volume production for various methods; right, comparison of melt volume production for various impact velocities and mantle potential temperature (see the text for more details).

Extended Data Figure 5 Melt spreading over the first 100 Myr of Earth history.

Mollweide projections of the cumulative record of craters at four different times. There are portions of the Earth’s surface that are not affected by impact-generated melt at each time step, except for the first 25 Myr (or >4.475 Gyr). However, there is no significant fraction of the Earth’s surface that is unaffected by impacts before 4 Gyr ago (see also Fig. 2b). Impacts therefore set the stage for the environmental conditions on the Hadean Earth and have implications for the origin and development of life (see, for instance, discussion in refs 78,79,80).

Extended Data Figure 6 Minimum impact time for projectiles larger than 500 km.

Blue dots indicate the minimum impact time for impactors larger than 500 km recorded in 163 successful Monte Carlo simulations (see Fig. 1). The vertical axis reports the number of impacts (in bins of 25 Myr each) normalized by the number of simulations. The lowest y values shown in the plot correspond to one impact. The median time is 4.32 Gyr ago, and the mean is 4.27 Gyr ago. The earliest evidence of life on Earth (3.8 Gyr ago), and the start of the Late Heavy bombardment (4.15 Gyr ago; see the text) are also indicated. About 10% of the simulations have a minimum time of 4 Gyr ago or less.

Extended Data Table 1 Various parameters used for iSALE simulations

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Marchi, S., Bottke, W., Elkins-Tanton, L. et al. Widespread mixing and burial of Earth’s Hadean crust by asteroid impacts. Nature 511, 578–582 (2014). https://doi.org/10.1038/nature13539

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