What came after the giant impact?

One of the nice things about our regular Wednesday research group meeting is that it provides a great opportunity to exchange information about what’s new and exciting in the field. It all helps to keep you up to date, in what is, a fast moving and exciting area of research. So it was last Wednesday, while discussing two important new papers about the giant impact (see recent blog), Sam Faircloth brought my attention to a really neat paper by Simone Marchi, Robin Canup and Richard Walker.

In their paper published in Nature Geoscience, Marchi and colleagues present the results of smoothed-particle hydrodynamic impact simulations looking at the bombardment of Earth by planetesimals, just after the Moon-forming giant impact. Based on highly siderophile element (HSE) data, it has long been known that Earth received a disproportionate amount of late arriving material compared to the Moon. The suggestion has been that this can be best explained in terms of the impact with Earth of a relatively small number of large asteroids, probably at least 1,500 km in diameter. Based on the assumption that all this material remained in the Earth’s mantle, a figure of 0.5% of the present day Earth’s mass is generally accepted as being the amount added by this so called “late veneer”.

Marchi and colleagues point out that such large asteroids are likely to have been differentiated into a metallic core and silicate-rich crust and mantle. Their simulations show that a large proportion of these planetesimals’ cores would likely have descended to the Earth’s core and so not have remained in the mantle. The simulations also show that a fraction of these planetesimal cores would have been scattered back into space, but could have re-impacted at a later date. The loss of planetesimal core material to the Earth’s core has important implications for estimates of how much material was added to the Earth after the giant impact, because in a differentiated asteroid the HSEs will largely be in the core.

The study by Marchi and colleagues suggests that we may have significantly underestimated the amount of material added to Earth after the giant impact. They point out that it could have been as much as five times greater than previous estimates, based of HSE abundance data. A further important implication of the study is that localized zones in the Earth’s mantle could have become enriched in projectile material. This may have resulted in small-scale deviations in the isotopic composition of tungsten. Previously such anomalies have been regarded as leftovers from the main phase of Earth’s accretion prior to the giant impact. This interpretation is problematic for high-energy giant impact scenarios, which would be expected to result in almost complete mantle homogenisation.  By proposing that such anomalies are a relatively late feature, Marchi and colleagues provide an interesting solution to this puzzling paradox.  It will certainly generate a great deal of debate.

Image: NASA

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