The simulations that captured the localized effects of individual large impacts also produced wholesale recycling of crust back into the mantle, with material dripping down to depths of at least 600 kilometers. Johnson thinks this recycling explains why so little Hadean crust survived to the present. It also explains, he argues, the near-total absence of shock-deformed Hadean zircons in the geological record. The researchers suggest that with so much melt present at shallow depths, it would have absorbed and scattered shock waves before they left lasting deformation in surviving crystals.
A turning point
The impact flux didn’t stay high forever; it declined more or less exponentially. Between 3.9 and
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