The end of the bombardment

We haven't discussed the "Late Heavy Bombardment" on this blog. As of 2024 we've rather been skeptical. We may, however, have some timeline for the end of some kind of bombardment: Universe Today reports from Tim[othy] Johnson. And from Hamed Gamaleldien and some others.

The issue here is that the Hadean has, in fact, ended - but the crusts came later than that ending, agreed 4.03 Gya. Crust first formed 3.9 Gya. There should have been, what, 400?500? My after the great Theia crash and all its debris; Earth should have cooled enough by then. Johnson argues that the crust had been heated exogenously. That is, Theia wasn't the last crash.

The "L" part of the LHB implies a surge at the 4.03-3.9 Gya ... I guess we can call it a "period", 130 My as it would be. We've been marking this period as the Archaean beginning. Johnson doesn't need a surge. A continual pounding from our uncaring universe would do the job just as well.

The constant heat over this extent of time continued the process of differentiation, at least at the crustal layers. We have a mostly "felsic" land surface up here, dominated by silicon and aluminium and all those lithophiles... just like the Lunar regolith. Granite looks compositionally felsic. The "mafic" rock, of iron and its siderophiles, clusters in some rare upwellings and on the ocean floor. That's básalt or should we say défault.

Our Hadean is defined by committee, basically: Ten oldest zircon U-Pb ages in the Acasta Gneiss up in Canada's NW Territory. Gneiss is metamorph granite so it was a crust once, exposed by glacier. These zircons are however preserving siderophilic (heavy) metals. So I dunno. They might be mafic and precede our crust - and the crust of Acasta.

Johnson and Gamaleldien earlier had evidence for water in those transCanadian zircons, which they haven't disavowed. So I guess they can continue to have a waterless Hadean, just like in the Gospel of Luke. Archaean is, then, defined by still-no-crust but now with uneasy pools of water. Maybe a steam period.