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The Hadean Aeon

The Hadean Aeon is the time between the formation of the planet, some 4½ thousand million years ago, and it beginning to take on a permanent appearance. The Hadean is now deemed to have ended 4 thousand million years ago.

This picture comes from Scientific American, October 94. It is intended to depict this transition from hellish to inhabitable.

It used to be thought that the young planet’s hellish phase lasted for a very long time – almost permanently molten, with a thick sulphurous atmosphere and a constant bombardment of mountain-sized chunks from space to boot. Nothing should have lasted long in such an environment. No land, no sea and certainly no life.

So it blew the geologists’ minds when they found some territory in Greenland which is very nearly 4 thousand million years old (more). For this land to have survived, it meant that our planet had already begun to settle down into old age. There can have been no serious upheavals from that day to this, or this territory would have been tipped back into the melting pot, like all the earlier land will have been.

But that wasn’t all. There are deposits of carbon in these rocks. And carbon normally means life. Not all scientists agree that it does in this case. But there no longer seems to be any reason why life shouldn’t have got going this early.

But that still isn’t all. The oldest things by far found so far are the zircons. And they speak of a planet that has remained reasonably intact ever since 4.4 thousand million years ago. That’s only around 100 million years after the planet was formed. Not only that, but the oldest zircons appear to have been formed with water around. Nobody had dreamed that the planet could have settled down so fast. Maybe life got started that early too.

We must be careful though. Once zircons have formed they are almost indestructible. We can say that the planet was reasonably cool when these zircons were formed. But all we can say about the next few hundred million years is that it was not so hellish as to completely destroy the rocks and the zircons that were hiding in them. Indeed, it’s highly likely that they had some pretty exciting adventures over that time.

Any life that did get started this early is quite likely to have been destroyed. Some scientists believe that life probably had to have several attempts, before conditions settled down enough for it to survive long term.