Robert Farrar sent in this link to www.sciencedaily.com/releases/2015/10/151017152343.htm … durable crystals known as zircons are used to date some the earliest and most dramatic cataclysms that occurred in the solar system including a super collision that is thought to have produced the Moon. This was somewhat prior to another big event involving a hypothetical mass bombardment of the Earth and the Moon by meteors, comets and asteroids. All this is safely dated 4 billion years ago.
These are consensus statistics we might say but are actually unproven hypotheses. This is the big thing to keep in your head. Did they really happen? No reason why not but in dating them the properties of zircon are used. Geoscientists look for clues – and this includes studying zircon. This was picked up from the Moon by the Apollo astronauts and has been studied intensively. It is also found in ancient meteorites on Earth, and projected impact sites. Zircon is used to date the worlds in collision that produced the Moon to 4.3 billion years ago and the bombardment episode to 3.9 billion years ago. As is the custom such dates very quickly become fixed and are reproduced in all manner of other research articles as if they are facts – but are they? Well, it seems some scientists have sought to test the ability of zircon to date such events by looking at an impact site in South Africa that is dated 2 billion years ago but they are coming up with dates a billion years older. It would therefore seem there is just a smidgeon of faith involved in accepting the dates around 4.3 billion years ago for the origin of the Moon.
See also www.sciencedaily.com/releases/2015/10/151015132119.htm
Meanwhile, at http://phys.org/print364487583.html …UCLA scientists have found evidence of life on Earth existing as early as 4.1 billion years ago – by using zircon dating. They are saying that life on Earth began earlier than the bombardment event at 3.9 billion years ago. Early Earth, in their view, was not a hell hole (the consensus picture). It was not dry and inhospitable or at boiling point. It was quite nice in fact – even if it was rained on by a lot of cosmic bodies. Apparently it was something like today's Earth. To arrive at such a conclusion they looked at zircons found in magma formations in Australia.
See also www.dailygalaxy.com/my_weblog/2015/10/-life-on-earth-may-have-started-al… …and makes the point the scientists involved are confident in the zircon dating methodology.