schwit1 writes: A new study has raised questions about the methods scientists have used to date the late heavy bombardment in the early solar system. According to the University of Wisconsin-Madison: "A study of zircons from a gigantic meteorite impact in South Africa, now online in the journal Geology, casts doubt on the methods used to date lunar impacts. The critical problem, says lead author Aaron Cavosie, a visiting professor of geoscience and member of the NASA Astrobiology Institute at the University of Wisconsin-Madison, is the fact that lunar zircons are ex situ, meaning removed from the rock in which they formed, which deprives geoscientists of corroborating evidence of impact. 'While zircon is one of the best isotopic clocks for dating many geological processes,' Cavosie says, 'our results show that it is very challenging to use ex situ zircon to date a large impact of known age.'" The problem is that the removal of the zircon from lunar rocks changes the data enough to make the dating unreliable. The method might work on Earth, but the dating done on Apollo samples can be questioned. This means that much of the supposed history of the solar system, centered on what planetary scientists call the late heavy bombardment, a period 4 billion years ago when the planets were being hit by innumerable impacts as they cleared the solar system of its dusty debris disk, might not have happened as dated from lunar samples. If so, our understanding of when that bombardment ended and life began to form on Earth might be considerably incorrect.