The deformed lunar
zircon at center was brought from the moon by Apollo astronauts.
The fractures
characteristic of meteorite impact are not seen in most lunar zircons,
so the ages they
record probably reflect heating by molten rock, not impact.
Photo: Apollo
17/Nicholas E. Timms
(October 17, 2015) Phenomenally
durable crystals called zircons are used to date some of the earliest and most
dramatic cataclysms of the solar system. One is the super-duty collision that
ejected material from Earth to form the moon roughly 50 million years after
Earth formed. Another is the late heavy bombardment, a wave of impacts that may
have created hellish surface conditions on the young Earth, about 4 billion
years ago.
Both events are widely accepted but unproven, so
geoscientists are eager for more details and better dates. Many of those dates
come from zircons retrieved from the moon during NASA's Apollo voyages in the
1970s.
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.
This highly
shocked zircon, from the Vredefort Dome in South Africa,
shows thin, red
bands that are a hallmark of meteorite impact.
Photo: Aaron
Cavosie
"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."
Although many of their zircons show evidence of shock,
"once separated from host rocks, ex situ shocked zircons lose critical
contextual information," Cavosie says.
The "clock" in a zircon occurs as lead isotopes
accumulate during radioactive decay of uranium. With precise measurements of
isotopes scientists can calculate, based on the half life of uranium, how long
lead has been accumulating.
If all lead was driven off during asteroid impact, the clock
was reset, and the amount of accumulated lead should record exactly how long
ago the impact occurred.