Widely accepted but never actually proven, the massive collision that caused material to be ejected from the Earth to form the moon may not have happened 50 million years after the planet’s formation, experts from the University of Wisconsin-Madison now claim.
Likewise, the late heavy bombardment, a wave of impacts which could have caused hellish conditions on the surface of the young world, may not have occurred four billion years ago, despite longstanding claims by the scientific community. It’s not that researchers behind the study doubt the events occurred; rather, it’s the dating process they call into question.
These cataclysmic events are dated using exceptionally durable crystals called zircons, Aaron Cavosi of UW’s NASA Astrobiology Institute and his colleagues explained. Many of the details of both events were discerned from the analysis of zircons collected by Apollo astronauts on the moon four decades ago, and the researchers believe there may be issues with this method.
Specifically, they write in the journal Geology, the lunar zircons are “ex situ” (meaning that the minerals were removed from the rocks in which they formed). This is problematic since it means that geoscientists are unable to review other evidence that could corroborate the impacts.
Zircons may reveal age not of impacts, but of surrounding rocks
“While zircon is one of the best isotopic clocks for dating many geological processes, our results show that it is very challenging to use ex situ zircon to date a large impact of known age,” noted Cavoisie. While many zircons show evidence of shock, he said, “once separated from host rocks, ex situ shocked zircons lose critical contextual information.”
A zircon’s so-called “clock” occurs as lead isotopes accumulate during the radioactive decay of uranium, the UW researchers explained. Using precise isotope measurements based on the half-life of uranium, scientists can calculate how long lead has been accumulating. If the lead was all lost during impact, it resets the clock and should reveal how long ago a collision took place.
Using this technique, the date of the late heavy bombardment was placed at between 3.9 billion and 4.3 billion years ago. However, Cavosie and colleagues collected and tested zircons from an impact site in South Africa, and found that while the zircons contain signs of shock deformation, they recorded the age of the rocks they formed in, which were about one billion years older than the impact itself. The discovery could force a re-evaluation of the planet’s early history.
“For a long time people have been saying if zircon is really involved in a major impact shock, its age will be reset, so you can date the impact,” explained UW-Madison geoscience professor John Valley. “Aaron has been saying, ‘Yes, sometimes, but often what people see as a reset age may not really be reset.’ Zircons are the gift that keep on giving, and this will not change that, but we need to be a lot more careful in analyzing what that gift is telling us.”
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Feature Image: Apollo 17/Nicholas E. Timms
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