Earth is often hit by rocks from our solar system, most of them from the asteroid belt, between Mars and Jupiter; these rocks usually disintegrate as they enter the atmosphere and do not have major consequences for the inhabitants of our planet. However, sometimes these rocks are big enough to trigger a cataclysm capable of destroying most species, which is what happened 66 million years ago. We know that this impact opened a crater that 180 kilometres in diameter.
The recent discovery of the oldest crater is also related to the end of the most intense and lasting glaciation that Earth experienced: the period "Snowball Earth”.
The remains of the impact in question have been found in western Australia, and correspond to a crater 70 kilometres in diameter, more than 229 million years old. The Yarrabubba crater, as it has been named, is 200 million years older than any other known crater on Earth.
But there's more.
Not only has this unprecedented crater been identified; it has been linked to one of the most interesting events in Earth’s history.
The "Snowball Earth" hypothesis still generates controversy. There is still no evidence that the Earth was ever covered with ice in its entirety, but what scientists are sure of is that during that period there were harsh glaciations, if not all, in most of the planet. The thaw brought with it a great explosion of marine life: the Cambrian period, in which organisms like trilobites, anomalocaris (insect ancestors) emerged and picaias, the first organisms with similarities to spines.
Researchers believe that the Yarrabubba crater is the vestige of an ancient impact that vaporised billions of gallons of water, freeing the Earth from its icy prison, and ushering in an unprecedented explosion of life. Until now, the hypothesis used by scientists to explain the end of this glacial period was an intense volcanic activity. However, with this discovery, we may have to rewrite not only the history of our planet but also our own.
How do scientists know how old the Yarrabubba crater is?
Identifying the age of ancient craters is not easy: these places tend to be poorly conserved because erosion and tectonic events such as earthquakes gradually erase in the geological past, as the researchers detail in their paper, published in the magazine Nature Communications.
The team searched for evidence of shock recrystallization in minerals such as zircon and monazite inside the crater. However, finding these things in the minerals involved searching for microscopic grains. Uranium in the grains helped scientists determine a precise date, which coincided with the great glaciation.
Researchers speculate that when the meteorite hit Yarrabubba, this place was covered in ice, like much of the rest of the Earth at the time: "The impact adjusts within the Earth’s context, coming out of cold conditions," they explained.
The massive attack could have sent billions of tons of vaporized ice into the atmosphere, according to the team: "On an ice sheet, the impact would have released a lot of water vapor, which is a greenhouse gas even more efficient than carbon dioxide,” according to lead author Timmons Erickson, from NASA’s Johnson Space Center and Curtin’s School of Earth and Planetary Sciences “This, in turn, could have caused global warming.”
What is actually a fact is only the crater’s age. Yarrabubba is, in fact, the oldest proof of a meteorite impact; but that this crater triggered the melting of the glaciers just a hypothesis that scientists have ventured.
Those sceptical about the findings of this study claim that large meteor impacts are more often associated with cooling events rather than with global warming. In addition, they question whether an object that leaves a crater of only’ 70 kilometres is capable of melting such an amount of ice, and triggering such decisive climate change.