Asteroids may not hit our planet at regular intervals after all. Scientists have reached this conclusion after analyzing impact craters formed in the last 500 million years, concentrating on precisely dated events.
As late as 2015, researchers have indicated that impact craters were formed on Earth around every 26 million years.
“This question has been under discussion for more than thirty years now,” says Matthias Meier from ETH Zurich’s Institute of Geochemistry and Petrology. “We have determined, however, that asteroids don’t hit the Earth at periodic intervals.”
In the past, some researchers have suggested the existence of a companion star to the Sun—a dim dwarf star, believed to draw near to the Sun every 26 million years and cause asteroids to bombard Earth. If the theory were true, this would next occur in around 10 million years. However, the star, named Nemesis, after the Greek goddess of revenge, has never been found.
Today, scientists know of around 190 impact craters on Earth, with diameters ranging from a few meters to more than 100 kilometers. They range from just a few years to billions of years old.
For the study, published in Monthly Notices of the Royal Astronomical Society, Meier and his former doctoral student Sanna Holm-Alwmark now at Lund University restricted their analysis to craters formed within the last 500 million years, around the time the first complex life forms emerged.
Holm-Alwmark then discovered that some of the dates used in previous studies were false and ultimately came up with a list of 22 craters whose exact age is known to within one percent.
Meier then analyzed these impacts using circular spectral analysis (CSA) which places the timeline of events in a circle with a particular range—in this case, 26 million years. If events repeated themselves regularly within this timespan, the points would have arranged themselves in a particular area of the circle. The findings showed no pattern.
The researchers also determined that some of the impact craters were almost exactly the same age.
“Some of these craters could have been formed by the collision of an asteroid accompanied by a moon,” suggests Meier. “But in other cases, the impact sites are too far away from each other for this to be the explanation.” An example is the 66 million-year-old Chicxulub crater in Mexico, which has been linked to the extinction of the dinosaurs, and the Boltysh crater in the Ukraine, which was formed at almost exactly the same time.
“We have no definitive explanation for that,” says Meier. One possible cause could be a collision between two fragments in the asteroid belt, forming debris which might then have quickly found its way to Earth.
One thing is certain, however: craters with similar ages could distort the results of the analysis. “Our work has shown that just a few of these so-called impact clusters are enough to suggest a semblance of periodicity,” says Meier, explaining that because the researchers of the 2015 study overlooked the formation of these clusters, their statistical method led them in the wrong direction.
Source: ETH Zurich