Asteroids Crash Into Earth Over Twice as Often as 290 Million Years Ago

Earth is being bombarded by over twice as many asteroids as it was 290 million years ago, according to research published Thursday in Science.

The study’s authors, led by University of Toronto planetary scientist Sara Mazrouei, speculate that this uptick might be the result of an ancient collision between objects in the asteroid belt that lies between the orbits of Mars and Jupiter. Such a disruption could produce “long-lived surges” of “kilometer-sized impactors” on Earth, the team said. These kilometer-scale asteroids are thought to impact Earth every 600,000 years on average.

The finding was made using a novel technique that reconstructed Earth’s impact history by examining the Moon’s craters. To get a sense of how much more bombardment Earth and the Moon have been experiencing, check out this sonic version of the impact rate over over the past billion years. The video, made by the Toronto-based group SYSTEM Sounds, assigns bell sounds to asteroid impacts that left lunar craters larger than 10 kilometers in diameter.

Space rocks have left craters and meteorites on Earth over the eons, but our planet destroys much of this evidence with geological activity like plate tectonics and erosion. Whatever traces survive have a “preservation bias,” the researchers said, meaning they don’t reflect the real rate or number of asteroid impacts.

The Moon is thought to be hit by space rocks at a similar rate to Earth, but it offers a more pristine record of these dust-ups. The trick, for Mazrouei’s team, was to find a reliable method for estimating the ages of lunar craters in order to calculate the rate of impacts over long timeframes, and apply that to Earth.

Co-author Rebecca Ghent, a planetary scientist at the University of Toronto and the Planetary Science Institute in Arizona, cracked this part of the problem using the Diviner thermal radiometer on NASA’s Lunar Reconnaissance Orbiter (LRO).

Ghent studied 111 lunar craters measuring over 10 kilometers (6.2 miles) in diameter with LRO, and found that some craters radiate more heat during lunar night than others. This is because rocks and boulders kicked up by recent collisions are warmer than those from older craters, which have been eroded by temperature fluctuations and micrometeorite impacts. In this way, Diviner provides a thermal method for dating craters up to one billion years old, which is when weathering makes age distinctions harder to spot.

After crunching the numbers, Mazrouei and her colleagues found that impacts have spiked by about 260 percent since Earth’s Permian period 290 million years ago.

Though the team suggests this heightened impact rate could be caused by asteroid belt disturbances, the reason is not known for sure. However, it can be assumed that this shift probably affected Earth’s life, given the havoc that large impacts create on biological systems.

Dinosaurs, for instance, began flourishing during the Triassic period 250 million years ago. As big animals, they “were particularly vulnerable to large impacts from the get-go," said study co-author Thomas Gernon, an Earth scientist at the University of Southampton, in a statement.

"It's perhaps fair to say it was a date with destiny for the dinosaurs,” he added. “Their downfall was somewhat inevitable given the surge of large space rocks colliding with Earth.”

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