A bombardment of meteors that dwarfed that which killed the dinosaurs likely plunged the Earth into an ice age which saw the entire planet covered in snow and ice, new research suggests.
Between 800 and 700 million years ago the Earth entered the Cryogenian Age, scientists say, believed to have been caused by the breakup of the supercontinent Rodinia. A massive outpouring of magma pulled carbon dioxide from the atmosphere, weakening the greenhouse effect and cooling the entire planet.
But evidence meteors were actually to blame has come from an unlikely source - the moon.
The Earth's only natural satellite is covered in craters dating back hundreds of millions of years - without an atmosphere, weather or plate tectonics, once a crater is there, it stays.
One particular crater, Copernicus, is known to have been created about 800 million years ago - the Apollo 12 mission brought back soil samples. Scientists in Japan found eight other massive craters about the same age on the far side of the moon, measured by seeing how many smaller craters they contained from later bombardments.
The Earth must have been hit by millions of billions of kilograms of asteroids at the same time, they said - the evidence of which has been eroded away in the years since.
"To date, the impact history and subsequent effects on the environment [that long ago] have not been understood because terrestrial craters are not well preserved due to erosion," the study, published in Nature Communications, reads.
"Based on crater scaling laws and collision probabilities with the Earth and Moon, we suggest that at least [40 to 50 quadrillion kilograms] of meteoroids, approximately 30-60 times more than the Chicxulub impact, must have plunged into the Earth-Moon system immediately before the Cryogenian, which was an era of great environmental changes."
The Chicxulub impact is the one which killed the dinosaurs. Its impact crater is about 100km wide - slightly bigger than the Copernicus crater on the moon, which is just one of several made about 800 million years ago.
Asteroid strikes can cause ice ages by sending up dust and debris which blocks sunlight reaching the ground.
Oddly enough, it's possible one of the asteroids in the group that struck Earth back then missed, and continues to pose a threat today. The scientists managed to trace back the bombardment to a family of asteroids called Eulalia, one of which is believed to be Bennu.
Astronomers have calculated Bennu, 490m across, has a one-in-2700 chance of hitting Earth next century. It's the subject of numerous conspiracy theories, and NASA sent a probe to it - OSIRIS-REx - last year.
If Bennu hit the Earth, it would be bad - striking with the power of about 23 hydrogen bombs - but not civilization-ending.
"We're not talking about an asteroid that could destroy the Earth," OSIRIS-REx principal investigator Dante Lauretta said prior to the probe's launch in 2016. "We're not anywhere near that kind of energy for an impact."
Earlier this year, scientists in Australia said they had evidence an asteroid strike ended a prior ice age, the impact evaporating huge amounts of water and ice into water vapour, a strong greenhouse gas.
