What did the Earth look like with rings?
Oliver Hull
After a near collision with an asteroid 466 million years ago, Earth formed a Saturn-like ring of debris that persisted for tens of millions of years and may have had a major impact on the planet’s climate.
Andy Tomkins of Monash University in Melbourne, Australia, and his colleagues identified 21 craters around the world that were caused by meteorite impacts 466 million years ago during what is known as the Ordovician impact peak. The team says the craters were the result of a large, previously unidentified ring-shaped object losing its orbit and colliding with Earth.
Taking into account the movement of continents due to plate tectonics, all of the locations would have had to have been located near the equator at the time, the team said, which is consistent with rings, since rings typically form above a planet’s equator, the researchers said.
The team also relied on previous studies that had identified meteorite signatures consistent with several limestone deposits that existed at the time and that once existed near the equator.
Tomkins said the team calculated that if all these craters were the result of unrelated, random impacts, the chances that they’d be located near the equator were just one in 25 million.
But where did the ring come from? The team speculates that an asteroid, perhaps more than 12 kilometers in diameter, came so close to Earth that Earth’s gravity ripped it apart, creating the ring of debris.
The shadow cast by the rings could have led to global cooling, resulting in the coldest conditions on Earth in the past 500 million years, the team said, although the exact nature of this is still unknown. “We don’t know what the rings would have looked like from Earth, how much light they blocked, or how much debris would have been needed in the rings to cool Earth down,” Tomkins said.
Tomkins said it’s not uncommon for planets to capture asteroids, with Earth thought to pull a kilometre-sized object into temporary orbit around the planet about once every 10 million years.
For smaller planets like Earth and Mars, it is much rarer for a large asteroid to pass within the region known as the Roche limit, the point at which the tidal forces of a larger body would tear a smaller body apart.
The exact distance depends on the properties of the two objects: for a solid asteroid approaching Earth, the Roche limit is just over 3,000 km, but for asteroids made of loosely compressed rubble, they break apart at 15,800 km.
Birger Schmitz of Lund University in Sweden said the team’s proposal is “a new and original idea that explains some of the observations.”
“But we don’t yet have enough data to say for certain that Earth had rings,” Schmitz says. One way to test the hypothesis, he says, would be to look for specific particles from the asteroid in the craters the team identified, or in nearby deposits of a similar age, to see if there are tell-tale signs of craters connected by rings.
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