Earth may once have had a ring like Saturn

After a near miss with an asteroid 466 million years ago, Earth may have developed a Saturn-like ring of debris that lasted for tens of millions of years – and may have significantly affected the planet’s climate.

That is according to Andy Tomkins and his colleagues at Monash University in Melbourne, Australia, who have identified 21 crater sites around the world that were created by falling meteorites, during a period known as the Ordovician impact spike 466 million years ago. The team say that these crater sites were the result of larger objects in a previously unidentified ring being pulled out of orbit and crashing into Earth.

Taking into account the movement of the continents due to plate tectonics, the team says that, at that time, all the sites would have been located close to the equator. This is consistent with a ring because these typically form above the equators of planets, the researchers say.

The team also relied on previous research having identified a consistent meteorite signature in a number of limestone deposits, also from that time and also once close to the equator.

Tomkins says the team has calculated that the likelihood of all these crater sites being positioned close to the equator if they were the result of unrelated, random impacts is just 1 in 25 million.

But where did this ring come from? The team propose that an asteroid, perhaps over 12 kilometres in diameter, passed so close to Earth that it was torn apart by the planet’s gravitational pull, creating a ring of debris.

The resulting shadow created by the ring may have led to global cooling and the iciest conditions experienced by Earth in the past 500 million years, say the team, but its exact nature is still unclear. “We don’t know how the ring would have looked from Earth or how much light it would have cut out or how much debris there would have had to be in the ring to lower the temperature on Earth,” says Tomkins.

It isn’t unheard of for planets to capture asteroids, says Tomkins, and it is thought that Earth pulls a kilometre-scale object into temporary orbit around once every 10 million years.

Much rarer for the smaller planets like Earth and Mars is for a large asteroid to pass within what is known as the Roche limit – the point at which the tidal forces of the larger body tears apart the smaller one.

The exact distance depends on the characteristics of the two bodies. For a solid asteroid approaching Earth, the Roche limit may be just over 3000 kilometres, while an asteroid made up of loosely compacted rubble would disintegrate at 15,800 kilometres.

Birger Schmitz at Lund University in Sweden says the team’s suggestion is a “new and creative idea that explains some observations”.

“But the data are not yet sufficient to say that the Earth indeed had rings,” says Schmitz. He says that one way to test the hypothesis would be to search for specific grains from asteroids in the craters the team has identified and in other nearby similarly aged deposits, to see if the ring-linked craters show a clear signature.