A Close Asteroid Encounter May Have Once Given Earth A Ring

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Rings are a pretty common phenomenon in our solar system: All of the large planets have them, as do several dwarf planets. Now, a new look at old impact craters suggests Earth may once have had a temporary ring created by the breakup of a passing asteroid that got too close.

"In the Ordovician, 466 million years ago, there's a cluster of impact craters," said lead researcher Andrew Tomkins , a planetary scientist and petrologist at Monash University in Melbourne, Australia. "It's the only time in the Earth's impact crater record when there was a distinct spike in the cratering rate."

The flurry of impacts occurred over a 40-million-year period between 485 and 443 million years ago. Limestone around the world geochemically records the beginning of this spike with a heavy enrichment of L chondrite meteorite and micrometeorite debris. There are also 21 impact craters scattered across the continents that date back to this period.

The team used six plate tectonic reconstruction models to turn back the clock on Earth's surface and trace the impact craters from their present-day locations back to their original locations. The models showed that 466 million years ago, all of the craters were concentrated within 30° of the equator. Plenty of crater-preserving crust was outside that narrow strip, so it's not likely that nonequatorial Ordovician craters escaped notice.

"The impact structures are really obvious," Tomkins said, "and we wouldn't miss them if they were there." The equatorial concentration is "quite unusual," he added.