Our planet Earth may have smashed an asteroid to pieces and worn its scattered debris like a ring for a while.
What was Earth really like?
466 million years ago, the Earth It was an alien world. Trilobites swam across the world’s first coral reefs, along with strange, jawless, armored fish. And high above the alien seas, a ring of slowly crumbling rocky debris circled our planet’s equator, occasionally raining meteors down on the ground below. All this is true if Monash University geologist Andrew Tomkins and his colleagues are right about the origins of 21 ancient meteor craters, that is.
When Tomkins and his colleagues pieced together what the constantly reshuffling puzzle of our planet’s surface looked like 466 million years ago, they noticed that a series of 21 meteor craters from an event called the Ordovician Impact Spike were all aligned along the ancient equator. They also found a sharp increase in material from a common type of asteroid mixed in with layers of limestone that formed during the same period. According to Tomkins and his colleagues, the asteroid craters and debris are all that remains of a ring that once adorned our planet.
The Ordovician Impact Spike was a 40-million-year period when more meteors than usual bombarded the Earth, but, strangely, they all seemed to land in tropical latitudes near the equator rather than crashing into random spots on the globe. Tomkins and his colleagues put the odds of this happening at about 1 in 25 million, or, as they put it, “highly unlikely.” Clearly, something unusual was happening to our planet (regardless of how unusual our planet itself was at that point).
Tomkins and colleagues’ 1-in-25 million odds only apply if the meteorites that formed these 21 craters came from the asteroid belt. But if they came from closer to Earth, such as a debris ring that circles the planet’s central section, then it makes much more sense that they would hit Earth near the equator as their orbits gradually decayed and the ring crumbled.
Planetary rings are beautiful, but they are also the result of violent destruction. Many of the iconic rings of Saturn They are the shredded remains of moons that ventured too close to the gas giant and were torn apart by its powerful tidal pull. And Tomkins and his colleagues say the same thing probably happened to an asteroid that passed too close to Earth, inside what’s called the Roche limit. Our planet’s gravity tore the asteroid apart and strung its innards together like a necklace, essentially.
Based on the number and size of the craters, Tomkins and his colleagues estimate that the asteroid that formed Earth’s ring must have been about 7 miles across. And over the next 40 million years or so, the ring slowly crumbled as the orbits of the debris deteriorated, occasionally raining down huge rocks on the tropics below.
If Tomkins and his colleagues are right, the ring may have been more than an elegant, if somewhat macabre, accessory. It may also have altered Earth’s climate for about 40 million years, triggering what geologists and paleontologists call the Hirnatian Ice Age: a drop in average global temperatures of about 8 degrees Celsius.
The Hirnacian Glacier has put a strain on all the strange plants and animals of the Ordovician seas, but life, uh, finds a way, and in this case the cold snap appears to have directly led to a dramatic increase in biodiversity, as species have found new ways to adapt and evolve.
The ring, according to Tomkins and his colleagues, may have been dense and wide enough to cast its shadow over the hemisphere tilted away from the Sun during the winter, leading to colder winters and a greater temperature difference between winter and summer.
As pieces of the ring began to fall from the sky, the dust kicked up by their impacts may also have blocked the Sun, intensifying the cold. Tomkins and his colleagues say this could “[risolvere] the puzzle of why such intense cooling occurred despite high atmospheric carbon dioxide.”
Climate simulations, along with computer models of asteroid orbits, will help test the idea.
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