Back when the Earth was crawling with trilobites and other strange shelled creatures, our planet may have had a ring just like Saturn's.
This ancient ring system is thought to have formed about 466 million years ago, at the beginning of a period when Earth was regularly hit by meteorites, according to a new paper in the journal Earth and Planetary Science Letters.
This era of intense bombardment, known as the Ordovician impact spike, may have resulted from meteorites falling from the ring rather than flying in from space, which would explain the strange placement of craters.
The researchers' idea that Earth once had rings comes from reconstructions of Earth's plate tectonics from the Ordovician period—which ran between 485.4 million years and 443.8 million years ago—including the positions of 21 impact craters from the Ordovician impact spike.
The Ordovician impact spike was previously thought to have been caused by the breakup of a large asteroid in the belt between Mars and Jupiter. However, all 21 of the craters are located within 30 degrees of the equator, leaving 70 percent of the Earth's surface more or less untouched. This is incredibly unlikely to occur by chance, but previous theories cannot explain why this may have happened.
The researchers suggest that this unlikely pattern of impacts could be explained by an asteroid having broken apart after a close call with the Earth and the debris forming a ring around the planet, from which the meteorites fell to the ground and created the craters.
To come to this conclusion, the researchers calculated the area of the Earth at the time that would have been capable of preserving impact craters. They found that much of the Earth would have preserved craters if something had impacted, with only 30 percent of this suitable land located close to the equator. Despite this, all the impact craters are close to the equator.
"Over millions of years, material from this ring gradually fell to Earth, creating the spike in meteorite impacts observed in the geological record," study lead author Andy Tomkins, a professor at Monash University's School of Earth, Atmosphere and Environment, said in a statement. "We also see that layers in sedimentary rocks from this period contain extraordinary amounts of meteorite debris."
This ancient asteroid must have come closer to the Earth than a distance known as the Roche limit, which is the minimum distance at which a celestial object, such as a moon or asteroid, can orbit a larger body (like a planet) without being torn apart by the larger body's tidal forces. The Roche limit varies depending on the size of the object and the mass of the planet.
A moon or large asteroid venturing too close to a planet and crossing its Roche limit is one of the most common ways rings form, with this being the way that Saturn's rings are thought to have originated.
This ancient ring around Earth may have blocked out some of the heat of the sun, possibly contributing to a major global cooling event known as the "Hirnantian Icehouse."
"What makes this finding even more intriguing is the potential climate implications of such a ring system," Tomkins said.
This cooling event occurred during the late Ordovician period, around 445–443 million years ago, and is characterized by a significant global glaciation that led to widespread ice sheets, especially over the southern supercontinent Gondwana, which was positioned near the South Pole during the Ordovician. As one of the coldest periods in Earth's history, the Hirnantian Icehouse played a crucial role in the second-largest mass extinction in Earth's history, known as the Late Ordovician Mass Extinction, which wiped out roughly 85 percent of marine species.
"The idea that a ring system could have influenced global temperatures adds a new layer of complexity to our understanding of how extra-terrestrial events may have shaped Earth's climate," Tomkins said.
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References
Tomkins, A. G., Martin, E. L., & Cawood, P. A. (2024). Evidence suggesting that earth had a ring in the Ordovician. Earth and Planetary Science Letters, 646. https://doi.org/10.1016/j.epsl.2024.118991
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