Earth may have once had glittering rings, millions of years ago
An odd pattern of impact craters across Earth's surface may indicate that our planet once had a ring, like Saturn, in its distant past.
Saturn-like rings shine in the sky over Griffith Observatory in this simulated image. (Kevin Gill (CC BY-SA 2.0))
New research proposes that Earth once had a ring of asteroid debris orbiting around it, which may have caused a global climate upheaval before that debris eventually rained down onto the surface.
Around 460 million years ago, during the Ordovician period, Earth experienced two very odd events that have left researchers scratching their heads. The first was an intense period of meteorite bombardment, known as the Ordovician impact spike. The second involved the rapid growth of glaciers, despite the concentration of carbon dioxide in the air being at least seven times higher (and possibly over 20 times higher!) than it is today.
According to a new study, these two events may actually be linked!
Three researchers from Monash University, in Melbourne, Australia, examined the positions of the 21 meteorite craters that are known to be related to the Ordovician impact spike. Turning back the clock, they traced where those craters would have been 460 million years ago, taking into account the movement of the continents due to plate tectonics over that time. Their results showed that all of the craters had formed within 30 degrees of the equator.
That far back in Earth's history, there were only a few small areas of land around the equator. Roughly 70 per cent of the exposed land mass on the planet that could have preserved impact craters was located outside of that band. Still, none of the craters associated with the Ordovician spike are found in any of that larger area of land.
The positions of the Ordovician impact craters are plotted on this map of the globe with the continents in their locations between 467-450 million years ago. The 3-letter labels on the land masses denote current continental regions: AMZ, Amazonia; ANT, Antarctica; AS, Arabian Shield; AUS, Australia; AZ, Azania; BAL, Baltica; IND, India; LAU, Laurentia; NC, North China; SA, Southern Africa; SAH, Saharah; SC, South China; SEA, South-eastern South America; SIB, Siberia; SSA, Southern South America; T, Tarim; WAF, West Africa. (Tomkin, et al., 2024)
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Previously, attempts had been made to explain this impact spike by suggesting that a collision of asteroids out in the asteroid belt beyond Mars may have pelted Earth with debris. If that was the case, though, the researchers point out that it's unlikely the impact craters would have been so concentrated. Instead, they probably would have been spread out much farther across the surface. Also, such a remote origin for a debris stream would also likely have impacted Mars. Despite the Red Planet not having plate tectonics to mix things up, they found no similar pattern of craters on its surface.
However, if a rocky rubble-pile asteroid, similar to Bennu, had wandered close enough to Earth, it could have crossed the planet's Roche limit distance. According to the researchers, the Roche limit distance for such an object would be just over 3,100 kilometres above the surface. The resulting gravitational shear on the asteroid would have torn it apart, causing it to spread out into a glittering ring around the planet.
Asteroid Bennu, imaged by NASA's OSIRIS-REx spacecraft. Bennu is considered to be a 'rubble pile' asteroid. It is composed of multiple boulders, smaller rocks, and dust, all loosely held together by gravity. It gets its nickname from the fact that if such an asteroid were instantaneously transported to just above the surface of Earth, it would collapse into a giant heap of debris. (NASA Goddard)
"Over millions of years, material from this ring gradually fell to Earth, creating the spike in meteorite impacts observed in the geological record," Professor Andy Tomkins, the lead author of the study, said in a Monash University press release. "We also see that layers in sedimentary rocks from this period contain extraordinary amounts of meteorite debris."
At roughly the same period in Earth's history, there was an odd period of glacier growth, known as the Hirnantian Icehouse. It was odd because studies have shown that atmospheric carbon dioxide levels at that time were somewhere between 3,000 and 9,000 parts per million (ppm). That is significantly higher than the current concentration of 422 ppm. Still, during the Ordovician period, Earth saw a rapid expansion of its polar ice caps, followed by a warming event that melted the ice once again, returning ocean levels to what they were prior to the glaciation event.
Attempts have been made to explain this strange event event, but it still remained a mystery. It's possible, though, that Tomkins and his colleagues may have found the answer.
A simulated image from space of Earth with a ring system, similar to Saturn's. (Kevin Gill (CC BY 2.0))
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According to their study, the ring system produced by the breakup of this asteroid would have remained in orbit for roughly 40 million years. During that time, the debris would have reflected incoming sunlight, thus casting a cooling shadow over the planet's equatorial regions.
The warming due to atmospheric carbon dioxide happens because the CO2 absorbs outgoing infrared radiation from the planet. This outgoing radiation mainly comes from the surface of Earth, which first absorbs incoming sunlight, and then re-radiates that energy as heat.
With less sunlight reaching the planet's surface due to this ring, there would have been less overall heat radiation for the CO2 to absorb. Therefore, the significantly higher concentrations of CO2 in the air at that time would have much less of an impact. Temperatures would have fallen significantly for as long as the ring was in orbit, shading the planet. The basic idea behind this is so solid that similar strategies have been explored, at least theoretically, to potentially help cool the impacts of recent anthropogenic global warming.
"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.