If astronomers had been walking the Earth 466 million years ago, they may have had something special to see. The moon and the planets and the stars and the sun would have looked pretty much the same as they do today. But Earth itself would have been a lot gaudier—decorated with a ring system similar to the ones that circle Jupiter, Saturn, Uranus, and Neptune. That’s the conclusion of a new study in Earth and Planetary Science Letters, one that not only explains what was going on in the near-space region surrounding our planet, but on the surface too, where a global deep freeze saw average temperatures rapidly plummet by 8°C (14.4°F)—a climate shift known as the Hirnantian Icehouse.
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The research, led by Andrew Tomkins, a professor in the School of Earth Atmosphere and the Environment at Australia’s Monash University, took a close look at the ancient Earth, back in what is known as the Ordovician Period, when the continents were mostly clustered in the southern and lower northern hemisphere. It was a time of robust biological expansion, with vertebrates, invertebrates, algae and sea plants thriving in oceans and seas, though the land was still devoid of life. Within that epoch there was also a 40 million year span known as the Ordovician Impact Spike that saw the Earth fairly tattooed with meteors raining down from space.
Most of the evidence for these collisions comes from high levels of chondrites—meteoric ingredients made up of silicates, sulphides, iron-nickel, and more—in limestone that dates back to that period. Nearly all of the craters themselves have long since vanished—some covered by ice at the South Pole, some erased by erosion, some broken up by tectonic plates crashing together and disrupting the ground above them. Today, only 21 of the impact scars survive. Their location told a revealing story.
The best place to look for ancient craters is in regions of the Earth known as cratons—large, geologically stable stretches that have changed little over time. Cratons are found all over the world—in western Australia; North America and Greenland; Scandinavia and the Baltics; Siberia; India and Sri Lanka; southern Africa; and eastern South America. Where those continents are today, however, was not where they were during the bombardment. When Tomkins and his colleagues began mapping the location of the 21 Ordovician craters preserved in the cratons, they noticed something intriguing: all of them were found within 30 degrees of where they were relative to the equator at the time of the impacts. That’s despite the fact that 70% of the land that made up the ancient continents was located outside of that band.
If the meteors were raining down randomly from all points in the sky, the investigators calculated that there would be only a 1 in 25 million chance that all of the surviving craters would be found where they were. Something was concentrating the incoming ordnance into a narrow latitude, and Tomkins and his colleagues suspected they knew what it was.
The prevailing explanation for the Ordovician Impact Spike had long been that a large body in the asteroid belt between Mars and Jupiter broke up as a result of a collision and the debris from that crack-up eventually found its way to Earth. But that is not a good fit given the location of the craters. A better scenario—and the one the new paper argues for—is that that body escaped intact from the asteroid belt and wandered toward the inner solar system. Ultimately, it flew close enough to Earth that it breached our planet’s Roche limit, the line of demarcation about 1,900 miles above the surface where Earthly gravity would, effectively, rip it apart, creating a flying rubble field. The flock of rocks would eventually settle into orbit, forming a neat ring around the planet’s equator. But the ring could not last: both the pull of gravity and the drag of Earth’s exosphere, which extends more than 6,000 miles into space, would eventually pull the rubble down to the surface, eliminating the ring.
“Over millions of years, material from this ring gradually fell to Earth, creating the spike in meteorite impacts observed in the geological record,” said Tomkins in a statement that accompanied the paper’s release. “We … see that layers in sedimentary rocks from this period contain extraordinary amounts of meteorite debris.”
Before that happened, however, the ring would have a profound effect on the climate on the ground. The 23-degree tilt of the Earth’s axis would have caused the ring to present its surface to the sun, casting a shadow in the atmosphere and on the ground below and causing global temperatures to plunge by the 14.4°F that defined the Hirnantian Icehouse. As the rubble began entering as meteors, dust kicked up into the atmosphere by the collisions would increase this darkening and cooling.
“The idea that a ring system could have influenced global temperatures adds a new layer of complexity to our understanding of how extraterrestrial events may have shaped Earth’s climate,” said Tomkins.
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