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Planetary rings may be one of space’s many spectacles, but in our solar system, they’re a dime a dozen. While Saturn’s rings are the brightest and most extensive, Jupiter and Uranus and Neptune have them, too. What’s more, four icy minor planets—Chariklo, Chiron, Quaoar, and Haumea—that orbit among or beyond our gas giants, also host ring systems. Even so, it would be fanciful to imagine that Earth once had a ring system of its own, wouldn’t it? I mean, that just seems almost too cool to be true.
Or is it?
Rings are likely the dwindling remains of shredded asteroids or comets, and when you think about the turbulence Earth experienced around half a billion years ago, the reality of a bygone ring system around Earth seems less farfetched. That’s the case researchers make in a new study published in Earth and Planetary Science Letters.
If Earth went through one ringed phase, there’s a good chance it went through several.
About 466 million years ago (long before the dinosaurs), the rate of stuff falling on Earth that was large enough to leave craters spiked sharply. Researchers have identified 21 impact craters from this spike, with sizes between a few and 50 kilometers in diameter. And sedimentary rocks from this period show a huge increase—a factor of 100 to 1,000—in the concentration of elements associated with a specific group of meteorites, called L chondrites. This period, the mid-Ordovician, also included an extreme drop in global temperature, roughly 10 degrees Celsius, which coincided with increased seismic and tsunami activity. Also, a mass extinction eliminated 85 percent of marine species, after which the temperature rebounded. What could possibly explain this?
The authors of the new study, led by Andy Tomkins, a geoscientist at Monash University in Australia, claim that you can account for the craziness of the Ordovician period if Earth had a system of rings that it captured from an asteroid. And if it went through one ringed phase, there is a good chance that it went through several. And for good measure, the other rocky planets may have done the same.
“This opens up a new way of thinking about the history of the Earth,” Tomkins wrote in an email. He and his colleagues focused on the distribution of craters on Earth during the Ordovician’s impact spike. The craters are spread across Earth’s surface, but its tectonic plates—the movement of which pushes the continents around—have shifted considerably in the last 450-plus million years.
The researchers determined where the impacts actually took place by “rewinding the clock” of the continents’ movements using computer models of how the Earth’s surface has rearranged itself. This showed that all of the crater-forming collisions took place within a narrow band centered on the equator. They gathered the data on where craters are located today (across the globe, especially in areas that were not likely to have been covered in ice during the Ordovician, given that ice could prevent crater formation), while also taking into account the regions where such craters could not have been found (due to present-day ice coverage, such as in Antarctica), and where craters could not have been preserved because of processes such as burial and erosion. Putting all of these factors together, Tomkins and his colleagues calculated that it would be highly improbable—we’re talking about a 1 in 25 million chance—that the impacts were randomly distributed across the Earth’s surface.
To explain why the impacts are concentrated at the equator, Tomkins and his colleagues proposed that the Ordovician Earth had a system of rings. The Ordovician impacts came from objects within the rings that crashed down onto Earth. Saturn’s rings are thought to have formed from a large icy object that passed so close to Saturn that it was torn into pieces in a process called tidal disruption. When a comet or asteroid passes very close to a planet, the difference in gravity across the object is strong enough to stretch it to its breaking point.
We have seen tidal disruption in action: In 1992, the comet Shoemaker-Levy 9 passed so close to Jupiter that it was torn into a string of about 20 fragments. Most of these rained down onto Jupiter in 1994. In the case of Saturn’s rings, the shards of the disrupted object were captured into orbit. The orbits of captured fragments would have started off being extremely stretched-out, or eccentric, and aligned with the initial orbital trajectory of the icy parent body. In time, the ring fragments collided with each other and were ground down to smaller pieces, and the collection of objects settled to a circular system of rings aligned with the plane of Saturn’s equator, where we see them now.
The ring-producing asteroid must have been an L chondrite, a type of ordinary meteorite (“ordinary” because they are the most common type of meteorites in our collections). Tomkins and his colleagues suggest that the L chondrite parent body passed extremely close to Earth—within just a few thousand kilometers of the surface, scratching the edge of the atmosphere—and was tidally disrupted, just like the object that broke apart and formed Saturn’s rings. Its fragments settled to Earth’s equatorial plane, and dust from the ring slowly rained down on Earth, along with the occasional impact of a larger fragment, mostly near the equator. This explains the high concentrations of L chondrite meteorite-like dust in Ordovician sedimentary rocks, as well as the distribution of craters across the globe.
The sharp drop in temperature after the Ordovician impact spike—called the Hirnantian global icehouse—may also have been a consequence of the Earth’s rings. The rings’ shadow would have fallen on whichever hemisphere was in winter at that time, and would have had a net cooling effect, which the dust in the atmosphere may have amplified. This drastic cooling perhaps played a role in the late Ordovician mass extinction: The global temperature bounced back up after the end of the Ordovician impact spike, presumably due to the dissipation of the rings.
“It seems plausible that the planets have seen multiple cycles of ring formation and loss,” Tomkins wrote in an email. These cycles would be triggered by the rare, very close passage of a comet or asteroid. Some planets are more likely to have ring phases than others, based on the frequency of asteroid flybys and the planets’ properties. Jupiter, Saturn, Uranus, and Neptune seem to have more frequent, or longer-lived, ring phases because they interact far more frequently with asteroids and comets than the rocky planets, and so have far more chances of disrupting one and capturing its fragments. Ring phases usually last for millions of years at a time, with considerable consequences for a planet’s geology, climate, and potentially life.
For Tomkins, the search is underway for other potential pieces of evidence that Earth has undergone previous ring phases. The Ordovician spike is the clearest sign so far. “The cratering record gets weaker the further back in time we look,” he wrote in an email. “What other evidence can we look for that Earth may have had this and other rings in the past?”
It’s a good question. And I’m excited to know someone like Tomkins is out there chasing down an answer. 
Lead image: MarcelClemens / Shutterstock
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