Did Earth once have a ring?

Amid the cold silence of the main belt, a giant rock drifts through space. It has existed for billions of years unchanged, but today, it will be irrevocably broken.

Another rocky object hurtles toward it, smashing the asteroid and sending a shower of shards outward. One dangerously large fragment careens toward the Sun on a path that threatens Earth. After a months-long journey, it reaches the planet — but there’s no impact. The asteroid chunk’s trajectory has brought it so close to Earth that it can’t escape, but not near enough for a direct hit. Instead, it loops around the planet, caught by a new gravitational tether.

Its new orbit is too extreme. Each lap around the planet tears the asteroid from the inside out, pulling and stretching it until it breaks apart. Some of the debris rains down to Earth, creating the most spectacular meteor shower the planet has experienced in millions of years. But much remains in orbit, gathered into a halo of dust and rubble — a ring around Earth.

Although it sounds like science fiction, scientists think this may have happened. We could have seen it for ourselves if humans were around several hundred million years ago; because we weren’t, scientists had to piece together ancient clues preserved in the geological record. There was already a consensus that a collision in the main belt shattered a giant space rock around 466 million years ago. Earth rocks from that time period, the Ordovician, record an increase in earthquakes and tsunamis, which we’d expect if Earth was getting peppered with space rocks.

Scientists have found debris from that pelting in the form of asteroid dust and meteorites preserved in limestone dating back to the Ordovician. And 21 craters date back to right around the same time, too.

However, scientists thought the debris from the cosmic crash reached Earth directly — no pit stop in orbit. A study published in Earth and Planetary Science Letters last November challenges that assumption.

Forensic planetology

After reading about how Mars’ moons may have formed from material in a ring, Andy Tomkins, professor of earth and planetary sciences at Monash University in Australia, wondered how we might be able to tell whether Earth once had a ring, too.

“Ring formation is a relatively common thing,” Tomkins says. “Mars may have had one, all the outer planets have them. Why not Earth?” And if Earth did have a ring in the past, Tomkins wondered, how would we know?

The surge of tsunamis and earthquakes, surplus meteorite debris (100 to 1,000 times more than usual), and so many craters during the Ordovician all signal that something major happened then.

“It’s just a question of what it was,” Tomkins says. Space rocks do strike Earth periodically and sometimes leave craters, but it would typically take tens or even hundreds of millions of years to accumulate 21 of them. Scientists think the Ordovician craters all formed within a period of thousands to hundreds of thousands of years.

The combination was suspicious enough to warrant a closer look. Tomkins reasoned that the craters could have been made by material that fell from a ring. If that were true, their locations should indicate they came from a restricted region above the planet. The trouble is that over the intervening 466 million years, plate tectonics have shifted the craters around the planet.

Careful studies enable scientists to essentially rewind continental drift. Tomkins and his team used plate tectonic reconstructions to see where the 21 craters from the Ordovician meteor event were when they were created. If they were made by asteroid fragments that smashed directly into Earth, the craters should be randomly distributed all over the globe. 

But if they were created by a fallen ring, they should be concentrated near the equator. Orbiting debris would be drawn into an equatorial orbit by Earth’s gravitational field and rotation. The material would stabilize there, the way the rings around the outer planets encircle their equators.

Over time, pieces of the ring would fall as their orbits decayed. Their trajectory would send them near the planet’s midsection, with larger chunks leaving craters. Tomkins and his team found that all 21 craters from the Ordovician meteor event were originally within about 30° of the equator — the middle third of the planet.

Phil Plait, an astronomer who was not involved in the study, says that while this isn’t ironclad proof of a collapsed ring, it’s strong evidence. “Statistically speaking, only about 30 percent of Earth’s surface within 30° of the equator was covered by land at the time the ring is presumed to have existed, so it’s unlikely this would occur by chance,” he says.

Breaking point

The scientists who conducted the study say the asteroid fragment that would ultimately create Earth’s ring ventured too close to the planet, past a point called the Roche limit beyond which the asteroid simply couldn’t remain intact. 

“You may have heard of the term spaghettification, which is used to describe an object getting stretched and ripped apart as it gets too close to a black hole,” Tomkins says. “The same sort of principle applies when a body like an asteroid passes too close to a planet.” The only difference is the degree of stretching and tearing, which is lower because the gravity is less extreme than near a black hole.

When a small object that’s loosely held together experiences tidal stretching from a close encounter with a planet, the effect can break the object into a stream of fragments. That’s likely what happened to the asteroid fragment as it approached Earth.

The fragment had to pass through an extremely narrow region of space to be shredded by Earth’s gravity. “But over time — billions of years — close passes by asteroids, even decent-sized ones, are inevitable,” Plait says. “So it’s not too far-fetched to think this has happened at least once in Earth’s history.”

There could have been several prior renditions earlier in the planet’s history. While those may be mostly unknowable due to a geological record that weakens with age, Tomkins still thinks the ring from 466 million years ago was at least Earth’s second. Most theories about the Moon’s formation explain that it was created following a collision between Earth and a Mars-sized body, as debris from the crash coalesced in orbit from a short-lived ring.

Although the asteroid that was ripped apart near Earth 466 million years ago was likely just one piece of a larger body from the main belt, it was probably still a significant size — perhaps just larger than the one that wiped out the dinosaurs some 66 million years ago. Had it been on a slightly different trajectory, the outcome could have been quite different.

Life on Earth was still mainly confined to the ocean 466 million years ago. There were some land plants back then, but according to fossil records, land animals wouldn’t evolve for a few dozen million more years.

A direct impact by the asteroid fragment probably wouldn’t have caused the same degree of mass extinction as the later dino-killing impact, since the effects are more extreme on land than underwater. But there’s reason to think the formation of a ring may have dramatically altered the course of life on Earth.

Cracking under pressure

The world saw an object disintegrate due to tidal forces in the early 1990s, when Comet Shoemaker-Levy 9 was torn apart upon close approach to Jupiter. Not quite spaghettified, the comet was broken into a “string of pearls” that impacted Jupiter within a couple years of its discovery, marking the first time anyone had ever witnessed an object impacting a planet.

The comet’s original path around the Sun brought it close enough to Jupiter that its gravitational allegiances changed, and it began orbiting the giant world instead. But its new orbit was unstable and brought it too close to the planet (within the Roche limit), and tidal forces were too strong for the comet to hold itself together. After it crumbled into pieces, it continued in a gradually decaying orbit before finally raining down onto the planet in 1994.

Scientists think the same phenomenon may have created the outer planets’ rings when moons were crushed under the pressure of tidal forces (although some could have been shattered by impacts with other objects). Those rings have lasted a long time — estimates range from millions to billions of years — but they, too, have an expiration date. Eventually gravity will pull the orbiting dust and rocks down to their parent planets’ surfaces, and the rings will dissipate unless they’re replenished by new material. 

Cold case

Earth’s ring likely persisted for millions of years, perhaps as a faint, dusty band arcing across the sky during the day and subtly glowing at night. It may not have directly caught the attention of the trilobites, corals, and early ancestors of animals like squid that inhabited the planet back then. But the ring would have cast shade on Earth, blocking sunlight like a visor and cooling the planet.

“You can see how Saturn’s rings cast a shadow on the planet,” Tomkins says. “The same sort of thing would have happened on a smaller degree, since this ring for Earth would have been smaller.”

Thanks once again to ancient rocks, scientists have studied the ratios of isotopes in marine sediments, which act like natural thermometers. (Isotopes are different “flavors” of the same element; they’re atoms that contain the same number of protons but a different number of neutrons in their nuclei.) Studying these records of past water temperature indicates that there was a strange supercooling that roughly coincides with the time of the ring’s formation.

When Earth’s surface temperature plunged, parts of the planet you wouldn’t expect to be frozen became ice-encrusted (as indicated by glacial deposits), and sea levels fell rapidly. The weirdest part is this ice age actually occurred during a time when an abundance of carbon dioxide in the atmosphere should have caused our planet to warm.

Scientists have suggested several possible explanations for this freeze, ranging from continental drift to weakened solar activity. But perhaps a crown of cosmic rubble played a role in lowering Earth’s temperature so severely and abruptly. While marine animals probably would have been at least somewhat insulated from a direct asteroid impact, they couldn’t escape the effects of a cooling planet. So in this case, by not striking Earth, the asteroid likely caused a lot more deaths than if it had.

Fossil records show that around 445 million to 443 million years ago, three-fourths of animal life was wiped out, unable to survive the chillier temperatures. That’s around the time of another major temperature shift. From about 444 million to 437 million years ago, Earth rapidly warmed up again — perhaps as the ring dissipated and the cooling effects caused by it lifted.

Somewhat counterintuitively, the same big change in Earth’s temperature may have been a boon to life as well. The Great Ordovician Biodiversification Event occurred around the same time the ring is thought to have formed. During this pivotal period, many new types of life arose as some species were able to adapt to the altered conditions or move into different environments.

Many ancestors of the life-forms we see today appeared during this time, laying the foundation for modern marine ecosystems. And increased complexity of ocean ecosystems helped stabilize the planet’s biogeochemical cycles, which helped pave the way for terrestrial life to thrive. So while Earth’s ring was temporary, it may have had substantial, lasting effects on life on Earth.

Why do the outer planets all have rings?

To make a ring, you need material and opportunity. The outer regions of our solar system are rich with both.

“You know how many moons the outer planets have — totaling in the hundreds, compared with only three for the terrestrial planets,” Tomkins says. “That’s a clue.”

Farther from the Sun, in the main belt and especially the Kuiper Belt beyond Neptune, it was cold enough for lots of small, icy objects to form. That means there are more potentially ring-forming objects around, and greater chances for something to drift close enough to a planet to break apart. 

Re-ringing Earth

A chance encounter with an asteroid that led to the creation of a ring around Earth that cooled the planet and supercharged evolution — it sounds wild. But rings are commonplace. They may be a one-in-several-million result, but the solar system has been around for 4.6 billion years.

“I’d say this wasn’t the first time Earth had a ring and won’t be the last, most likely,” Tomkins says.

Some might even feel tempted to give a small asteroid a nudge in our direction. If its trajectory were precise enough, it would break apart and potentially create a new ring around the planet. Goodbye, global warming!

“The problem — besides the cost of finding and towing an asteroid into Earth orbit, which is a lot — is that we don’t know what all the effects would be,” Plait says. “Changing the climate globally almost overnight could cause a lot of instability in our ecosphere that could add to the damage we’re already doing.” Not to mention the possibility of accidentally sending the asteroid crashing into Earth. Plus, global warming is a human-made problem we should be capable of solving, so we should keep working on that “before trying out an untested experiment on our entire planet,” Plait says.

Scientists aren’t even entirely sure about the effects a ring 466 million years ago may have had on the planet, or whether the ring existed for certain in the first place.

But, Plait says, “Even if it turns out to be wrong, I love hypotheses like this. We still don’t know a lot about Earth’s deep past, and allowing our imaginations to explore unusual paths is a good way to open up more exploration of the topic, inspire more ideas. If it’s correct then it provides a direction for us to pursue more research, and if not, well, disproving something is valuable as well because it means there’s a different reason for the observations we’re making.”

Cosmic questions sometimes feel far removed from earthly affairs, a distinctly separate category. But studying them helps us understand how Earth’s environment has shaped its formation, climate, and even perhaps the course of the evolution of life.

Research drives technological innovations and scientific awareness. Working together they can help us better protect and sustain life on Earth, and perhaps someday, elsewhere too.