An artist's impression of Earth with a ring - a phase suggested by new evidence
Saturn's rings are iconic, but new evidence suggests Earth might once have sported one of its own. This ring would have caused chaos on the surface.
We know Earth has gone through a lot of different phases in its lifetime. Early on it was covered in magma oceans, and much later it was a giant snowball for a while. Landmasses have broken up, drifted apart and come back together over hundreds of millions of years. But a period where it sported a ring is a relatively new idea.
Researchers from Monash University in Australia have now hypothesized that a rocky ring formed around Earth about 466 million years ago, and it persisted for a few tens of millions of years. This conclusion was the result of some rigorous geological detective work.
Around that time, during what's called the Ordovician period, there seems to have been an increase in impact craters on Earth. The team mapped the locations of 21 known craters of this age, and used models of tectonic plates movements to track them back to where they were at the time of the impact.
Intriguingly, all of them seemed to have struck within 30 degrees of the equator. Normally, they'd be expected to hit at completely random locations around the world. That's an odd finding, but it could potentially be a sampling error.
To find out, the researchers then calculated how much of the continental surface from the time would be able to preserve craters for so long. They focused on stable, undisturbed sections of crust that date back to before the mid-Ordovician, excluding regions that have been buried, eroded or affected by tectonic activity. The best results were found in Western Australia, Africa, and sections of North America and Europe.
The team then used the same tectonic models to rewind time to find where on the planet those prime locations were back in the Ordovician. And sure enough, only 30% of this suitable land was found to be near the equator. Having all 21 craters hit within this minority of land is a statistical anomaly, the team says.
What would cause an influx of meteorites to be concentrated around the equator? If Earth captured a passing asteroid around 466 million years ago, it could have ripped it to shreds and formed a ring. This debris would then rain down on the planet, focused on the equator, over a few tens of millions of years.
The ring theory could also explain a few other mysteries of the time too. Meteorite debris in those craters shows signs of not having spent much time in space before crashing to Earth, which is consistent with material from a relatively recent breakup of a large asteroid into a ring.
About 20 million years later, Earth entered the Hirnantian Ice Age, where temperatures plunged to their lowest in the last half-billion years. Because of the Earth's tilt relative to the Sun, a ring around the equator would have shaded part of the Earth's surface, potentially causing a global cooling event.
"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," said Professor Andy Tomkins, lead author of the study.
It's an intriguing idea, and even raises the possibility that rings are a phase Earth and other planets go through multiple times in their lifespans. After all, Saturn's rings could be as young as 10 million years - meaning Earth was the trendsetter in this case - and they could be gone in another 100 million. Mars is also currently tearing one of its moons apart, which could form a new ring in 20 to 40 million years.
Next, the team plans to model how asteroids break up and form rings, and how that ring evolves over time. Later work will attempt to model the effects that rings could have on the climate.