A recent study published in the journal Earth and Planetary Science Letters reveals evidence that Earth may have had a ring similar to Saturn’s around 466 million years ago, during the Ordovician period. The research team, led by Andrew G. Tomkins from Monash University in Australia, reached this surprising conclusion after meticulously analyzing the geographical distribution of 21 asteroid impact craters dated to the Ordovician.

What they discovered was truly intriguing: all these craters are concentrated within an equatorial band of no more than 30 degrees latitude, despite the fact that approximately 70% of the exposed Earth’s crust that could potentially preserve craters lies outside this band.

This unusual crater distribution coincides with a period known as the “Ordovician Meteor Event”, a time span of about 40 million years during which Earth experienced a significant increase in meteorite impact frequency. The onset of this period is marked by a notable rise in the accumulation of L chondrite material (a common type of meteorite) in sedimentary rocks, precisely dated to 465.76 ± 0.30 million years ago.

Traditionally, scientists have attributed this increase in impacts and meteorite material flux to the breakup of the L chondrite parent body in the asteroid belt. However, the new study challenges this interpretation. The researchers performed binomial probability calculations that indicate it is extremely unlikely (with a probability of only 4 × 10^-8) that the observed crater distribution was produced by bolides originating directly from the asteroid belt.

Areas of continental crust proximal to the equator during the Ordovician. Areas older than Ordovician are indicated in salmon colour, Ordovician rocks are dark blue, and younger are grey. Light blue indicates lakes in Europe and Russia. The labelled pink points are the recognised Ordovician impact spike craters
Areas of continental crust proximal to the equator during the Ordovician. Areas older than Ordovician are indicated in salmon colour, Ordovician rocks are dark blue, and younger are grey. Light blue indicates lakes in Europe and Russia. The labelled pink points are the recognised Ordovician impact spike craters. Credit: A. G. Tomkins et al.

Instead, Tomkins and his colleagues propose a bold hypothesis: a large fragment of the L chondrite parent body came dangerously close to Earth about 466 million years ago, passing within the Roche limit (the minimum distance at which a celestial body can approach a planet without disintegrating due to tidal forces). This close encounter would have caused the asteroid to disintegrate due to intense gravitational forces, forming a debris ring around our planet.

This hypothetical terrestrial ring would have persisted for millions of years, gradually releasing material that impacted Earth’s surface, thus explaining the prolonged duration of the Ordovician Meteor Event and the extended accumulation of L chondrite debris in sediments. The equatorial distribution of the resulting craters would be explained by the tendency of orbiting objects to fall preferentially near the equator due to the planet’s rotation.

The researchers go even further in their speculations, suggesting that the shadow cast by this ring could have triggered the global cooling that led to the glaciation period known as the Hirnantian Ice Age, which marked the end of the Ordovician around 445 million years ago.

Areas of continental crust distal to the equator during the Ordovician. Areas older than Ordovician are indicated in salmon colour, Ordovician rocks are dark blue, and younger are in grey. Light blue indicates lakes in Africa. The labelled point (L) in Africa is the Luizi impact structure, which has a very poorly defined age that could plausibly be Ordovician, constrained to be between 0 and 573 Ma
Areas of continental crust distal to the equator during the Ordovician. Areas older than Ordovician are indicated in salmon colour, Ordovician rocks are dark blue, and younger are in grey. Light blue indicates lakes in Africa. The labelled point (L) in Africa is the Luizi impact structure, which has a very poorly defined age that could plausibly be Ordovician, constrained to be between 0 and 573 Ma. Credit: A. G. Tomkins et al. / Schmieder and Kring, 2020

To arrive at these conclusions, the team used sophisticated paleogeographic reconstruction techniques, employing six different tectonic plate models to determine the positions of the continents and impact craters during the Ordovician. They also conducted an exhaustive analysis of continental areas capable of preserving impact craters from that era, considering factors such as tectonic stability, erosion, and ice cover.

The study does not just propose this hypothesis but also provides a rigorous statistical analysis to support its claims. Using multi-distance spatial clustering analysis (Ripley’s K-function), the researchers demonstrated that the distribution of Ordovician craters shows a much higher degree of clustering than observed in craters formed over the last 40 million years, reinforcing the idea that their distribution is not random.

The Ordovician period witnessed an explosion of biodiversity known as the Great Ordovician Biodiversification Event, and some scientists have suggested that the increase in meteorite impacts could have played a role in this phenomenon.

The authors acknowledge that further studies are needed to confirm their conclusions and explore all the implications of a potential terrestrial ring during the Ordovician.

SOURCES

Andrew G. Tomkins, Erin L. Martin, Peter A. Cawood, Evidence suggesting that earth had a ring in the Ordovician. Earth and Planetary Science Letters, Volume 646, 15 November 2024, 118991. doi.org/10.1016/j.epsl.2024.118991

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