Hidden craters reveal Earth may have had a ring, like Saturn

Saturn’s rings are among the most famous and spectacular features in the solar system. Earth may have once had something similar.

In an article published in Letters on Earth and Planetary SciencesMy colleagues and I present evidence that the Earth may have had a ring.

The existence of such a ring, formed about 466 million years ago and persisting for several tens of millions of years, could explain several mysteries of our planet’s past.

Plea for a Ringed Earth

About 466 million years ago, many meteorites began to strike the Earth. We know this because many impact craters formed over a geologically short period of time.

At the same time, there are also limestone deposits in Europe, Russia and China containing very high levels of debris from a certain type of meteorite. The meteorite debris in these sedimentary rocks shows evidence that it was exposed to space radiation for a much shorter period of time than is observed in meteorites falling today.

Numerous tsunamis also occurred at this time, as evidenced by other unusually mixed sedimentary rocks.

We think that all of these characteristics are probably related to each other. But what connects them?

A pattern of craters

We know of 21 meteorite impact craters that formed during this period of strong impacts. We wanted to see if there was a trend in their locations.

Using models of how Earth’s tectonic plates moved in the past, we mapped where all of these craters were when they formed. We found that all of the craters were on continents that were close to the equator at that time, and none were in locations closer to the poles.

So all the impacts happened near the equator. But does that give a true sample of the impacts that happened?

We measured the proportion of the land area suitable for crater preservation that was close to the equator at that time. Only about 30% of suitable land was close to the equator, compared with 70% at higher latitudes.

Under normal circumstances, asteroids hitting Earth can strike at any latitude, randomly, as seen in craters on the Moon, Mars, and Mercury.

It is therefore extremely unlikely that the 21 craters from this period would have formed near the equator if they were not connected to each other. One would expect to see many more craters at higher latitudes as well.

We think the best explanation for all of this is that a large asteroid broke up in a close encounter with Earth. Over several tens of millions of years, debris from the asteroid slammed into Earth, creating the pattern of craters, sediment, and tsunamis we describe above.

How rings are formed

You may know that Saturn is not the only planet with rings. Jupiter, Neptune, and Uranus also have less visible rings. Some scientists have even suggested that Mars’ small moons Phobos and Deimos could be the remains of an ancient ring.

So we know a lot about how rings form. Here’s how it works.

When a small body (like an asteroid) passes close to a large body (like a planet), it is stretched by gravity. If it gets close enough (within the Roche limit), the small body will break into many smaller pieces and a small number of larger pieces.

All of these fragments will be moved and gradually evolve into a ring of debris orbiting the equator of the larger body. Over time, material from the ring will fall toward the larger body, where the larger pieces will form impact craters. These craters will be located near the equator.

So, if Earth destroyed and captured a passing asteroid about 466 million years ago, this would explain the anomalous locations of impact craters, meteorite debris in sedimentary rocks, craters and tsunamis, and the relatively brief exposure of meteorites to space radiation.

A giant parasol?

At the time, the continents were in different positions due to continental drift. Much of North America, Europe, and Australia were near the equator, while Africa and South America were at more southern latitudes.

The ring would have been located around the equator. And because the Earth’s axis is tilted relative to its orbit around the Sun, the ring would have shaded parts of the Earth’s surface.

This shading could in turn have caused global cooling, as less sunlight reached the planet’s surface.

This brings us to another interesting puzzle. About 465 million years ago, our planet began to cool dramatically. By 445 million years ago, we were in the Hirnantian Ice Age, the coldest period in the last half-billion years.

Is the shadow of the rings on Earth responsible for this extreme cooling? The next step in our scientific investigation is to create mathematical models of how asteroids fragment and disperse, and how the resulting ring evolves over time. This will pave the way for climate modeling that will study how much cooling such a ring could cause.

This article is republished from The Conversation under a Creative Commons license. Read the original article.