Hidden craters suggest Earth once had rings like Saturn

Saturn's rings are one of the most famous and spectacular objects in the solar system. Earth may have once had something similar.

In a paper published last week in Earth and Planetary Science Letters, my colleagues and I presented evidence that Earth may have a ring.

The existence of such rings, formed about 466 million years ago and lasting for a few hundred million years, could explain many mysteries about our planet's past.

The case for a ringed Earth

Around 466 million years ago, a lot of meteorites began hitting the Earth. We know this because many impact craters were formed in a geologically short period of time.

From this same period, we also find limestone deposits in Europe, Russia and China that contain very high amounts of debris from a particular type of meteorite. The meteorite debris in these sedimentary rocks indicates that they were exposed to space radiation for a much shorter period of time than meteorites that fall today.

Several tsunamis also occurred at this time, as can be seen from other unusually confused sedimentary rocks.

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

a pattern of craters

We know of 21 meteorite impact craters formed during this high-impact period. We wanted to see if there was a pattern in their locations.

Using models of the movement of Earth's tectonic plates in the past, we figured out where all these craters were when they first formed. We found that all the craters are on continents that were close to the equator during this period, and none of the craters are in places that were close to the poles.

So all the impacts happened close to the equator. But is this a fair sample of what actually happened?

Well, we measured how much of the Earth's land was suitable for preserving craters near the equator at that time. Only 30% of the suitable land was close to the equator, while 70% was at higher latitudes.

Under normal circumstances, asteroids that hit Earth would hit at random, at any latitude, as we see in craters on the Moon, Mars, and Mercury.

So it is extremely unlikely that all 21 craters from this period would have formed close to the equator if they were unrelated to each other. We would expect to see many other craters at higher latitudes as well.

We think the best explanation for all this evidence is that a large asteroid broke up during a close encounter with Earth. Over many millions of years, the asteroid's debris rained down on Earth, creating the pattern of craters, sediments, and tsunamis that we describe above.

How are rings formed?

You may know that Saturn isn't the only planet with rings. Jupiter, Neptune and Uranus also have less obvious rings. Some scientists have even suggested that Mars' small moons Phobos and Deimos may be remnants of ancient rings.

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

When a small body (such as an asteroid) passes close to a larger body (such as a planet), it is pulled in by gravity. If it gets close enough (within a distance called the Roche limit), the smaller body will break up into many small pieces and a few larger pieces.

All those pieces will bump around and gradually turn into rings of debris orbiting the larger body's equator. Over time, the material in the rings will fall onto the larger body, where the larger pieces will form impact craters. These craters will be located closer to the equator.

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

A giant umbrella?

At that time, the continents were in different locations because of continental drift. Much of North America, Europe, and Australia were closer to the equator, while Africa and South America were at higher southern latitudes.

This ring would have been around the equator. And since the Earth's axis is tilted relative to its orbit around the sun, the ring would have shadowed parts of the Earth's surface.

This shadow would have caused global cooling, as less sunlight would have reached the planet's surface.

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

Was a ring casting a shadow on Earth responsible for this extreme cooling? The next step in our scientific exploration is to create mathematical models of how asteroids break up and scatter, and how the resulting ring evolves over time. This will create scenarios for climate modelling that will explore how much cooling such rings could cause.

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