Here’s an extremely cool question: Did Earth ever have rings like Saturn’s? 

New research just published makes a good case that yeah, maybe we did.

Awesome. 

Ring systems are pretty common. Saturn has the most glorious set in the solar system, of course. Jupiter has one as well, though it’s faint. Uranus and Neptune do, too, which points toward a trend of big planets being able to have rings, maybe for a long time (though not forever).

You may not know that smaller objects have rings too, like Chariklo, Haumea, and Quaoar, three smaller rocky/icy bodies in the outer solar system. That means it’s not just the big guys that sport rings. There’s some thought that Mars may have once had rings, too.

So it’s not too silly to wonder if Earth ever had them as well. The problem has been the lack of evidence for them.

Until this new research, that is [link to journal paper]. The work is pretty clever. It’s been known for a while now that there was an uptick in meteorite impacts on Earth in the Ordivician Period. Volcanic ash from that time has high levels of minerals commonly found in L-type chondrite meteorites. The levels sometimes reached hundreds or even thousands of times higher than usually found on Earth, indicating a pretty heavy rain of asteroids. This “spike” in impacts started about 466 million years ago, and lasted very roughly 40 million years (though the endpoint of the impacts isn’t well constrained).

The leading hypothesis is that an asteroid in the asteroid belt suffered an immense impact from another asteroid, causing it to break up. The pieces eventually made their way closer to the Sun, with a lot of them hitting Earth.

But the new work proposes a different idea. Instead of the event happening in the asteroid belt out past Mars, instead an asteroid passed very close to Earth. It got so close that the tides from Earth’s gravity ripped it apart. Normally an asteroid passing by Earth would just keep going, but it’s possible during a tidal breakup to have lots of debris stick around, which would eventually form a ring around our planet. The ring would’ve been decently dense if it were able to stick around for tens of millions of years… and during that time big chunks could have fallen to Earth.

Cool idea! But what’s the evidence? The scientists looked at craters on Earth that were dated sufficiently accurately to be placed around that time. They found 21, ranging in size from a little over a kilometer to more than 50 km across (which is terrifyingly huge). And here is the very clever bit: Using different models of continental drift, they looked at where those craters were 460 million years ago. Their finding made me gasp: They were all within 30° of Earth’s equator at the time.

That’s a big deal. A ring of debris might start off tipped to Earth’s equator, but our old friend tides would rather rapidly force the debris to circle over the equator — the Earth’s rapid spin makes the equator bulge, putting excess mass in that bulge. That means there’s more gravity in the bulge, which torques the ring and rapidly forces it to orbit over the equator. Any chunks of ring that then fell to Earth would do so close to the equator in latitude, which is just what they found.

If the old idea that a collision in the asteroid belt formed these chunks, we’d expect to see impacts all over the world, at all latitudes. We don’t, but instead see them arrayed as you’d expect from an equatorial ring. Wow.

The material would have been dark, but then the Moon’s surface is pretty dark material too, about as reflective as a blackboard. So the ring, sitting in full sunlight during the day, would’ve been pretty bright, easily visible. And at night, well. It would’ve been easy to read by ringlight. Imagine that view!

But it’s not all good news. The dense ring would also block sunlight, casting a shadow over a wide range of the planet depending on the season. Summer is when the axis of Earth is tipped toward the Sun, and the rings would then shadow the other hemisphere, the one experiencing winter. That means winters would have been extra cold. Reflected sunlight would warm the summer hemisphere, but only a little, and not nearly enough to offset the winter cooling. Not only that, but dust from the impacts would cool the Earth as well, though in general that doesn’t last long.

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