It lasted a little over forty million years, from about 485 million years ago to 444 million years ago. Coming out of the Cambrian Period, there was incredible diversity in marine life, especially invertebrates like arthropods, mollusks, and brachiopods; and at the beginning it was very hot -- not far off from the Paleocene-Eocene Thermal Maximum, which had a global average temperature almost ten degrees higher than it is today. But over the next forty million years, the climate went into a slow slide, ending with what is called the "Hirnantian Icehouse," a period of widespread continental glaciation. The climate shift triggered a mass extinction, one of the "Big Five," and an estimated eighty percent of marine species went extinct.
But unlike the later Permian-Triassic and Cretaceous Extinctions, this one wasn't at all sudden -- suggesting that the causes of the other two mentioned, a massive volcanic eruption and an asteroid strike respectively, might not have been responsible.
So what triggered the climatic shift and die-off?
One thing was simply plate movement; by the end of the Ordovician, Gondwana (what is now Africa, South America, Antarctica, and Australia) were near the South Pole, which led to the formation of glaciers. But it's hard to see how that by itself would have had such an enormous effect on life worldwide.
A paper this week in Earth and Planetary Science Letters proposes a curious solution, hinging on a peculiar observation; there was a meteorite barrage around 466 million years ago, during the middle of the Ordovician. Extant rocks of that age show dozens of impact craters. But... those craters are almost entirely limited to regions that were within thirty degrees of the equator at the time.
The researchers estimate that the likelihood of that occurring by chance is equivalent to flipping a coin 21 times and getting tails every time. But if they were connected, there's the problem that the extinction didn't occur right after the barrage; there was an almost twenty million year gap between the impact array and the icehouse/extinction. It's apparent that the strikes didn't directly trigger the extinction.
What the researchers propose is a near strike by a large asteroid -- one that, had it hit square on (as the Chicxulub Meteorite would do almost exactly four hundred million years later) would have been in the planet-killer category. But it did pass inside the Roche limit, the distance a smaller object can pass a planet at which the gravity holding the passing asteroid together is exceeded by the tidal forces trying to tear it apart.
So rather than going into orbit, or crashing into the Earth in one piece, the asteroid got shredded. The larger chunks went into decaying trajectories and ultimately impacted Earth near their orbital planes (parallel to the Earth's equator -- resulting in the odd distribution of craters), and the rest got spun out into...
... a ring system.
[Image credit: Oliver Hull]
The researchers think the rings shaded the Earth from enough of the Sun's warmth and light that it precipitated a slow decline into an ice age, and coupled with the movement of a big section of the Earth's crust down to near the South Pole, a worldwide icehouse. But because it was a gradual drop in temperature, the hit on biodiversity didn't happen all at once -- although by the end, it certainly was big enough to rank amongst the largest mass extinctions ever.
But -- a ring system. Can you imagine what that'd have looked like?
Of course, it's not like taking a time machine back to the late Ordovician would be all that hot an idea, and I mean that both literally and figuratively. Notwithstanding how gawdawful cold it'd have been, there would also be the problem of finding food. Plants had yet to colonize the land -- that wouldn't happen until the next geological period, the Silurian -- so the continents were basically one barren expanse of rock, dirt, and sand.
But still. Standing there in that empty landscape, and you look up, and arching over your head, spanning the entire dome of the sky, are these broad rings, a circular belt shining in reflected light.
We used to think rings were uncommon; for a long time, Saturn was the only planet known to have them. But better telescopes and (especially) flybys have found ring systems around all four of the gas giants. Now, if the current paper bears up under scrutiny, we might add Earth to the list. Eventually, the Earth's ring system scattered and decayed away -- gravitational interactions between multiple objects of similar sizes are inherently unstable -- allowing the Earth to warm again, leading into the swampy, hot Silurian and Carboniferous Periods.
But for a while, we had what must have been an awe-inspiring adornment.
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