ByRobin George Andrews
Published February 9, 2024
If you travelled back in time many millions of years, Earth would resemble the planet Hoth from Star Wars: a frigid, dry air would sweep over a world of endless ice, covering almost every inch of land and sea. This state of global refrigeration, known as Snowball Earth, has happened at least twice, both times more than 600 million years ago. And something must have gone seriously awry with the world’s thermostat to turn it into a giant ball of ice—but what?
Various ideas, from rogue volcanism to supercontinental destruction, have been suggested. A new study, published today in Science Advances, explores another idea that has been largely ignored: a cataclysmic asteroid impact.
When large asteroids careen into the planet, they can liberate vast quantities of rock and send it screaming into the sky. Plenty of this ejecta can be composed of sulfur-bearing minerals, which turn into sunlight-reflecting aerosols in the stratosphere, the layer of the atmosphere above the lowest one. Get enough aerosols up there, and Earth can get seriously chilly extremely quickly.
For this new study, scientists simulated the injection of sulfate aerosols into the stratosphere at varying concentrations—the sort generated by a massive asteroid strike—at various points during Earth’s past, from its swelteringly hot eras to its already chilly chapters. They found that warmer times could bear the brunt of an asteroid impact without freezing over, but already cold climates could be pushed into a Snowball state by an extraterrestrial sucker punch.
There is presently no geologic evidence that shows that this happened. But this study shows that asteroids should be looked at as potential suspects. “It’s a very interesting thought experiment,” says Thomas Gernon, a geoscientist at the University of Southampton who wasn’t involved with the study.
It’s also a study that makes you appreciate current efforts to develop a planetary defense system—one that’s combining asteroid-spying observatories and asteroid-deflecting technologies to make sure dangerous asteroids never reach the planet’s doorstep.
“The effects of a large impact followed by global glaciation would be disastrous to complex life and could lead to the extinction of humanity,” says study author Minmin Fu, a climate dynamicist at Yale University.
A force strong enough to remake Earth
Block out sunlight for long enough, the planet cools and its icy areas grow. Ice reflects sunlight back into space, so as you get more of it, the planet cools further, triggering more ice formation—and if this reaches a certain icy threshold, the planet inexorably becomes a snowball.
Although it’s unquestionable that during its multi-billion-year history Earth has gone through hotter times and colder times, not every scientist agrees that Earth has become fully enveloped in ice. But a wealth of strange, ancient geologic features —for example, squashed layers of sediment and rocky debris, typically crafted and transported by glaciers, found at the equator—has convinced plenty that Earth was enveloped by snow and ice at least twice—between 720 and 635 million years ago, during the aptly named Cryogenian period of the Neoproterozoic era.
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Ascertaining why they happened (and why they ended) is of paramount importance; not long after the second snowball thawed, an explosion of complex life took place, something known as the Cambrian Explosion.
“It is therefore critical to understand why they happen—to understand the history of life on Earth and the potential for life on other planets,” says Fu.
Volcanoes have been considered prime Snowball-making suspects: perhaps they erupted too much sulfur dioxide (which becomes an aerosol in the atmosphere), engendering a cooling effect—or maybe, another theory suggests, Earth once had far fewer volcanoes belching carbon dioxide into the sky, reducing the greenhouse effect.
“Both hypotheses are feasible,” says study author Alexey Fedorov, a climate modelling expert at Yale University. But it isn’t clear if volcanoes could erupt such high quantities of sulfur dioxide quickly enough, or experience a dramatic drop in carbon dioxide output, to kickstart Snowball Earth.
An asteroid impact, though, hits differently. “An impact is a geologically instantaneous event,” says study author Christian Köberl, an impacts expert at the University of Vienna. And they are known to hastily fling plenty of sulfates into the atmosphere.
The Chicxulub impactor, the six-mile-long asteroid that struck Earth 66 million years ago, caused a litany of environmental and climatic troubles, enough to trigger a mass extinction. The sulfate aerosols it created also contributed to years of global cooling and sea ice expansion. This didn’t trigger a Snowball state—but, the authors wondered, what if something similarly catastrophic happened at other points during Earth’s history?
An extraterrestrial antagonist
To test their asteroid theory, the team created detailed simulations of various chapters of the planet’s past, each with different continental, oceanic, and atmospheric arrangements: the temperate pre-Industrial era (pre-1850), the frigid Last Glacial Maximum (20,000 years ago), a hot Cretaceous-like era (145 to 66 million years ago), and during the Neoproterozoic era (750 million years ago)—a warmer and a colder recreation.
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They then injected plausible, Chicxulub-like amounts of sulfur dioxide gas into the stratospheres of these time periods–—6.6, 200, and 2,000 billion tonnes of the stuff— and watched what happened. Would a Snowball Earth, defined here as 97 percent global sea ice coverage, transpire?
A fully frosted world did not occur in the balmy pre-Industrial and Cretaceous eras under any conditions. But adding 200 billion tonnes of sulfur dioxide to a cooler version of the Neoproterozoic era, and to the Last Glacial Maximum, was sufficient to see ice cover all the world’s seas in less than a decade.
“It is a hell of a lot more difficult to induce a Snowball when it’s so warm on Earth,” says Köberl. But this study suggests that when the planet is already cold, “it’s possible.”
The only way to corroborate this idea would be if a crater similar in size to Chicxulub (110 miles across) or an impact’s ejected, sulfur-rich remnants, were found and dated to the onset of these icy periods. Gernon suspects that, even after nearly a billion years of erosive activity by water, volcanism, biology, and tectonic mashups, such a large crater could still be found hiding on one of Earth’s continents.
“It’s tantalizing, and their modelling is kind of compelling,” he says—but he will remain skeptical until that smoking gun geological evidence is found.
For now, this remains a theoretical exercise. “Impacts don’t explain everything,” says Köberl. “But you’ve still got to keep an open mind,” because this study, and events like Chicxulub, demonstrate just how forcefully asteroids can change Earth’s fate.
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