Giant Impacts Might Have Triggered Snowball Earth Events – GWC Mag

Earth has, at times, resembled an alien world. At least twice in its history, ice reached the equator, and the planet looked more like the Star Wars ice world Hoth or the icy moons of Jupiter and Saturn than our familiar blue marble.

The development of these “snowball Earth” states still puzzles geologists.

“This is a totally unsolved and important problem in terms of planetary stability, in terms of climate stability,” said geologist Francis Macdonald of the University of California, Santa Barbara.

A new study points to a dramatic answer: These icy periods may have been kicked off by large asteroid impacts, on the scale of the Chicxulub event that killed off the dinosaurs.

Large impacts can blast titanic quantities of aerosols and soot into the atmosphere, blocking out sunlight and temporarily chilling the globe. This impact winter usually resolves in a few decades. But using climate models, researchers found that a run-in with a sufficiently large space rock during a period that’s already sufficiently chilly—as it would have been during the Last Glacial Maximum some 21 million years ago—could have kicked off a catastrophic feedback loop that encased Earth in ice. The results were published in Science Advances.

Deep Freeze

Snowball Earth events happen because snow and ice reflect sunlight and contribute to cold temperatures, and cold temperatures contribute to more snow and ice. The more reflective ice covers Earth’s surface, the chillier Earth becomes over time. Colder climes make it easier for yet more ice to form. This feedback between shiny ice and cold temperatures makes Earth’s climate unstable; polar ice creeps beyond a certain critical latitude, thought to be around 30°, kicking off catastrophic cooling, and Earth’s surface freezes right to the equator.

Why Earth froze over twice within this relatively narrow slice of geologic time is a stubborn geological puzzle.

Rocks left behind by glaciers in what were once tropical locales point to two snowball Earth events: one that kicked off between 718 and 716 million years ago and another between 650 and 639 million years ago. Why Earth froze over twice within this relatively narrow slice of geologic time is a stubborn geological puzzle.

“Recently, there’s been some suggestions that perhaps volcanism could have gotten you into a snowball Earth state,” said climate scientist Minmin Fu of Yale University in New Haven, Conn., the first author of the new study. Diverse explanations of lower greenhouse gas emissions or faster uptake of carbon dioxide from the atmosphere have also been proposed, he added.

But most triggers suggested so far can’t effectively overcome the stabilizing influence of Earth’s natural thermostat—the balance between carbon dioxide emissions by volcanoes and carbon dioxide removal through chemical weathering, which happens when rock reacts with water and air.

Fu and his colleagues explored the idea that the sudden shock of an impact winter might be enough to push Earth’s climate into a snowball runaway if the background climate was cool enough to begin with. The idea was originally proposed in 2002, but this is the first time that researchers have used sophisticated global climate simulations to put this hypothesis to the test. “It’s been just pretty loosey-goosey before this,” said Macdonald, who wasn’t involved in the new study.

Fu and his colleagues used models to simulate the climatic aftermath of collisions comparable to the notorious Chicxulub impact for four different starting climates: a preindustrial modern climate, the Last Glacial Maximum’s chilly climate, the dinosaurs’ steamy Cretaceous climate, and a cold climate similar to the conditions preceding the known snowball Earth events.

“Whether or not the climate goes into a snowball state in response to a large impact is mostly sensitive to how warm the background climate is.”

“We find that whether or not the climate goes into a snowball state in response to a large impact is mostly sensitive to how warm the background climate is,” Fu said.

Even though the preindustrial modern climate was rather cool by Earth’s historical standards, a Chicxulub-like impact wouldn’t have been enough to kick off a snowball Earth episode. But the Last Glacial Maximum was cold enough. “If the impact that killed the dinosaurs occurred 20,000 years ago, we could have gone into a snowball state, which is kind of interesting and surprising,” Fu said.

It’s possible that impact-derived aerosols could rain out of Earth’s atmosphere much faster than modeled here, Macdonald noted, which would weaken impact winters. But overall, the study shows that an impact is a feasible trigger for snowball Earth events, he said. And unlike some of the other proposed onset scenarios, a large impact has a good chance of leaving distinctive evidence in the rock record—craters, glass droplets formed when impact debris melted upon falling back to Earth, and enrichment of elements such as iridium that are more common in space than on Earth would all be strong clues.

Macdonald said he hopes modeling studies like this one will nudge geologists to get out into the field to look for that hard evidence. “I just think there’s going to be a lot more surprises and interesting things that come out of having serious modeling,” he said. “That gives geologists more things to test.”

—Elise Cutts (@elisecutts), Science Writer

Citation: Cutts, E. (2024), Giant impacts might have triggered snowball Earth events, Eos, 105, https://doi.org/10.1029/2024EO240120. Published on 15 March 2024.

Text © 2024. The authors. CC BY-NC-ND 3.0
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