Image 1: Art by Nina (Yurou) Liu
At several points in Earth’s history, our home world looked nothing like the blue planet we know today—instead, it was a frozen wasteland. During these periods, nearly all of Earth’s oceans were covered by thick sheets of ice ranging from hundreds of meters to kilometers thick.
This phenomenon of extreme global glaciation, called “Snowball Earth,” occurred at least twice over the past billion years. The first and most extreme of these episodes, the Sturtian glaciation, took place approximately 710 million years ago and lasted for nearly sixty million years, permanently altering the Earth’s atmosphere and biodiversity. The second known snowball event occurred roughly 640 million years ago and was followed by a surge in oxygen levels in the Earth’s atmosphere. This increase in oxygen enabled the evolution of aerobic metabolism, a biological process reliant on oxygen to convert nutrients into energy. Aerobic metabolism’s efficiency in energy production and storage was instrumental to the emergence of multicellular life forms and the evolution of the first animals during an era known as the Cambrian Explosion around 540 million years ago.
Yet the causes of Snowball Earth remain unknown. “Even though [‘Snowball Earth’] represents the most dramatic changes that Earth’s climate has ever experienced, it’s actually not fully understood why they occurred,” said Minmin Fu, a Flint postdoctoral fellow at Yale’s Ocean, Atmosphere, and Climate Modeling group. Investigating the potential causes of Snowball Earth has been a point of interest in the research group, which is run by Alexey Fedorov, a professor of ocean and atmospheric sciences at Yale. “Understanding why [snowball events] occurred in Earth’s past is definitely an important problem, and this could also have implications for understanding life on other planets,” Fu said.
What could push the Earth to become a massive snowball? To address this question, Fu teamed up with Fedorov and collaborators from the University of Chicago and the University of Vienna. Their study, which was published in Science Advances earlier this year, suggests that under the right conditions, a massive asteroid impact—one similar in size to the collision that wiped out the dinosaurs—might just be a key to finding the answer.
A Long-Standing Debate
Many scientists believe that Snowball Earth events occur due to a positive feedback loop known as the ice-albedo feedback. This occurs as the Earth becomes increasingly covered in ice, causing the surface to become more reflective and reducing the amount of sunlight absorbed by the Earth’s surface. Consequently, the planet’s temperature drops, allowing sea ice to expand even further. Eventually, the Earth freezes over completely, with glaciers expanding from the tips of the poles to the equator, freezing everything in their path. The current challenge lies in explaining why global temperatures dropped so severely, thus initiating ice-albedo feedback—and there has been no shortage of scientific disputes over potential mechanisms.
One popular theory proposes that increased volcanic eruptions could have initiated Snowball Earth. This could have occurred through two potential mechanisms. First, the weathering of basaltic rocks, which are volcanic rocks released during eruptions, may reduce atmospheric carbon dioxide (CO2) via a process called silicate weathering, thereby diminishing the greenhouse effect and leading to global cooling. Another possible mechanism is that eruptions emit large amounts of sulfate aerosols into the atmosphere. These aerosols act as a reflective barrier, deflecting solar radiation back into space and reducing the amount of heat reaching Earth’s surface, effectively lowering the planet’s temperature.
However, Fu and Fedorov were puzzled by how volcanism alone could lead to a Snowball Earth since volcanic eruptions would not release enough sulfate to initiate Snowball Earth. “You would need many [powerful] volcanic eruptions, uninterrupted, happening all the time,” Fedorov said. “In some ways, these explanations are unsatisfactory. So we wanted to explore a different and exciting new mechanism, which is the possibility that [asteroid] impacts could cause these events,” Fu added.
Fu and collaborators looked at an alternative hypothesis, called the impact theory, which suggests that Snowball Earth was triggered by a large-scale extraterrestrial impact. When an asteroid slams into the Earth, the energy from the impact instantly vaporizes nearby rocks, ejecting tons of sulfur aerosols into the atmosphere—enough to initiate snowball conditions. While the impact theory was first proposed by two Danish researchers in 2002, it was largely overlooked due to its reliance on simpler modeling techniques compared to the technology available today. Recognizing the frequency of collision events in Earth’s history, Fu and collaborators found merit in revisiting and expanding upon this lesser-known theory using current, more sophisticated climate models.
Finding a Recipe
Climate models that simulate Earth’s atmosphere and oceans have long been valuable tools for everyday weather forecasting and climate science research. “We used a global climate model which is similar to the ones that are used to project future climate change, except here, we’ve adapted it to study the paleoclimate deep in Earth’s past,” Fu said. In their study, the researchers simulated various historical climates—including preindustrial (150 years ago), Last Glacial Maximum (21,000 years ago), Cretaceous-like (145 to 66 million years ago), and Neoproterozoic climates (1 billion to 542 million years ago)—along with extraterrestrial impacts of varying magnitudes. To simulate these paleoclimates, the team reconstructed the conditions during each period of interest, which included factors such as temperature, paleogeography, and concentration of greenhouse gasses in the atmosphere.
The climate model suggests a possible recipe for ideal conditions for a Snowball Earth. First and foremost, the Earth must already be sufficiently cold. Out of the climates simulated, only large-scale impacts under the Last Glacial Maximum (LGM) and Neoproterozoic climates—both of which had colder oceans compared to the warmer preindustrial and Cretaceous climates—resulted in snowball events. While smaller-scale impacts under LGM conditions did not trigger snowball events, these collisions were sufficient to cause a sizable increase in sea ice coverage, which was not seen in warmer climates. “It seems that the global temperature is the most significant variable for determining whether or not you’re vulnerable to snowball initiation after a large impact,” Fu said.
The asteroid must also strike the Earth on land rather than the ocean. Impacting the ocean would mostly release water vapor, which would not cool the planet significantly. In addition, the asteroid must be sufficiently large—over ten kilometers wide—to generate at least two hundred gigatons of sulfates. Finally, atmospheric CO2 concentration cannot be too high; the researchers found that higher concentrations of CO2 in the colder LGM and Neoproterozoic climates amplified the greenhouse effect and prevented snowball events. “If you satisfy all of these conditions, then it’s possible that you could initiate snowball conditions,” Fu said.
The recipe for a Snowball Earth aligned with the scientists’ expectations. Based on a rough estimation of the frequency of massive asteroid impacts, the researchers concluded that there is a fifty-three percent chance that an impact-induced snowball event occurred at some point in Earth’s history. However, the rapid speed at which the Earth snowballed in the model was quite surprising.
“I was thinking, maybe it would take a century or maybe even longer, but after ten years, the entire ocean was covered with ice,” Fedorov said. In the researchers’ simulation, the process took only around seven to eight years to reach ninety-five percent ice coverage—a stark contrast to the millions of years that the volcanism hypothesis posits it would take for Earth to reach the threshold for global glaciation.
Searching for Impact
Moving forward, the Yale researchers aim to gather additional evidence to solidify their theory. Currently, there is no geological evidence to support the theory that previous Snowball Earth events were caused by impacts. The most recent Snowball Earth occurred over seven hundred million years ago, which means that any potential evidence is literally buried deep in time. Rock fragments from impact craters have likely undergone heavy erosion and compression under ice, making them nearly impossible to identify and analyze.
Nevertheless, Fedorov remains hopeful that evidence of an impact might still be out there—and history is on his side. At the Chicxulub crater, where the impact responsible for the demise of the dinosaurs struck Earth, scientists identified a rock layer containing high levels of iridium dating back to sixty-six million years ago. Given that natural iridium is not commonly found on Earth, its presence likely originated from an extraterrestrial source. If the impact theory holds, there may be traces of elements still lingering from snowball-inducing collisions as well. Fedorov is currently in discussion with scientists at Yale who study the deep geological past to determine what clues they can search for.
In addition, Fu and Fedorov hope to test other hypotheses, which could further strengthen the theory that impacts indeed played a role in snowball events. For instance, coupling the climate modeling approach with a geochemical model could aid in quantifying how CO2 levels decrease in the volcanism hypothesis, offering opportunities to test and validate the competing hypothesis. According to Fu, linking how several different hypotheses fit within each other may be the key to understanding how Snowball Earth events were initiated: while volcanism alone may not fully explain how Snowball Earth was initiated, it may have been an important factor. “If you had a CO2 drawdown that wasn’t sufficient to get you into snowball conditions by itself, but led you to a much colder climate that was much more vulnerable to an impact, then there could be a combination of multiple mechanisms at play,” Fu said.
Asteroid or not, Snowball Earth most likely resulted from the blend of several key ingredients, all working in harmony to freeze the globe and create life as we know it today.