More than 600 million years ago, the Earth would have been unrecognizable, with frigid, dry air sweeping over a planet of endless ice. This global state of refrigeration, called "Snowball Earth," was believed to have happened at least twice.
Mystery of the Earth's Past
Since the late 1960s, climate scientists have known that if the Earth were to get sufficiently cold, it would make a positive feedback loop resulting in a runaway cooling until the entire planet got covered in ice. Based on their models, they assumed that glacial sediments, along with other evidence, had led the Earth to enter a snowball state at points between 720 and 635 million years ago.
The Snowball Earth state started when the ice sheet spread out further from the poles. Since ice is more reflective than land or water, it increases the amount of sunlight thrown back into space. As a result, it leads to the formation of more ice and colder temperatures, a series of events that would only stop when the entire planet got completely covered in ice and had finally entered a state of equilibrium.
What sets off these episodes of runaway cooling, however, has remained a mystery for several years. There is a conventional hypothesis which states that the concentration of greenhouse gases decreased to a point that the "snowballing" process began.
Unveiling the Cause of Snowball Earth
At Yale University, researchers decided to explore an alternate possibility. They believed that an extraterrestrial impact might have caused the abrupt climate change transition. Their study is discussed in the paper "Impact-induced initiation of Snowball Earth: A model study".
Led by climate dynamicist Minmin Fu, the research team modeled the climatic consequences of a large asteroid impact at four points in ancient Earth. The same model was also used in predicting future climate scenarios by simulating both atmospheric and ocean circulation, as well as sea ice formation.
In this study, Fu and her colleagues investigated the aftermath of a hypothetical large asteroid collision during four different periods of ancient Earth. These include the Neoproterozoic Era, the Cretaceous Period, the Last Glacial Maximum, and the pre-industrial period.
In the two warmer periods that they examined - the pre-industrial and Cretaceous Periods - the researchers determined that it is unlikely that an asteroid impact would have been able to trigger a Snowball Earth state. Meanwhile, the scenario is very different when they consider an impact during the Neoproterozoic Era or at the Last Glacial Maximum. During these periods, the Earth may have been cold enough to warrant calling them an ice age.
The scientists noted that the effects of an asteroid impact may have pushed the Earth over a climate tipping point and into a global glaciation state. They also noted that this shift to a frozen Earth might have been relatively rapid.
When large asteroids strike the Earth, they can liberate vast amounts of particles and send them into the sky. This ejecta can be made up of sulfur-bearing minerals, which can turn into aerosols that reflect sunlight. When there is enough aerosol in the stratosphere, the Earth can get very chilly in just a small amount of time.
It was also suggested that a hypothetical impact crater which preceded past global glaciation might have been destroyed by burial, erosion, or subduction of the corresponding crust back into the Earth's mantle. The researchers also believe that in the future, the risk of a global glaciation would be unlikely, should the Earth be struck by an asteroid.
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