Understanding the history of asteroid impacts on Earth is important because large impacts play a critical role in the evolution of Earth’s natural systems such as its atmosphere, biosphere, and hydrosphere. Unfortunately, there is very little evidence of impacts from early in Earth’s history from 2.5 to 4 billion years ago, referred to as the Archean age, because geologic processes such as erosion and plate tectonics have erased the distinct features of craters. In fact, the oldest confirmed impact structure on Earth, the Yarrabubba crater located in Australia, is 2.23 billion years old, only about half of Earth’s total age. However, impact ejecta found on Earth range from 2.6 to 3.47 billion years old. As a result, scientists have only small pieces of evidence with which to reconstruct the environment of a heavily bombarded, young Earth and understand how impact processes contributed to Earth’s evolution. The Maniitsoq region of West Greenland has recently garnered interest because several studies have hypothesized that it represents a 3-billion-year-old impact structure, nearly 750,000 years older than the Yarrabubba crater. A recent study, led by Chris Yakymchuk at the University of Waterloo, Canada, sought to test this hypothesis.
The study generated several new datasets including chemical analyses, age data, and microscopic mineral structures from more than 5,000 zircon grains collected from the Maniitsoq region. Upon closer examination of these datasets, Yakymchuk and colleagues concluded that the evidence does not support the hypothesis that the Maniitsoq region of West Greenland is an impact structure. For example, ages obtained for a sample that was originally thought to be produced by impact melting were too young to be associated with the hypothesized 3-billion-year-old impact event. Additionally, minerals that typically deform at the atomic scale when subjected to an impact event lacked those unique structures in the studied samples. Ultimately, the study concluded that their observations could be explained by the long-duration (approximately 80 million years), high-temperature metamorphism and magmatic activity that has been reported to have affected the area. In repudiating the hypothesis that the Maniitsoq region of West Greenland represents a 3-billion-year-old impact structure, the study confirms that the 2.23-billion-year-old Yarrabubba impact in Australia is the oldest known impact structure on Earth. These results highlight the continued elusiveness of Archean-age impact structures and serve as motivation for future studies that seek to improve our understanding of early Earth’s evolution through its impact record. READ MORE