Earth’s oldest craters could provide valuable information about the early Earth’s structure and the composition of celestial bodies in our solar system, as well as help scientists understand the crater records on other planets. However, according to a new study published in the Journal of Geophysical Research Planets, it is unlikely that geologists will ever be able to find these ancient craters.
Geologists have discovered evidence of impacts from over 3.5 billion years ago, such as ejected materials, melted rocks, and high-pressure minerals. However, the actual craters themselves have remained elusive. The oldest known impact structures on the planet, which scientists refer to as mega-craters, are only about 2 billion years old, leaving a gap of two and a half billion years without any record of these massive craters.
This gap is due to the continuous erosion caused by the passage of time. Matthew S. Huber, a planetary scientist at the University of the Western Cape in South Africa who led the study, explains that it is almost by chance that the existing old structures are preserved. He points out that geologists have to work with the limited available information in this field, and there are many unanswered questions that could be resolved if older craters were discovered.
Geologists sometimes use geophysical tools like seismic imaging or gravity mapping to identify hidden or buried craters. Once a potential impact structure is identified, scientists search for physical remnants of the impact process, such as ejected materials and impact minerals, to confirm its existence.
The study led by Huber aimed to determine how much erosion can erase a crater’s geophysical traces. While geophysicists suggest that even the largest impact structures could be erased by 10 kilometers of vertical erosion, this threshold had never been tested in the field before. To investigate, the researchers studied the Vredefort crater in South Africa, one of the planet’s oldest known impact structures. This crater, which is about 300 kilometers across, was formed around 2 billion years ago when a 20-kilometer-wide asteroid crashed into Earth.
The impact created a long-term dome, elevated the crust and mantle, and caused rock ridges, mineral transformations, and melting. Over the course of two billion years, erosion has worn away about 10 kilometers from the surface of the structure. Today, only a semicircle of low hills southwest of Johannesburg indicates the center of the structure, while gravity maps provide some evidence of the uplift of the mantle. However, geophysical signs of the impact are missing beyond the center.
To evaluate the reliability of the remaining deep layers as records of ancient impacts, the researchers analyzed rock cores and compared the physical properties between impacted and non-impacted rocks. They also created models to simulate the impact event and its effects on rock and mineral physics, aligning these models with their observations.
The results were not encouraging for the search for Earth’s oldest craters. Although some impact melt and minerals were still present, the rocks in the outer ridges of the Vredefort structure were indistinguishable from the non-impact rocks when viewed through a geophysical lens. Huber describes this as an unexpected outcome, confirming the previous estimates of geophysicists that 10 kilometers of erosion is enough to remove all geophysical evidence of an impact, even in the case of the largest craters.
According to Huber, the study caught the Vredefort crater just in time. Any further erosion would result in the complete disappearance of the impact structure. Given the rarity of finding preserved impact structures from over 2 billion years ago, the chances of discovering Earth’s oldest craters are low. Huber acknowledges that Earth is full of unexpected conditions, so scientists will continue the search for these ancient craters.