Scientists Rush to Uncover Ancient Craters Before They Vanish

The traces of impact craters and their encompassing structures may be observable on a geologic map, similar to a bullseye, but what about the lingering geophysical marks at the extremities of these structures? (Image Credit: Huber et al. 2023, Journal of Geophysical Research: Planets)

Earth’s primitive history continues to puzzle geologists, with ancient craters potentially holding the key to unraveling some of these enigmas. Scientists are now urgently trying to locate these craters.

The oldest craters on Earth could furnish scientists with crucial data about the early Earth’s structure, the composition of objects in the solar system, and how to interpret crater records on other planets. However, locating these primeval craters proves challenging for geologists and there’s a chance they may never be discovered, as indicated by a recent study. This study was published on August 1, 2023, in the Journal of Geophysical Research Planets, AGU’s official journal for planetary formation and evolution research, including moons and objects in and beyond our Solar System.

The Mystery of the Ancient Craters

Impact evidence such as ejected materials, melted rocks, and high-pressure minerals from over 3.5 billion years ago have been unearthed by geologists. However, the actual craters from this time period remain obscure. The oldest known impact structures, as scientists refer to these gigantic craters, are merely about 2 billion years old, leaving us with a missing 2.5 billion-year record of such massive craters.

The Role of Time and Erosion

The constant passage of time and the unceasing process of erosion are to blame for this gap, says Matthew S. Huber, a planetary scientist at the University of the Western Cape in South Africa, who studies impact structures and is the lead author of this study.

The preservation of the ancient structures we do possess seems serendipitous, says Huber. There are many questions that could be answered if we had access to those older craters. But that’s typical in geology, where we are often left to piece together a story from what we have at hand.

Decoding Buried Craters

Occasionally, geologists can identify hidden, subterranean craters using geophysical tools such as seismic imaging or gravity mapping. Once potential impact structures are identified, the scientists can look for physical remnants of the impact process, like ejecta and impact minerals, to confirm its existence.

Erosion’s Impact on Craters

Huber and his team were keen to understand how much of a crater could be eroded before the last remaining geophysical traces vanish. Geophysicists have theorized that 10 kilometers (6.2 miles) of vertical erosion would obliterate even the largest impact structures, but this idea had never been put to a practical test.

The Vredefort Crater Investigation

The scientists analyzed one of the planet’s oldest known impact structures to discover this: the Vredefort crater in South Africa. This structure spans approximately 300 kilometers (186 miles) and was formed about 2 billion years ago when a 20-kilometer (12.4 miles) wide impactor crashed into the planet.

Erosion Over Time

The impactor hit with such force that the crust and mantle rose up at the point of impact, forming a long-lasting dome. Further from the center, ridges of rock protruded, minerals altered, and rock melted. Over the course of two billion years, time and erosion wore down about 10 kilometers (6.2 miles) from the surface.

Impact Remnants Today

All that’s left today is a semicircle of low hills southwest of Johannesburg, indicating the center of the structure, and some smaller, subtle signs of impact. The bullseye, caused by the uplift of the mantle, is visible on gravity maps, but beyond the center, geophysical evidence of the impact is absent.

That pattern is one of the few remaining geophysical signatures detectable and that only occurs for the largest impact structures, Huber explains. As only the deepest layers of the structure remain, the other geophysical traces have vanished.

Despite this, Huber aimed to understand how reliable those deep layers are in documenting ancient impacts from both a mineralogical and geophysical viewpoint.

Study Methods

Erosion erases these structures from the top down, explains Huber. Hence, we studied them from the bottom up.

The team sampled rock cores across a 22-kilometer (13.7-mile) transect and examined their physical properties, looking for variations in density, porosity, and mineralogy between impacted and non-impacted rocks. They also modeled the impact event and its effects on rock and mineral physics, comparing these predictions to their sample observations.

Outcomes of the Study

The results were disheartening for those searching for Earth’s oldest craters. While some impact melt and minerals persisted, the rocks in the outer ridges of the Vredefort structure were essentially identical to the non-impact rocks surrounding them when examined through a geophysical lens.

That was not the outcome we anticipated, Huber admits. The difference, when present, was incredibly subtle. It took us some time to truly comprehend the data. Ten kilometers of erosion and all the geophysical evidence of the impact vanishes, even with the largest craters, thus validating the earlier estimations of geophysicists.

Looking Ahead

The researchers found Vredefort just in time; any more erosion and the impact structure would be eradicated. The likelihood of uncovering buried impact structures over 2 billion years old is slim, says Huber.

To have an Archean impact crater endure until today, it would have had to experience incredibly rare preservation conditions, Huber adds. But Earth is filled with extraordinary conditions. So, perhaps there’s something unexpected out there, and so we continue our search.

Research Citation: “Can Archean Impact Structures Be Discovered? A Case Study From Earth’s Largest, Most Deeply Eroded Impact Structure” by M. S. Huber, E. Kovaleva, A. S. P. Rae, N. Tisato and S. P. S. Gulick, 1 August 2023, Journal of Geophysical Research Planets. DOI: 10.1029/2022JE007721

Authors’ Affiliations:

Matthew S. Huber (corresponding author) and E. Kovaleva, Department of Earth Science, University of the Western Cape, Bellville, South Africa; E. Kovaleva also at the Helmholtz Centre Potsdam, GFZ, Potsdam, Germany

A.S.P. Rae, Department of Earth Sciences, University of Cambridge, Cambridge, UK

N. Tisato and S.P.S. Gulick, Department of Geological Sciences, Jackson School of Geoscience, University of Texas at Austin, Austin, TX, US; they are also affiliated with the Center for Planetary Systems Habitability, University of Texas at Austin, Austin, TX, US; and the Institute for Geophysics, Jackson School of Geoscience, University of Texas at Austin, Austin, TX, US

Frequently Asked Questions (FAQs) about Ancient Impact Craters

More about Ancient Impact Craters

• Journal of Geophysical Research: Planets
• Understanding Impact Craters
• The Vredefort Crater
• Erosion and Its Impact on the Environment
• University of the Western Cape, Department of Earth Science