Earth Impactors Remain A Catastrophic Threat, Says Leading Geologist
Earth impactors of the sort that brought down the dinosaurs are now usually looked upon as relics of the distant past. Indeed, most of Earth's asteroidal incursions took place hundreds of millions of years earlier than the Chicxulub impactor that hit Earth some 66 million years ago.
But each year astronomers detect new asteroids, and their impact threat remains real. The United Nations has even declared 2029 for as the 'International Year of Asteroid Awareness and Planetary Defense.'
Even if a small 50-meter diameter object hits a large city, it could easily kill a million people, Christian Koeberl, a planetary scientist at the University of Vienna, tells me at the European Geosciences Union (EGU) General Assembly 2025 in Austria. We now know of 40,000 Earth crossing or near-Earth asteroids, he says. In a decade's time, we may have detected as many as 100,000 near-Earth asteroids, says Koeberl.
Yet on Earth there are only 200 currently known impact craters and only three are known to be truly ancient between 2.2 to 2.3 billion years old. And less than half of the 200 have precisely determined ages.
That's not very many, but Earth is an active geological body, and so things change on the surface over time, says Koeberl.
On Earth, there's constant weathering, erosion, volcanism as well as plate tectonics, the means by which our planet recycles its crust.
Even though Galileo first identified what we now know are impact craters on the moon in 1610, geologists didn't definitively link those lunar craters to impacts until rock samples from Apollo 11 were analyzed in Earth laboratories, says Koeberl.
But in the 1980s, when evidence for the dinosaur-killing impactor was discovered, geologists realized that all you need is a very small asteroidal object to make a crater that is least 20 times larger than the impacting body, says Koeberl.
How important has satellite remote sensing been in identifying Earth impact craters?
It used to be somewhat useful, but it has run its course, because by now, we have identified all the circular structures that are obvious of impact origin, says Koeberl.
Even so, Koeberl notes that he's constantly bombarded by amateur Google Earth impact sleuths who think they have found a new impact crater. But as Koeberl explains nearly all such photos have been formed by wholly Earth-based geological processes.
Impacts are the highest energetic geological process that we know, says Koeberl. Each event per area, per affected rock is somewhere between a hundred to a few thousand times more energetic than the largest volcanic eruption possible, he says.
In fact, they are so energetic that they cause changes in the affected rocks' mineral structure.
I could name a dozen other geological processes that form circular crater-like features on Earth's surface, says Koeberl. This is what brings us to the very important point of shock metamorphism, which is how you identify an impact crater, he says.
Koeberl takes out his laptop and shows me a magnified image of a quartz crystal rock that has the telltale signatures of impact shock.
No normal quartz crystal will look like this, says Koeberl. The lines that go through here are what we call shock lamellae, and they only form from an impact and no other geological process, he says.
The paucity of ancient craters also correlates with major episodes of extensive 'Snowball Earth' glaciation phases, with its related subglacial erosion some 650 to 720 million years ago, Koeberl and colleagues note in a 2024 paper in the journal Precambrian Research. It's thought to have removed kilometers of material from the continents, enough to erase most existing impact craters except for the large ones, they write.
Despite their potential for calamity, serendipitously, a few impacts have inadvertently revealed precious metals buried beneath Earth's surface.
Located in the center of the Witwatersrand gold fields in present day South Africa, the Vredefort impact event formed the largest impact structure that remains at least partly preserved, the authors note. Some two billion years ago, the impact uplifted a massive gold cache that since the 1880s has generated about a third of the total gold ever extracted from our planet, they write.
Trouble is, most of us fail to realize that we live in a dynamic solar system with asteroidal and cometary leftovers from its formation that potentially threaten life here in untold ways.
Past impact craters on the surface of our planet serve as a reminder that we are constantly bombarded from space, often with devastating consequences, says Koeberl. Such events happened in the past and will happen in the future, he says.
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