Forbidding Planet: Scientists Find Remains Of Monster Asteroid Impacts On Early Earth

Artist’s view of Earth several billion years ago during the Late Heavy Bombardment, when the planet is thought to have been battered by impacts of comets and asteroids. Credit: Chris Butler/SPL

Earth 3.5 billion years ago was a terrifying place. Picture a rocky landscape pounded by meteorites and asteroids with a surface resembling that of the moon. Volcanoes spewed water vapor but also a toxic mix of carbon dioxide, sulfur dioxide and methane. If you could whisk yourself back to this world by time machine, you’d need to be fully protected by a spacesuit and lucky enough to not get picked off by a falling space rock. Oh, and bring a boat too. Hot-water oceans likely covered a fair portion of the planet back then.

This time-lapse illustration of the Nice (pronounced ‘neece’) model of solar system evolution shows how outer planet migrations kick asteroids into the inner solar system

Scientists call the period from about 3.8 billion to 1.8 billion years ago the Late Heavy Bombardment (LHB), a time when the number of asteroids and their fragments pelting the inner planets and their moons spiked. Why then? No one’s absolutely certain, but the leading theory posits that the migration of the giant outer planets to their present positions “stirred the gravitational pot”, slinging boatloads of asteroids into the inner solar system, where they rained down on Earth and its neighbors in hellish monotony for millions of years.

Anyone with a small telescope can see resulting devastation to this day. Just take a long look at the moon’s battered and cratered surface and thank your lucky stars you’re around during a more peaceful time. Finding Earth’s craters is trickier because water and wind erosion, along with the continual recycling of much of our planet’s crust through plate tectonics, has erased much of our violent past.

The Vredefort Dome – these concentric hills, which rebounded after the impact that created Vredefort Crater – are what remains after an asteroid about 3-6 miles wide struck Earth 2 billion years ago. Credit: NASA

About 180 craters are known on Earth today, but we’re aware of only three resulting from the Late Heavy Bombardment. The oldest, estimated at 3 billion years old and 62 miles (100 km) wide, is also the most recently discovered. Found in western Greenland in 2012, all that remains of the impact are rocks rattled by the massive shock wave that penetrated 15 miles (25 km) deep within Earth’s crust.

You can still see the remains of the impacts that formed the 112-mile-wide (180 km) Vredefort Crater in South Africa, which is 2 billion years old, and the youngest LHB member, the 155-mile (250-km) Sudbury crater in Canada dated at 1.85 billion years.

Map of South Africa with the Barberton greenstone belt shown in red. Shock waves from the impact of an asteroid 3.26 billion years ago created telltale formations within the belt. No one knows yet where the impact happened.

Now, a group of scientists have announced they’ve found evidence for an even older impact, one that occurred 3.26 billion years ago and left its signature in a South African region known as the Barberton greenstone belt.

A recent press release describes the huge impactor as between 23 and 36 miles wide (37- 58 km). Colliding with the planet at 12 miles per second, the jolt delivered was bigger than a 10.8 magnitude earthquake and propelled seismic waves hundreds of miles through the Earth, breaking rocks and setting off other large earthquakes. Tsunamis thousands of feet deep swept across the oceans that covered most of the planet at that time.

A graphical representation of the size of the asteroid thought to have killed the dinosaurs (left), and the crater it created, compared to an asteroid thought to have hit the Earth 3.26 billion years ago and the size of the crater it may have generated. A new study reveals the power and scale of the event some 3.26 billion years ago which scientists think created geological features found in a South African region known as the Barberton greenstone belt. Credit: American Geophysical Union

“We knew it was big, but we didn’t know how big,” Donald Lowe, a geologist at Stanford University and a co-author of the study, said of the asteroid.

The collision would have blasted out a crater some 300 miles (500 km) wide, filled the atmosphere with fiery rock vapor and set the surface of the ocean a-boil. We’re talking serious cataclysm. Somehow life found a way through the heat and crater-punching to gift us with the rolling green hills, coral reefs and forests that characterize Earth today.

Table from the book “Near Earth Objects – Finding Them Before They Find Us” by Donald Yeomans showing average asteroid impact results and probabilities by size. Credit: Donald Yeomans

I try to imagine the dark days of the LHB to help me appreciate these calmer times. Yet we know in our gut – and in fact, thanks to probability – that we’ll never truly be out of the woods. Asteroids lurk in the deep that could one day cause a similar scenario. Don’t let it worry you too much – the chance of a 10-mile-wide space rock striking Earth is once every 89 million years. You’ve still got time to take a nap, catch a show and enjoy a few nights out on the town. Probably.

8 Responses

    1. SamL

      If I may. Have you heard of the idea that our moon was formed by an impact long ago that kicked up a great proportion of the object and the Earths early mass up to then form our moon. Well that impact you can think of as the kick starter for plate tectonics and the shifting around of the continents repositioning themselves since that past impact that made the entire Pacific Ocean.

      How’d I do A.B.?

      1. SamL

        Ps, that seems too obvious once you recognize that cosmic – geologic history. The impact cracked a large section of land that remained on the crust, that became the Mid Atlantic Ridge. Initially cracked from below, then it has been spreading apart ever since.

      2. astrobob

        The leading theory for the formation of the moon involves a very early grazing impact of a Mars-sized planet with Earth. Material from the collision eventually coalesced to form the moon. The theory that the moon arose from a chunk of Earth where the Pacific Ocean is now has been discredited.

  1. SamL

    Astro Bob, I love your page so much. I have a question about one table in this story. You have “no crater” listed for objects up to 30m in size. This brings to mind the question, what is the requirements to determine that a crater is before you? A 30m rock is going to leave a mark. no matter how far from above it is dropped. Is there an International Crater Assessment Panel that must certify a hole or indentation before it can be called a crater? I believe a 30m bubble of water would leave a mark too. The “no crater” distinction seems off to me.

    Thanks for all the entertaining cosmic information over the years.

    1. astrobob

      Hi SamL,
      Thanks for writing. Great question. Yes, a 30-meter object would definitely make a crater — if it fell intact. Yeomans table lists it and smaller objects as “no crater” because most meteoroids that size and smaller break up in the atmosphere and fall to the ground as fragments (meteorites) that don’t leave craters so much as indentations or holes in the ground. It’s a stretch but you could still call these craters even if they’ll be gone with the next heavy rain. The Carancas fall in 2007 left a crater in the mud 15 feet deep and 43 feet around. They estimate the meteoroid’s size around 3-5 feet. I think Yeoman was talking mostly about significant craters that last more than a few months or years. As for your other question, there’s no special committee to name craters per se. Craters are usually named after the meteorites (if any are found) that created them. And meteorites are named for the nearest town or a significant feature in the local landscape.

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