# How often do we get smacked by space rocks?

By Phil Plait | November 14, 2006 11:25 pm

… or, perhaps more eloquently, how often does the Earth suffer major impacts from cosmic debris?

This is a terribly important question. Just look to Meteor Crater Arizona if you’re not sure… or the surface of the Moon. A rock a hundred meters across impacting at 30 kilometers per second could wipe out an entire county. If it hit over a major city, well, you can figure out what that would mean.

There are different ways to figure out how often a big rock hits us. One is to look out into space and count up the number of rocks out there that can possibly hit us. How many could cause, say, a 10 megaton explosion upon impact (for comparison, the largest nuclear weapons explode with a yield of about 10 Mtons)? Count ’em up, apply some math, and you can get a statistical rate of impact — very roughly one such impact every million years or so.

Another way is to look down, not up. Look for evidence of impacts here on Earth, like craters. But not just craters: 72% or so of the Earth is covered in water, and an ocean impact would create an enormous tsunami, one big enough to dwarf the one that killed a quarter million people in 2004. That leaves evidence too.

A maverick group of scientists have looked for just that kind of evidence, and they are concluding that impacts are far more common, maybe occurring 10 times as often as previously thought (that link goes to the New York Times, and you can register for free to read it). They have found structures called chevrons, arrow or wedge-shaped deposits of sediment that can be hundreds of meters high.

I couldn’t find too much information on this sort of geological deposit on the web, but I did find one pretty good paper about them. It had some drawings of that these chevrons look like:

You get these formations from normal tsunamis, but they can also be formed from impacts. One difference would be in the size of the chevrons, and also in their composition: impact-formed chevrons would have mineral mixtures more typical of asteroids than of the Earth. Some like that have been found! Also, chevrons tend to point in the same direction, indicating the direction of the tsunami. Tracing these backwards, the scientists have found that some of them point right toward impact craters in the bottom of the ocean.

Yikes.

Let’s be clear: this is brand new stuff, and the evidence is still shaky. And to be blunt, what they claim directly contradicts what is thought to be known about the frequency of impacts from counting asteroids. David Morrison, a NASA impact expert, was quoted in the NYT article and makes this clear. In the newsletter he sent out today, he says:

… the NY Times article reports on an international group of scientists who dispute the standard understanding of impact frequencies. They quote both early written documents and geological evidence to suggest a much higher impact rate during the Holocene (the past 10,000 years). As noted in this article, the most recent assertions involve evidence for mega-tsunamis within this period, on a scale that suggest one or more impacts in the 100,000 megaton range. Previously there have also been claims for smaller Tunguska-class impacts occurring at a rate of several per century. These impact rates are at least an order of magnitude greater than is calculated from the current population of NEAs [Near Earth Asteroids]. As I am quoted in this article, if they are right and they find convincing evidence for multiple recent tsunamis, we’ll have a real contradiction on our hands.

That’s pretty cool. It’s cool that we have a mystery on our hands, and it’s cool that an established (mainstream) scientist is so willing to admit that there may be trouble brewing if these new findings pan out. Like anyone else, I’m fascinated by the prospect of a major collision in the future — whether it’s an asteroid hitting the Earth, or two hypotheses smacking head on. I’ll be keeping my eye on this for sure.

CATEGORIZED UNDER: Astronomy, Cool stuff, Science

1. A Ler…-- Rastos de Luz | November 15, 2006
1. Tim G

A rock a hundred meters across impacting at 30 kilometers per second could wiping out an entire county? That’s equivalent to 100 Megatons of TNT, which doesn’t seem that great to me. Maybe that could wipe out Liechtenstein.

From the article:

Scientists…say the evidence… is strong enough to overturn current estimates of how often the Earth suffers a violent impact on the order of a 10-megaton explosion. Instead of once in 500,000 to one million years…catastrophic impacts could happen every few thousand years.

Until Project Space Guard becomes an international effort, the risk of populations within 100 miles of a coastline being wiped out my not be insignificant.

Novels and movies about impacts seem to always involve a comet or asteroid capable of wiping out all of humanity. Perhaps the far more frequent but still catastrophic smaller impacts are our biggest overall threat. If we can spot a medium-sized asteroid with the ocean’s name on it a week in advance, coastlines
can be evacuated.

2. Tim G

Strike out “could” in my first sentence in my first post.

3. The Russians detonated a 50 Megaton bomb (called “Ivan”, which was dropped from a plane) in 1961. It blew a hole in the atmosphere and the mushroom cloud went around the earth three times. The pilot barely got away…

I am a little bit skeptical about the reported results, though. Are these chevrons better preserved than impact craters on land? I can’t believe that meteors hit the ocean ten times more than they hit land.

4. jess tauber

No mystery- it was the Centauri and their planetary bombardment. Sure they claim they didn’t know we were here, but who wants to foot the remediation bill? But not to worry, the minions of the departed Shadows will give them theirs. And then Stephen Forster can rule the empire.

5. Robert Day

There need be no contradiction here. We are dealing with probability after all. The ‘looking down’ record could simply be the record of a particularly unlucky period where the number of impacts were much higher than usual. A period following (or indeed, preceeding) this of much fewer impacts than usual would redress the balance. In other words, an impact today does not make us immune from another one tomorrow and just because we haven’t had one for a long time doesn’t actually make it more likely that we will have one now.

6. jason B

PK. It was the shock wave that around the world three times.

Also we have Bruce Willis so there is absolutley nothing to worry about.

7. This cool. As far back as ’99, we were pointing out Bryant’s megatsunami deposits on the NSW coastal field trip for first years. In the Pacific, though, you have a competing hypothesis, as Hawaii megaslides can also cause large waves. This Indian ocean stuff, as well as the identification of possible craters, is very cool.

I had a minor quibble about the NYT writeup, which I posted here:
http://lablemminglounge.blogspot.com/2006/11/eye-of-newt-and-blur-of-science.html

8. Clearly, if scientists disagree about some aspect of the observed data, we can only conclude that all data in this area is also incorrect and the universe is only six thousand years old. I mean come on, if they disagree by a factor of ten, they could just as easily be off by a factor of … um … one million … see, the universe is only six thousand years old, and …

This Creationist Reality Break was brought to you by the makers of “Delusion-O’s”, the healthy and heavenly breakfast cereal guaranteed to warp your perception of the universe by lacing the sugar with crack!

9. Clearly, if scientists disagree about some aspect of the observed data, we can only conclude that all data in this area is also incorrect and the universe is only six thousand years old. I mean come on, if they disagree by a factor of ten, they could just as easily be off by a factor of … um … one million … see, the universe is only six thousand years old, and …

This Creationist Reality Break was brought to you by the makers of “Delusion-O’s”, the healthy and heavenly breakfast cereal guaranteed to warp your perception of the universe by lacing the sugar with crack!

10. Tim G

Lab Lemming,

It looks like Dr. Abbott made a slip of the tongue. Still, I think the journalist or editor should have noticed it before publication.

11. Mike

Do they address the possibility that some of these ‘chevrons’ could be created by tsunamis caused by massive earthquakes?

12. I just found some of these in Google Earth. Search for Carnarvon, Australia and then go north just a bit to where the “corner” of the island is.

13. Peptron

Mike said:
Do they address the possibility that some of these â€˜chevronsâ€™ could be created by tsunamis caused by massive earthquakes?

It’s covered in the article. They can make the difference by the mineralogy of those chevrons.

14. jason B, I got the cloud info from a fairly recent documentary (I don’t remember which one, so that severely weakens my case), and I just read on Wikipedia about the shock wave through the Earth.

However, if the mushroom cloud reached an altitude of 62 km, I can easily believe that it went around the Earth three times, because it would essentially sit relatively undisturbed in the stratosphere.

15. John Parejko, that’s very cool, thanks! The coordinates are roughly 24 deg 33 S 113 deg 30 E.

16. Brandon

Yep, gotta correct the Nuke reference. 10mt was the yield of the first full scale thermonuclear test (IvyMike) and 15mt was the largest yield the US ever created (7mt that “ran-away” to 15mt). The Soviets however, detonated much larger devices with 25mt and 50-60mt “Tzar Bomba.” The latter was actually a scaled-back version of a 100mt bomb, air dropped over Novaya Zemlya range. The shockwave travelled around the world three times actually, not the cloud (perhaps the fallout).

17. Brandon

Yep, gotta correct the Nuke reference. 10mt was the yield of the first full scale thermonuclear test..IvyMike.. and 15mt was the largest yield the US ever created ..7mt that “ran-away” to 15mt. The Soviets however, detonated much larger devices with 25mt and 50-60mt “Tzar Bomba.” The latter was actually a scaled-back version of a 100mt bomb, air dropped over Novaya Zemlya range. The shockwave travelled around the world three times actually, not the cloud..perhaps the fallout.

18. Brandon

Sorry, dupe post…

19. PaleoProf

I worked in the K/T tsunami deposits in Texas, and I’ve gotta say it’s VERY easy to look at the sediments and sedimentary structures and tell a dune deposit from a tsunami deposit (which seem to be the main issue in the Kelletat and Scheffers paper) they admit that it needs to be done but they don’t do it. It seems that before they go to all the trouble they went through that you would do a quick sediment analysis to rule out the possibility of them being dunes. It just makes me a little nervous.
I would love to go to the AGU meeting and hear what Abbot has to say but it’s during finals week.

Paul

20. Gary Ansorge

Doc Jerry Pournelle and Larry Niven (?) did a fine job explaining large impact dynamics in Lucifers Hammer, especially the consequence of an ocean strike. If we can’t impell humanity into space for resource acquisition reasons, perhaps the rationale of protecting our collective bums will get the point across?
At any rate of impact, even a single 100 meter asteroid is one too many, considering the delicate balance of technologies requirted to hold our civilization together, if it arrives before we’ve dispersed our tribes across the solar system.

GAry 7

21. Mark Martin

PK Said:

“I canâ€™t believe that meteors hit the ocean ten times more than they hit land.”

Why wouldn’t they strike the ocean more frequently on average than land? The surface area of Earth’s oceans is much greater than that of the land. This means that an Earth-intersecting asteroid chosen at random has a higher chance of intersecting water than land.

22. TheBlackCat

“How wonderful that we have met with a paradox. Now we have some hope of making progress.”
-Niels Bohr

23. Irishman

Mike Martin, I think the difference is looking at percentage of hits in ocean vs land, versus the rate of hits in ocean vs land. You’re looking at percentage, Gary is talking about rate. If you look at the Earth as an undifferentiated sphere, there’s every reason to assume the flux of impact at any region should be equal to the flux at any other region. The average around the globe should be constant. So if the rate of impacts in the ocean is higher than the rate of impacts on land, that flies in the face of the understanding. There’s no mechanism for impactors to preferentially select ocean as target other than the ratio of surface area. Take surface area out of the equation and still see a difference in rates, then you have an unexplained anomaly.

Tim G, that’s county, not country.

24. PaleoProf

Mark Martin Said:

“Why wouldnâ€™t they strike the ocean more frequently on average than land? The surface area of Earthâ€™s oceans is much greater than that of the land. This means that an Earth-intersecting asteroid chosen at random has a higher chance of intersecting water than land.”

70% of the surface of the earth is covered by ocean so persumably somethng hitting the Earth has a 70% chance of hitting ocean and a 30% chance of hitting land. So it’s greater but not 10 times greater.

25. PaleoProf

argh… typos… sorry in a hurry here…

26. Tim G

Oops! Anyway, here is what a 50-60 Megaton blast would look like.

27. Mark Martin

Irishman,

Yes, I see what you mean: “Where are all the high-energy impacts on land?” That’s a valid point. It then becomes similar to the theory that Earth is bombarded daily by dozens of miniature comets. The question prompted by that proposition is about the same: “Where are all the daily comet impacts on the Moon?”

28. Mark Martin

â€œHow wonderful that we have met with a paradox. Now we have some hope of making progress.â€

That is so true. It reminds me of an interview I saw at least as far back as the early ’90s with Steven Weinberg. It was a documentary about hunting for the Top quark, and whereas many of the run-of-the-mill scientists were all expressing hope that it would be verified soon, Weinberg said something more to the effect of, “What a lot of us are hoping for is that the Top quark won’t be found, either where it’s predicted or at all. That way we’ll have a failure of the Standard Model, and we’ll have something as a guide so we can make a better theory.”

That’s a class-act.

29. Bob

Tsunamis aside, one aspect that over gets overlooked is the secondary effects a large impactor can have. The above comment that stated that the shock wave went 3x around the world ought to give one pause. Shockwaves bouncing (multiple times) off of fragile danger spots…Yellowstone caldera? New Madrid Fault? San Andreas? Big island of Hawaii and the Azores both have large chunks of land (with existing cracks) just waiting to slide into the ocean.

30. Gary Ansorge

Tim G: That video reminds me what Warner Von Braun called the H-bomb:”Dot greaten biggen ear splitting lauden boomer,,,”

Cool video, but I’m sure glad it was never used in combat,,,

GAry 7

31. Tim G.– “wipe out Liechtenstein” – is a countRy catastrophe.

What is talked about in the article is a county – no ‘R’.

Smaller catastrophe, to be sure…
but changes the magnitude…
don’t ya think.

Relavent Implication is comment re: If it hit over a major city, well…

Max Al

32. ABR

Max Al — leaving population out of the magnitude discussion, I suppose it would depend on which county you had in mind to wipe out. At 13 square miles, Kalawao Co, Hawaii would make for a “smaller” catastrophe than Liechtenstein (62 square miles). However the median land area per county in the US is something like 620 square miles. Coconino County, Arizona where the Meteor Crater is located is a whopping 18,600+ square mile area. All I’m saying here is that country vs. county may be a slight oversimplification.

33. Irishman

Poor choice on the part of Phil to pick something that doesn’t have consistent sizes. But the word he said was “county”, and Tim G brought up Liechtenstein, which is a country. Not the same thing.

34. Shootist

There are demonstrated impacts of 1200 (Australia) and 4800 (SSE of Madagascar) years ago. Add to this the large bolide exploding over the Laurentide Ice Sheet 12,900 years ago.

http://www.pnas.org/cgi/content/abstract/104/41/16016

Yes, impacts are far more frequent than most believe. Anything remotely sounding of catastrophism has been kept out of journals for decades. I took one look at the 1000km2 strewn field in southern Egypt and was convinced the planet is whacked more often, and harder, than anyone guessed.

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