... 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.
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.