Contemplating Craters

Simon Mitton, a fellow of Saint Edmunds College at the University of Cambridge, recently sat down with Charles Cockell, geomicrobiology professor at the Open University in the UK. In part one of their interview, they discuss how Cockell came to study impact craters, as well as the importance of such impacts in Earth’s history. Large asteroid or comet impacts have proved to be fatal for life in the past, and they are still a concern for us today. But they also can provide new environmental niches for life once the dust from the massive explosion has settled.

Asteroid Impact
This painting depicts an asteroid slamming into the tropical, shallow seas of the sulfur-rich Yucatan Peninsula in what is today southeast Mexico. The aftermath of this immense asteroid collision, which occurred approximately 65 million years ago, is believed to have caused the extinction of the dinosaurs and many other species on Earth.
Credit: Donald E. Davis

Simon Mitton (SM): I know that you were a research scientist with the British Antarctic Survey, and conducted microbiology field work in Antarctica. Was that when your interest in astrobiology first started?

Charles Cockell (CC): Actually, the British Antarctic Survey was a continuation of my astrobiology interests. I’ve wanted to go to Mars since I was about seven. And I’ve been an amateur moth collector from a very young age — I inherited a moth collection from my grandfather, but that’s another story. But the connection between natural sciences and space exploration has been something that’s gone on for a long time, and I’ve always sought ways to bring those two things together.

I first got an opportunity to do that when I did my postdoc at NASA Ames under the National Research Council. After that, I went to the British Antarctic Survey. But it was really NASA that gave me the opportunity to indulge my astrobiology interests to the full. That was where I moved into microbiology, looking at extreme environments and trying to understand how microorganisms survive in impact craters in extreme polar environments. At the British Antarctic Survey I was more focused on Antarctic environments, looking at the interaction of microbes with ultraviolet radiation in these extremely cold places.

SM: I’d like to ask you about your work on cratering. Many craters on Earth were formed by asteroid impacts hundreds of thousands or millions of years ago. When an asteroid hits the Earth it’s a gigantic explosion, and it must destroy everything that’s living in or near the crater. What are the mechanisms by which life is restored to these craters?

CC: That’s an interesting question because, as you say, craters tend to result in the destruction of animals and plants and much other life. If the impact event is big enough there can be global extinctions — that’s how people think the dinosaurs were destroyed.

chesapeake_credt: Aaron Gronstal
Scientists examine a drill core at the Chesapeake Bay impact site. Click image for larger view.
Credit: Aaron Gronstal

One of the things I’m interested in as a microbiologist is the way in which asteroid and comet impacts might create new opportunities for life. We’ve been doing work in a number of craters, including the Haughton crater in the Canadian High Arctic, and also the Chesapeake Bay impact crater, which is an 80-kilometer diameter crater in the US. One of the things we find is that the massive energy of impacts can shatter rocks and create habitats for microorganisms. The microbes use the surfaces in these cracks as a home. That’s one of the ways in which impacts can provide new opportunities for microorganisms to proliferate. So we can’t just think of asteroid and comet impacts as destructive agents, but as agents offering benefits as well.

SM: One of the most recent craters on Earth is Meteor Crater near Flagstaff, Arizona. Gene Shoemaker used that crater to train the astronauts who landed on the moon. When I visited it, I was very impressed by the way the surface layers of the rock had been flipped head-over-heels, landing on the desert platform. Do you know if any work has been done by microbiologists on that crater?

CC: There’s not a lot of microbiology work that’s been done there, but we’re beginning to do some work on shocked sandstones. Some of the limestones in the crater have been heavily shocked. We want to understand the ways in which impacts might change different rock types, because not all rock types will necessarily be changed in the same way.

Meteor Crater is a very good example of a smaller impact crater. It still would have caused destruction for several hundred kilometers around the zone of the impact, but it’s the sort of crater that you might expect to be formed on the surface of the Earth every few thousand years. So it’s relatively common compared to impacts that cause global-scale extinctions. And that makes Meteor Crater a very sobering thing to view, because it makes you realize that you’re not just sitting on an Earth that’s isolated from the rest of the cosmos. In fact, giant objects come in every now and then and cause destruction.

An aerial view of Meteor Crater, Arizona.
Credit: Jim Hurley

It’s not something that most people think about on a day-to-day basis. It’s probably not very healthy if people do think about this often, but I think it’s enlightening to stand at the edge of that massive hole and realize it was created by an explosion something on the order of a thousand times the size of the atom bombs of the second World War. You don’t want to be standing anywhere near one of those things when it happens, but it’s a very useful education to stand on the edge of a crater several tens of thousands years after it has happened.

SM: Has anybody worked out what is the probability of an incoming asteroid or comet hitting a large city? I ask this question bearing in mind that something like 80 percent of the Earth’s surface is not inhabited land mass –- it’s oceans and ice caps, plus we’ve got all the mountain ranges and deserts.

CC: David Morrison has done a lot of work on this at the NASA Ames Research Center, but there’s still a lot of risk analysis to be done. I don’t know off the top of my head a number for the probability of an asteroid hitting a city. But of course, you have to bear in mind that three quarters of the planet is ocean, so most impacts would disappear into the oceans. But they could cause tsunamis that might hit coastal cities, so it’s not a simple calculation to work out the probability of a city being affected by an impact event. Generally, the probability of a direct hit on a city would be very small, because of the oceans and the fact that a majority of the land mass is not inhabited.

Read Part 2 of this interview.

Related Web Pages

Asteroid Riddling
Bombarded by Mysteries
What Happened at Meteor Crater?
2029 A Near Miss Odyssey
The Great Impact Debate