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Rough Science 4 Death Valley: Iain Stewart's diary: Impact

Updated Tuesday, 29th August 2006

Iain Stewart's diary about the challenge for the Impact programme, from the BBC/OU series Rough Science 4

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Kathy, Kate and Iain at the edge of the crater Copyrighted  image Icon Copyright: Production team

Day One

There are some places that as a geologist you want to go to. Today, I went to one of them. I couldn’t believe it when Kate revealed the challenge – go to a meteor impact site in Arizona, measure the size of the crater and estimate the size of the body that crashed into it. Immediately I knew we were talking about Meteor Crater – a feature whose image adorns almost every geology textbook and a classic example of how science works in mysterious ways. Geology undergraduates are brought up on how geologists argued for years about whether it was created by a volcanic explosion or an impact from space. A hundred years ago, the most respected geologist in the land had spent years carefully studying the problem, to conclude it was volcanic. A classic case of wrong answer for the right reason. Only in the last few decades did scientists come round to the idea that this really did come from outer space.

Poor Mike. Kathy and I have left him playing with marbles and sand while we fly off to Arizona. Jonathan and Ellen are happy enough – their common interest in astronomy is perfect for their task to measure the size of a crater on the moon, so they have their minds on higher things. Kate, obviously is coming with us to a small town in Arizona. Now there are a few things that Winslow in Arizona is famous for. (1) It was famed as one of the main stopping off points on the railroad and highway 61 heading west to California. (2) It has a dirty great hole in the ground a few miles out of town – more on that later. (3) Most importantly, there is a statue to The Eagles frontman, Don Henley, after he immortalised the town with the words “Standing on the corner of Winslow, Arizona…” in that famous song. You can guess it. I can’t say it – I’ll end up just singing it constantly like we did when we got there.

Anyway, back to the business at hand. The crater is spectacular, particularly when you get to approach it from the air. We flew in by helicopter, skirting low over the flat desert floor until you rise up over the elevated rims of the crater and then see its full extent. I’d seen it hundreds of times in photos, but I still couldn’t believe how enormous it looks. And another thing, it wasn’t really circular – I was surprised how square it looked from some angles. Until now, I hadn’t given much thought to measuring it – I had been more preoccupied with ways of estimating the size of the lump that crashed into it. Besides, I was sure Kathy would think of something on the measurement side.

Some geologists have spent decades studying the rocks in and around the crater for clues about what happened here. I had a day and a half. Thankfully, because the place is so famous I knew a lot of the clues that I needed to look for. First I knew that we didn’t have to bother looking for the meteorite in the bottom of the crater – it had vapourised on impact. Good news for us, but bad news for the guy who a century before had bought the crater to mine out the iron-nickel body he believed lay at its core. Still, you can see small dense metallic fragments littering the rim of the crater and hunting for them is a popular tourist past-time. And the evidence for the enormity of the explosion that vapourised it is all around – powdered rock and enormous boulders occur all around the outer edges of the crater walls. It must have been some bang. Still, there was nothing that could give a definite answer to the size of the body that struck, and we still had to measure the blasted thing.

Kathy, Kate and Iain at the edge of the crater Copyrighted  image Icon Copyright: Production team

Day 2

As expected, Kathy has a plan. We’re going to measure the diameter of the crater the easy way – by trigonometry from one part of the crater, sighting onto an opposing part. On one part of the crater wall we mark out a rough 100m baseline and then using giant protractors (knocked together Blue Peter style with a few bits of wood), we measure angles between the baseline and our target point at the two end points. The aerial view showed us how the crater was far from a perfect circle, so we know we should do the same measurements in as many different places as possible to get a decent estimate. Before hand we figured between five and ten measurements would be ideal; now on the ground we change our plan - we do one more for luck. I know, not very good but you should see the size of this thing!

Kate’s also having trouble taking this all in. It’s clear that a misunderstanding is creeping in. The challenge is what is the size of the object that hit, but me and Kathy keep going on about the energy, rather than the size, of the impact. That’s because it is the amount of the energy released by the impact that causes the crater, not simply its size. Two similar sized objects will create very different craters if they’re travelling at different velocities, and the combined effect of velocity and size is what we are meaning by energy. In the heat of Arizona, our explanations are getting frazzled. Kate can’t work out how we’re going to get at energy; to be honest, neither can we. We need to get back to the workshop and think of some way of getting at the explosive energy of the impact and then a simple way of extracting from that the size of the impactor body. So it’s back to the helicopter and off to see what Mike has been doing back at the ranch with his bucket of sand.

Mike, as it happens, has been doing some brilliant experiments dropping marbles and iron balls into flour and then sand at gradually increasing heights. This is great stuff - and really easy to try at home. If you do try it, you’ll find the same amazing thing that he did. That there is a nice linear relationship between crater size and drop velocity – his data points plot on a wonderfully convincing straight line. So, it should just be a simple case of scaling up from his small-scale results to our large-scale crater (once Kathy calculates the size). But I’m really not convinced that this ‘up-scaling’ will work. For me, the small-scale impacts are fundamentally different in terms of process to those that evacuate out the kilometre-sized craters. For one thing, Mike’s marbles don’t vapourise.

Enter a cowboy with a gun. We’ve got a hired hand to fire a bullet into sand to try to get a better simulation of a high-velocity impact and explosion. It’s still not perfect, since our bullet isn’t vapourising either, but we are certainly getting craters that are much larger than we would expect from the small bullets being fired. We plot it on Mike’s graph – it’s way off the line. The higher velocities are still not giving us the size of crater we need.

We’re at a bit of a loss, but while we’re stumped as to how best to proceed, we take time out to admire Jonathan and Ellen’s fantastic telescope and its incredible view of the Moon. Even then we can’t get away from our problem. All of the craters on the Moon appear perfectly round, but you might expect some to be elliptical since not all impacts would be direct hits - some would come in low angle. In fact, many of them probably do, but the explosion and the vaporisation of the meteorite body throws material out in all directions, producing a nice round crater. Back to that flipping problem. Perhaps a good night’s sleep will give me some inspiration.

Kathy, Kate and Iain at the edge of the crater Copyrighted  image Icon Copyright: Production team

Day 3

Nope. No inspiration. Me, Kathy and Mike discuss the problem as soon as we get to the workshop. Mike is convinced that the up-scaling ought to work, and sets about mathematically justifying why it should. I’m just convinced that we don’t have anything better to throw at the problem. Kathy is busy being practical – laying out sheets of paper to graphically draw up the angles measured at the crater and work out its diameter. As we thought, the two measurements are quite different, one is 900m and one is 1500m. An average of 1200m seems fine, but an error range of +/-300m looks pretty disappointing. For me and Kathy it’s OK – the crater wasn’t perfectly round so it won’t have a single diameter value anyway. I suspect Kate thinks we’re hedging our bets to hide sloppy field measurements, but Kathy stresses that it is just the way that scientists normally deal with natural variability and uncertainty. Kate nods, but the eyes say ‘sloppy, sloppy, sloppy…’

Mike has convinced us that the linear plot of impact energy against crater size for the small measurements is worth extrapolating up to the size of our Meteor Crater. There’s loads of discussion about whether this might be a minimum estimate, given the explosive nature of the high velocity impacts. When Kate puts us on the spot, we go for a range. Now this is hedging our bets. The magical envelope reveals that Kathy’s spot on with diameter measurement – 1200m. Amazingly – and I still can’t quite work out why – our estimate of the impact body being between 30m and 100m wide is also spot on; the scientific consensus says 50m. It shouldn’t have worked, but it did. Exhilarated and drained, we find comfort in some beers and wait for night to fall.

Our bit is done, but the fun is just starting for Jonathan and Ellen. The telescope is working brilliantly, and we all can’t help but get involved as we get deeper into the night doing repeated measurements of the diameter of little Archimedes crater on the Moon. Under Ellen’s flickering lights, the effort pays off. Eventually, reluctantly, we pack up and head back - tired and happy.





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