Friday, August 3, 2007

Meteor Crater

On one of my cross-country trips, I decided to visit Meteor Crater.
It was the first time I’d taken I-10 on my way across. There’s a point at which you can decide to take a side trip either north to the Grand Canyon or south to Meteor Crater. The Grand Canyon sidetrack is basically a day trip, while Meteor Crater is only like 20 miles off the Interstate, so I went to Meteor Crater.

Meteor Crater is seriously cool, a big hole out in the middle of nowhere. There was a debate originally about whether it was from a meteor or whether it was volcanic in origin. One of the arguments against it being meteoric was that it was so symmetrical, and people imagined that a meteor was more likely to come in at an angle, and that would show up as an asymmetry, an elongated gouge in the surface, in other words.

That seems intuitive, and it’s wrong. The problem is that our intuition doesn’t deal very well with the difference between momentum and energy. It drives me crazy in news articles about cosmic rays, where the energy of very high energy cosmic rays is compared to a well-hit golf ball. While this is technically true, when we think of a speeding golf ball, it’s the momentum that we’re visualizing, not its energy. The impact when it hits you, or the feel of it coming off the club, those are phenomena of momentum, rather than the amount of heat it would generate when it stops, which is a measure of energy. In the cosmic ray example, a better indication would be that it has an energy similar to what is required for a photoflash on a camera.

In the case of a meteor impact, the amount of kinetic energy involved totally dominates the behavior on impact. The meteor basically explodes, with the energy of many kilotons. It’s as if you lobbed a grenade into a sandbox. It doesn’t matter what angle it comes in at, the explosion is going to produce a circular crater.

All of which makes this last bit just a little embarrassing. There I was driving along a road in the Arizona desert, a flat, flat, desert. Then, after a while, the road began to climb. I looked ahead and saw the road going up a hill, the only hill in sight. I realized that the crater would be at the end of the road.

And I found myself thinking, “Gee, what are the odds that the meteor would have hit the only hill in the area?”

Duh.

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