Scientific Method / Science & Exploration

Using lasers to find meteor craters

It is a geologic mystery: there appear to be very few small impact events in …

It seems appropriate that as I write this, Fx is airing the movie Armageddon, in which a giant meteor threatens to wreak havoc on earth. Meanwhile, I'm reading a study in the current issue of Geology which presents a newly discovered meteorite crater in Alberta, Canada.

What makes this discovery interesting is both the size of the crater and the techniques used to find it. The crater is tiny, 36 meters in diameter, and a mere six meters deep. Craters this size are generally very difficult to find, although they should be rather common.

Meteor Crater

The much easier to find Barringer Crater, AZ

Based largely on the crater size distribution we see on other planets and on the number of larger craters on earth, we can infer the number of small craters there should be on earth. On average, there should be a new crater around 40m in diameter every 10 years but, of the 175 known impact craters, only 5 are from the last 10,000 years and have diameters under 100 meters. In contrast, we have recorded at least 20 new craters on Mars between 1999 and 2006.

So how did anyone find this tiny marvel? Lasers. The team used a technique known as LiDAR to map the topography as it would be without any plants on top of it. More common methods of mapping topography, such as using aerial photos or RADAR interferometery, cannot see beneath the trees.

LiDAR works by shining a laser pulse at the ground and measuring the time delay of the returning light. The first light to get to you comes from the top of the canopy, and the last light to get back comes from the few photons that reached the ground before reflecting back. The distribution of light in between the first and last returns revealing details about the plant canopy structure.

By scanning pulses across a landscape, one can build up a map of how the land surface looks with and without vegetation. When the team looked at the landscape without any vegetation in this spot, they saw the clear image of an impact crater, just as if it was on the moon.

Because it is not uncommon to find clusters of small craters together, the team set about searching LiDAR data from the surrounding area for more craters. Although they found "Two irregular depressions ~3m across", these turned out to be man made.

Unfortunately, LiDAR data are not as plentiful as we might like, so it will be some time before we can search the globe for small craters using this technique. In the mean time, it is fascinating to see such a clear image of a crater in what otherwise looks like unremarkable terrain. This impact probably occurred about 1000 years ago, but the search is on for the 10 ton, "billion-watt lightbulb"/meteor that lit up the Canadian sky on November 20th.

Geology, 2008. DOI: 10.1130/G25236A.1

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