Tuesday, January 09, 2007

Thinking About Craters

The world may turn topsy-turvy in an hour. ~ John Clarke

When people think of meteor impacts on Earth, they think of the crater in Arizona or the massive Chixulub crater that probably finished off the dinosaurs. Whatever they may think of, they probably don't think about Wetumpka, Alabama. If we really get down to it, there's probably not much else that makes them think of Wetumpka. Heck, I live a half-hour away, and I never think of the place.

There's nothing wrong with Wetumpka (well, it's local politics can get a little ridiculous, but that's not the focus of this blog), but there's nothing much about it to get anyone all that excited either, unless you're into meteor craters. I was reminded of that by an article in the January issue of “Alabama Living” magazine. Normally, this slim periodical is devoted to telling me how its publisher, the local electric co-op, is doing a great job (they're not, but that's not the point of this blog, either). But it also talks about interesting areas of the state, and this month it mentioned the ancient impact in Wetumpka.

Somewhere around 80 or 85 million years ago, 62 million tons of rock slammed into central Alabama. Of course, it wasn't Alabama back then, but we need to give you a geographical reference of some sort. The impact released energy equivalent to 2,300 megatons of TNT, no doubt ruining the day for a large number of southern dinosaurs.

(I wish I hadn't written that. Now I can't get rid of the image of a bunch of T-Rex-type dinosaurs sitting in rocking chairs, wearing white suits and sipping mint juleps.)

Now, by all means come on down and take a look at Wetumpka's crater. The Chamber of Commerce will love you for it. But don't come down expecting to see a big hole in the ground. Oh, there's one there, all right, but if you look out over the area where it is, you'll see a pretty scenic view of trees and rolling hills. It takes careful examination of aerial photographs to really see that there is a crater.

That's not the fault of anyone in Wetumpka. With rare exception, it's hard to detect most of the visible craters on our planet from the ground. It's easier (and more impressive) to see something like the Terminal Moraine (the place where the glaciers stopped) in Thompson, Ohio, with its clear ledges and huge drop stones. But that's how the Earth is. We live on a dynamic planet, and, over the long haul, everything changes.

Craters fall prey to erosion, mountain-building, volcanic events, even colliding continents. For a crater to stay visible, it has to either be relatively recent (geologically speaking) or in a good place. The crater in Arizona is a good example of both conditions.

As a result of this smoothing out process, we get rather blasé about meteor impacts, as if they're something that only happened long ago. That's hardly the case. The same article contains a sidebar about Ann Hodges, a resident of Oak Grove, Alabama. Ms. Grove, it seems, is the only recorded person who has actually been struck by a meteor. In 1954, she was peacefully napping when a small rock from space crashed through her roof and hit her on the hip and arm. Beyond being startled and bruised, she suffered no significant injury.

Too long ago for you? On January 2, 2007, a golf-ball sized lump of metallic rock came down in New Jersey landed in someone's bathroom. Fortunately, the facility was unoccupied at the time, sparing some serious embarrassment.

Those are small events, but there are bigger ones possible. One need only look at the Moon to see a record of massive impacts over the eons. In fact, part of the reason we're here is because the Moon helped sweep up a lot of the detritus left over from the creation of the Solar System. In fact, observations made during the recent Geminid meteor shower detected five or six impacts from Geminid meteors. Telescopes were watching for these events to try to get a handle on what sort of risks a lunar base might face from such collisions.

That impacts were once much more common is seen on a planet like Mars, where a combination of a thin atmosphere and geological inactivity have resulted in numerous impacts that have stayed visible on the surface. In fact, one of the great disappointments in the early days of Mars exploration were the Mariner photographs showing a heavily cratered surface. We've since realized that there's a lot to learn on Mars, despite all those craters, so we've sent many messengers back there. In fact, there's at least one case, as I recall, of a recently discovered crater that was not in photographs taken just a few months earlier.

There's still a lot of stuff out there.

We understand why we don't see many craters on Earth, but there are places that should have a lot of craters that don't. Io, the volcanic moon of Jupiter, has practically no craters, but that's easy to understand, given the nearly constant resurfacing going on there. Europa is also rather shy of craters, probably because the surface is floating on an ocean and is being remade as well, through a kind of plate tectonics, except that the plates are made of ice.

Then there's Venus. Venus, which has been thoroughly mapped by radar has craters, but they seem to be the same age. Really old craters don't seem to be there at all. It's certain that Venus has been geologically active during its history, but the implication is that the entire surface was overturned in some cataclysmic fashion, perhaps a massive lava outburst, akin to the Siberian or Deccan traps, only planet-wide. Alternatively, it could have something to do with sulfurous atmosphere eroding the planet's features.

Then there's Titan, everyone's favorite satellite these days (including me). Scientists were disappointed not to find oceans of methane on Titan, but they have found what appear to be lakes in the polar latitudes, based on radar mapping conducted by the Cassini probe. Another thing they haven't found much of on Titan is cratering. Some have been located, but not the amount that might be expected on a body circling the junk-filled environs around Saturn.

One reason for the lack of craters might be Titan's reasonably thick atmosphere, but another might be some sort of geological activity. Something is replenishing the methane in the atmosphere, which is why Titan-watchers were expecting hydrocarbon seas. It's unclear whether the lakes, if that is what they are, contain enough liquid methane to make up for the methane that is broken up by solar radiation. It's possible that the methane is produced from within as part of water-ice volcanoes. Or there may be some mechanism for both the methane and the lack of craters that we haven't discovered yet.

It's amazing how interesting some holes in the ground can be.

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