08 January 2006

Why are craters round?

At first glance, this seems like a dumb, dumb question: because meteors fall into them from above, duh?

Well, actually, they don’t — at least, the vast majority of them don’t. If you took a brick halfway to Lunar orbit and dropped it, it would fall (approximately) straight down through the atmosphere and possibly even survive to make a crater. But why a round crater, even so? Bricks are not round (at least, I have never seen a spherical brick, and I think any brickie who did would be screaming and running away). Think about all of the photos of comets and asteroids that you’ve ever seen: only the planet-buster sized ones are anything like round. The others are elongated, and often bent. You would expect many of these, falling straight down, to make an elongated crater, no? Yet very, very few craters are anything but round.

Then think about the descriptions of meteors you’ve read or hear. What were these meteors doing?

Invariably, they were streaking across the sky or some similar description was used. Very, very few hit straight down or anything like it, because the velocities they possessed long before they approached Earth far exceed anything they’d pick up from being dropped like our semi-lunar brick, and even when Earth’s gravity bends them towards the surface, it happens in a large, relatively gentle curve.

In fact, one of the (many) prime worries of the Apollo missions was that a re-entering capsule would “skip” off the atmosphere like a flat stone on a pond, and bounce away for weeks or months before returning for another go.

Places like the Moon and Mars don’t have much of an atmosphere to ablatively brake an incoming rock, either, so you’d expect to see even more oblique and glancing blows there — ovoid and vee-shaped craters with a debris fan instead of round ones — but the vast majority of them could be used for calibrating protractors. When something which looks like a glancing blow turns up, it’s news. Astronomy sites make specific comment about it.

In fact, Mars exhibits some examples of what could at first glance be mistaken for glancing blows — long chains of craters. However, the craters in the chain are often all of a size. Odd, isn’t it? How often do you think a skipping meteor would make the same sized (and circular!) dent on the second bounce? How about the fifth or sixth impact? How often do you think a meteor screaming in from the Big Dark bounces, bounces, bounces and changes course as it bounces? Yet many of the crater chains on Mars do exactly that. Consider the crater chains in the full-sized version of this image:

valles marineris, mars

And/or follow a course down the middle of a gully. Er... what? Yes, absolutely. And have a look at the bottom of Schroter’s Valley (Rille, really; near Aristarchus, on the Moon) for another example.

All of this shows lots of stuff not working out at all well for the idea of most craters being impacts, but one particular crater in the Schiaparelli Basin on Mars really raised my eyebrows:

layered crater, schiaparelli basin, mars

The official explanation says:

This old meteor impact crater in northwestern Schiaparelli Basin exhibits a spectacular view of layered, sedimentary rock. The 2.3 kilometer (1.4 miles) wide crater may have once been completely filled with sediment; the material was later eroded to its present form. Dozens of layers of similar thickness and physical properties are now expressed in a wedding cake-like stack in the middle of the crater. Sunlight illuminating the scene from the left shows that the circle, or mesa top, at the middle of the crater stands higher than the other stair-stepped layers. The uniform physical properties and bedding of these layers might indicate that they were originally deposited in a lake (it is possible that the crater was at the bottom of a much larger lake, filling Schiaparelli Basin); alternatively, the layers were deposited by settling out of the atmosphere in a dry environment.

But why would the erosion be least in the middle of the crater, and roughly symmetrical? On the other hand, there are plenty of craters kicking around all over the solar system which are in full possession of central peaks or mesas. These are often described as “rebounds”, but the geologists are pretty sure that the Chicxulub crater — also blamed for the lack of living dinosaurs — is an impact crater, and the rock structure under that is a mess, not the neatly deposited, even, symmetrical layers we see here with no evidence of underlying chaos. The layers are said to be a lake deposit — on a planet without significant water, and where are the layers outside the crater? And if they’re aeolian deposits, as also posited, what bound them? Why are there layers instead of a continuous deposit? And how were they deposited evenly across the entire crater and not unevently eroded between deposition episodes?

The electric cosmos people, despite other flaws in their theories, do have some excellent explanations for all of this. If a crater is excavated electrostatically or arc-machined by twisted, paired Birkeland currents, a central peak and round rim would be natural features, almost expected. The damage to the rock under the excavation would be (relatively!) minimal, so an excavation which exposes layered rock strata (or leaves them essentially undisturbed for later wind erosion to uncover) would be quite reasonable. While not as dramatic as the photo above, many of Mars’s craters do feature stratified terrain within the crater bowl.

Speaking of the above image, there is a crater to the lower right of the image which looks like it probably is an impact crater: it’s pushed the layers aside when it formed.

Another expected side-effect of arc-machining is (mostly) non-molten rubble flung far and wide, most of it with a strong vertical component (ie, it would return more or less straight down, the furthest-flung stuff producing more-or-less round impact craters where it fell). A prominent feature of most Mars landscapes is indeed shattered rubble.

If — as many of the electric cosmos people assert — Valles Marineris was electrically excavated, that would provide ample supplies (many, many cubic kilometers) of such rubble. Who knows, Phobos (with its spectacular and otherwise inexplicable — a “giant impact” would pulverise the whole moon — Stickney crater) may even be an unusually large piece of that rubble. Definitely something to be watching from a safe distance, if there was a safe distance at the time.

Since nobody was considerate enough to be in orbit with a video camera when it happened, we’ll just have to keep guessing for now. It’s interesting to see how well a barely-developed, hole-laced theory can still fit the observations when compared with obviously-inadequate traditional explanations.

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