04 October 2005

The Susan/Calvin Theory of Crater Formation

A random event today catalysed an idea which has been lurking half-formed in my gray matter (ie cement — at least, on bad days) foir soem considerable time.

The Earth system gets hit by meteors all the time; thousands or more substantial hits a day — so often, in fact, that we can use the meteor trails as a radio reflector, and a great many of these meteors are not much more then chunks of ice or whacking great snowballs. Some theorists even postulate that Earth’s oceans are the result of patient additions to our ecosystem by these soggy meteors.

Lurking in the back of my mind are pictures of many battered-looking moons, including Phobos (with the massive Stickney Crater at one end) and Hyperion, with its most amazing collection of craters and Comet Tempel, described as a ball of slush and yet quite well cratered also.

How can a ball of slush get cratered? Surely anything substantial hitting it at speed would either plough right through or destroy it?

Part of the answer lies in understanding sound. When an aircraft goes supersonic, what’s happening is that the air in front of the ’plane can’t communicate the message “get out of the way, aircraft coming through” fast enough, so the air kind of piles up in front of the vehicle. On terran re-entry, this piled-up air is being compressed hard enough to make it glow. When Wile E Coyote slams into the bottom of the canyon, you knew that he was going faster than the speed of sound in rock because he left a little silhouette as the too-sluggish rock piled up and compressed in front of him instead of getting out of the way.

When Deep Impact smacked into Tempel, the atoms of each body couldn’t avoid each other, so reconciled their differences of velocity by converting it to heat. That’s the standard cratering model. But there are many craters, and many of them are... odd. They come in squares and other polygons. They also come in chains; many of the “grooves” on Phobos look very much like chains of craters, as if something bounced and rolled.

Then today I dropped the small screwdriver bit from the magnetic holder on my drill, and it landed amid dust on the loosely-fitted wooden floor — and made a “crater”. The thing which first struck me was that despite the bit having landed on its side, the “crater” was very round, wider across than the full length of the bit, and even had a slight central peak as so many extraterrestrial craters do.

So, what if something really did bounce and roll on Phobos? What happens to moons and coments when a gentle impact takes place?

I’m having a look at the image of the huge crater on Hyperion, and wondering if at least the outer layers of hyperion are not loose ice slush, and the big crater represents a “real” hit ploughing straight through the soft slush, blowing a big hole under the slush by compacting it (and maybe outgassing some of it), leaving room for it to subside and producing the crater we now see.

IOW, maybe the moons, comets and asteroids are having a whopping big snowball fight, and Hyperion-as-Calvin was dumb enough to fling one at Susie Derkins (whichever body represents her) — and not miss? The larger planets’ atmospheres would both protect them from quite large snowballs and absorb most of the evidence.

Another feature which quite intrigues me is cycloids. I strongly suspect that the popular explanation (tidal forces under an ice layer) is inadequate to explain them.

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