Sunday, August 9, 2009

Of Holes in the Ground and Anaesthetized Pigs

Recientemente a meteorite, universally described as being the size of the Earth, crashed into Jupiter. Cue spectacular photos and general confusion from the press, which invariably fails to grasp even the most basic points of a story about science.

There seems to have been some debate about whether it is possible to leave a crater in Jupiter, since it is almost all gas, with, at the most, a small, solid nucleus. The answer is, yes, it is possible. Although the leaving is not permanent, the Earth is a big thing, and it takes a while for an Earth-sized hole to close up. Quite long enough to call on your friends on the interlab chatlines and tell them to have a look and take the above-mentioned photographs which aren't worth as much as Peter Mandelson emerging from the back door of a brothel but a few quid is always welcome.

The real point, though, is what a crater is. It isn't a hole in the ground. When you sling a stone into mud what you get is a hole on the ground. When a meteorite hits the Earth you get a crater. The difference is the energies involved.

You could read this in Wikipedia or some such source, or just ask someone who didn't stop doing planetary geology 20 years ago, but then you wouldn't get to hear about the anaesthetized pigs. Because this is not a rant about the iniquities of the press, nor is it, other than incidentally, a serious attempt to inform. It's about memories, from the days when I studied astrophysics at UCL, and learnt about craters from someone whose name escapes me now, but who was an international expert and had taken part in the seventies in the celebrated 'anaesthetized pig' experiments.

When a meteorite hits a planetary surface it is moving at several kilometres per second, and thus has enormous kinetic energy. Energy doesn't just disappear, so the old rock doesn't just go 'thwwunk' and sit there in the mud like an aging tortoise; its kinetic energy is transformed into heat which is shared between the rock and the ground it struck. The effect of this is to melt both the meteorite and the ground, and the system flows hydrodynamically. Thus, it's not like dropping a stone into mud, it's like dropping a stone into water. The transformation of kinetic energy into thermal energy is not intantaneous, and while it is going on a broadening ring of ejecta is thrown out, centred on the point of impact, which is frozen at the point where the energy drops to a value that won't melt the rock.

What happens when you drop a stone into water? You get a circular hole, a drop comes flying out of the middle, and a series of ripples spreads across the surface until the hole is filled and it's all smooth again. But very briefly, there was a crater in the water. In rock, the filling process stops, with a spike in the middle where the drop was coming up, or even a central ring where a second ripple had time to form before it froze. The meteorite itself is gone, consumed, become part of the ejecta surrounding the outer ring of the crater.

We're getting to the pigs.

In the seventies, academics from my alma mater designed an experiment involving particles with sufficient kinetic energy to create hydrodynamic flow at the point of impact. The machine was essentially a gun, which could fire a projectile, I seem to recall a very small ceramic sphere, at a speed of some seven miles per second (that's from memory, but it was very quick indeed). They also had a high quality, very high speed camera, several thousand frames per second (because it all happens very quickly). They tested it in a vacuum chamber by firing at a number of different materials, including water (yes, and including pig's flesh) and at many different angles.

The films are fascinating (and probably on Youtube or itunes- I can't check at the moment). In all cases the process looks exactly like what happens in water, except that it stops at the point where the energy runs out; the same excavation process outward from the point of impact, with matter thrown up over the rim, the consumption of the projectile, the central spike or ring depending on the original energy, the whole Arizona/Yucatán/surface of the Moon look of the final crater and so on.

When you're testing a wide range of materials someone is going to suggest flesh, and pigs were what they could get (possibly only one, in fact). It was anaesthetized for humanitarian reasons, though since the impact would have instantly scrambled its brain, and the experiment was done in a vacuum chamber there wasn't really much point. It kept it still, in any case.

Since you ask, the answer is 'messy'. It's mainly subcutaneous fat, you see. For the rest, it looks like any other crater in a soft solid with a low melting point. (On the other hand this may be an invented memory; I'm not certain they ever showed us the pig).

So yes, it is perfectly possible to form a crater on Jupiter, although it will not be permanent. And the vast meteorite that created it is not somewhere deep in the heart of the planet; it is no more, it has gone to the great rubble yard in the sky, it has become one with the five elements, as the Brahmins say.

If anyone else remembers these experiments or took part in them, I'd love to here from you.

No comments: