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Re: Mathilde - part 1 of 2
An explanation for the few big showers of chondrites has to do with strength
variations. This is discussed, for example, in "Physics and Chemistry of the
Solar System" by John S. Lewis, Academic Press, 1997. When they enter the
atmosphere at some speed V meteorites slow down in a way depending on their
angle, mass and size but at each new speed V' the pressure depends both on the
square of V' and the atmospheric density at that height, so you can reach some
pressures that are high relative to a meteorite strength even at high altitude
because the atmospheric density raises rapidly. While a typical iron or hard
achondrite may have crushing strengths of a few kilobars, for chondrites there is
wide variation. Lewis reports "62 bar to 3.7 kbar have been documented." As you
can see, some meteorites are apt to begin fragmenting at high altitudes into many
pieces. (Think of 5x10^6 N/m^2 strength; the density needed, for 10^4 m/s V', is
0.05 kg/m^2. Maybe at 20 or 25 kilometers such a weak chondrite could fragment
if it has retained appreciable speed at that altitude.)
Another interesting feature Lewis points out has to do with seize and a Weibull
(statistical) analysis of the variation of fragmentation with meteorite size.
Bigger pieces tend to have fractures at every size scale you go up, and there is
a relationship for how to scale down the crushing strength with mass or size. A
fairly weak specimen of an ounce may have come from an overall weaker few tons
preatmospheric meteoroid. At its largest size it is weakest so the fragments
should be a little stronger on average and penetrate deeper. But they have more
surface area and so experience more drag if the small meteorites start to move
apart. This makes them slow down, so the pressure tends to lower due to this
effect even it tends to rise due to increasing density. Its a competition to see
where fragmentation stops and how many pieces of various sizes you end up with
when its done. I've run a very simple estimate and I always tend to end up with
top final meteorite speeds of a few hundred miles per hour at ground for the
meteorites we have on our lab shelves, but that's a well-known result. What's
more interesting is looking at where the speed really begins to drop and that is
typically 15-30 km; i.e., showers are realistic for weak meteorites (whew, I'm
glad theory agrees with reality; had me worried for a minute- I might have had to
deny there are meteorite showers). Thinking of a rain of meteorites is proper
since when they are slowed by the atmosphere they tend to fall rather close to
straight down.
Jamie Platt wrote:
> "Asphaug and his colleagues began with a
> peanut-shaped asteroid with three different structures: solid rock, a
> pair of solid rocks, and a rubble pile. The team hurled house-sized
> rocks at each asteroid and measured the devastation.
> Asteroids made from solid rock shatter into smaller pieces, but they
> don't always disperse. They may break into several large pieces or form
> a cluster of debris. Asteroids made from a pair of rocks seem to resist
> impacts because one of the rocks absorbs most of the shock from the
> impact. And rubble-pile asteroids seem even less affected."
>
> Hello Bernd, List -
> I'm glad someone else came across the same study I read and commented
> about at the beginning of this month. I'd found it in an old Discover Mag
> from Sept 98. I thought it was rather interesting as a possible reason why
> we get large showers of many stones on occasion. That they entered as a
> single large, but loose, mass and separate in the turbulence with the usual
> spalation etc... Take care...
>
> Jamie Platt
>
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