Sunday, October 4, 2009

Wheelies in the sky

Just in case you were wondering, this is what I will look like when I die:

No, no, no... not an inappropriately posed well-meaning donated-my-body-to-science cadaver in the Sydney Museum. No, I will be doing one-handed wheelies on my horse in the sky. Because I won't fear horses in the after life. Because horses can't smell fear in the afterlife.

Died of boredom? Not likely at the museum. More likely causes of death were there to liven up the learning experience. Like these livestock leg bones which at some point rampaged through fencing. The bone is completely grown around the wire. I call these "Western Pearls."

On to the minerals section, while Karin looks at the ethnic art section.

Silver. The minerals section was really good. Definitely as good as the Smithsonian collection, but not as big - which is fine because the high-points of a gem collection pretty much make it. The opals were better but they didn't have any fluorescing specimens.

Natural tin foil and natural lung cancer. My basement had asbestos in it when I was growing up. Some contractor who's sole job was removing asbestos from basements came and took it all out without even having a mask on. He'd have been against socialized medicine.

This is the most thought-provoking thing I found - meteorites. Basically posed as a footnote in a poorly positioned glass case in a hallway next to some stacked chairs, it contained some real brain teasers. Sure, some of the meteorites looked like they let the astro intern at them with a dull band-saw. But check this out: the one on the left has recrystallised on re-entry. AND THE PROCESS FAVOURED THE GRAIN DIRECTION OVER THE THERMAL GRADIENT! what??
Ok, so what's going on? Let's assume that the crystalline structure away from the heat affected zone (HAZ) is the original structure as it was chunked off of some distant amazing metal-planet.
  • The temperature at the surface was close to T_melt because the outer surface is smooth
  • The temperature dropped rapidly from the surface to the interior due to the quick heating (falling stars are fast), despite the high conductivity of most metals.
  • There may have been some massive spin on this puppy.
Ok so I have an idea - basically, the outer surface was hot, but not for very long before it touched down. I think the thermal conductivity of the crystal was orthotropic - different in the grain direction. So the heat penetrated more easily along certain directions, determined by the pre-existing grain. Approximate penetration depth of the transient heat front: sqrt(5*K*time/density*C). Assuming nickel properties:
density: 8.88 g/cc (8800 kg/m3)
C: 0.46 J / kg.°C
K: 60.7 W/m.°C

Then the penetration depth of the thermal front after about a 1sec fall would be:
d = 0.274m - clearly larger than the item itself. So, if we assume the penetration depth was the ~1cm seen in the photo we can solve for the heated time during descent - 1.3 milliseconds!

Karin just made this observation: meteorites break up on descent. So, with what we know about the surprisingly rapid nature of this heating I think this:
The parent meteor completely broke up about 1.3ms before impact and thus this chunk was heated for an extremely short period of time. Perhaps it even continued recrystallising on the ground as the heat conducted towards its core.


Scientists don't get mad, they get even. He was at a entomology conference in NYC, she scorned him and he named a bug after her.

Mom! Son! (big hugs)


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