05 April 2002Diamonds in the sky ... and other stuff, too
by Art Winfree
Many years ago, early in December 1980, I drove to California for the first time, from Purdue University in Indiana. In northern Arizona there appeared on the horizon to the southwest a distant and peculiar-looking mound in the remote desert:
Finding a dirt road to the south, I investigated. Seemed the bottom of a circular hill maybe a mile in diameter, but above about 100 feet altitude it had all vanished. Will the insides be a plateau? a lake? Reaching the edge was like stepping into a dream: there are no insides, at least not until you fall 300 feet. It is a mile-wide crater! The inside walls are tumbled blocks of limestone, similar to the rubble outside that I had passed in vacant perplexity. Looks like a huge explosion site. Scrambling down to the bottom I found rusty excavation equipment and guessed this is or was once thought to be a meteorite hit, so potentially an iron mine.
The Sun was near setting in the next photo, taken to the east, then I climbed back out in the dark to search anxiously for dirt road and car under a starry moonless sky. Stepped on no rattlesnakes, and hadn't the wit to search for lumps of iron now called "Canyon Diablo meteorites" on Ebay.
Arriving at La Jolla and the Scripps Institute of Oceanography geology library, I read the fascinating history of "Barringer Crater", where roughly 60,000 tons of cold iron is supposed to have vaporized on impact 50,000 years ago.
Last time, we examined a 60-gram single crystal of iron from the sky, presumably older than the Earth itself. That utterly unexpected Discovery seems a fine reward for indulging curiosity with no apologies for lagging 1-2 centuries behind the times. While visiting the local Ace Hardware to replace lathe pulley belts, get abrasive paper, etc., all excited about unfamiliar and unexpected encounter with the crystal ready in my fist to show to anyone careless enough to catch my eye, I encountered a 1-ups-man who told of picking up boxes full of similar-looking iron rocks in a cattle field between Gila Bend and Casa Grande 3-4 years ago. He had crates full of them somewhere in his garage and promised to bring a sample when he gets time to excavate that garage. Hmmm. This sounds very interesting. Wasn't there a fuss on TV news a few years ago about a fireball seen over Interstate-10 between Tucson and Phoenix?
Back home, a web search revealed Yes, and that professors at my university's Lunar and Planetary lab had gathered all the visual fireball reports and guestimated landfall between Casa Grande and Gila Bend. The fall site was never identified, though. While rummaging, I found Bob Haag's web site ("The Meteorite Man" of Tucson): email exchange confirms that the fall was never found. But it was 7 June 1998: 2-3 years ago, not 3-4.
Drove "there" instantly anyway (4 hour round trip on a Saturday), found gorgeous desert flowers in bloom after a wet winter, plenty of gophers churning the dust on a scale of maybe 1 foot depth per year, some tin cans of antique vintage churned up, but no meteorites. Nor had local ranchers and farmers heard of such. Nor had they heard of the swarms of prospecting university people and collectors systematically combing through the mesquite in summer 1998. Have all the little pieces since gone underground during summer thunderstorms and gopher tunneling, or maybe got plowed into cultivated farmland? Did the fellow at Ace Hardware maybe pick up lumps of magnetite (which would be black, not rusty), such as are known to be present in that geological area? In any case, he never showed me a sample despite (or because of?) my persistent daily nagging. I gave up.
And went back to cultivating my own garden more aggressively: on Friday 30 March 2001, I cut off another tiny corner of putative Sikhote-Alin, polished it, and took it to LPL an electron probe for quantitative analysis of X-ray emissions. Kenneth Domanik found that it is indeed close to 5% nickel, plus 1/2 % cobalt. This is just the mix known to characterize the Sikhote-Alin mass. Iridium or gallium assays would require more expensive neutron activation analysis: not done, but if anybody has the means I would be happy to provide a sample. The tiny corner also has 20-micron-ish inclusions called "schreibersite", a phosphate of iron and nickel that is found only in meteorites (so far as I have learned. And "only" is fortunate: its hardness ruins saw blades) Primed to notice such things now, I also see a huge one (shiny metallic bronze-colored about 1x2 mm) on the big piece after removing some rust from an un-sanded side.
Conclusion so far: this really is a piece of Sikhote-Alin, or at least of a very similar fall. And this little bit of iron goes some way toward answering a question that probably fascinated most of us as children: How come there is no planet between Mars and Jupiter, but instead just a lot of rubble? Might there have once been a body, or several, even a planet ruled over by Emperor Ming and his minions? If a single body had exploded to create all these fragments, survivors would still be on orbits intersecting through that site, at least until they collide with each other. I don't think that is the case, but I also don't know how long it takes before mutual collisions alter such orbits. Anyhow (just guessing) maybe the irons falling from our skies are not from the interior of Ming's empire. Might they, however, be from some asteroids that aggregated 4-5 billion years ago only to sufficient size to segregate iron from silicon dioxide in a melt? If meteorites are samples of pulverized asteroids that subsequently collided, then this meteorite reveals that some of them were big enough to gravitationally segregate iron as a melted core big enough and buried deep enough to cool slowly enough to form monstrous single crystals of mighty pure iron such as we never see on Earth (except as recent falls). How big would such a body need to be? Why would they melt? How big must an asteroid be to sufficiently contain the heat of its radioactive decay? And why would it later cool off again? How much stuff is there now in the asteroid belt, to compose such large bodies? (and how much of what there once was has already been swept up by planets and the Sun?) Such questions are the railheads of paths into the forest, good branch points for further Adventuring.
Continued August 2001:
In July of 2001 my wife and I were fortunate to enjoy the wonderful hospitality of friends at Buenos Aires for the first time while I lectured at the University Physics Department. On the way back our flight went directly over the site of the 100-ton Campo de Cielo meteorite of 4000-6000 years ago. (Pure luck, but I was on the look-out with GPS, primed by Norton's book and encounter with Haag, who had a great adventure there: pp 297-99.) This piqued my curiosity enough to locate a trader on the web and acquire an 842-gram chunk by Ebay auction. Larry Asedo in the local machine shop was kind enough to belt-sand this heavily rusted monster for me. Its now-shiny surface reveals profuse cracks, their inner edges rusty. I imagine impact damage, then millennia of mud and rain (during all which time, by the way, the local people retained verbal memory of the day it fell from the sky to create craters as wide as 200 feet.)
Treated similarly to Sikhote-Alin, Campo results are much the same but there are more crystals in such a big rock, and they are individually somewhat smaller: Campo5, Campo6, Campo7, Campo4, Campo8. (For greater visual detail you can download the original files and view them at full scale and resolution, either from SAS or from http://marley.biosci.arizona.edu/~art/graphics/Campo[4..8].jpg).
These are 56x40 mm photos in sunlight from different directions, so you can see from the angle-dependent reflections that the islands of metal between "cracks" are single crystal grains of independent uniform orientations. Maybe some are kamacite, some taenite: Campo is reportedly 7% Ni, 1/2% Co, 1/4% P , which means is should crystallize in mixed hex and octa lattices and so should show "Widmanstatten patterns". Anyhow, now I think the cracks are not 5000-year-old impact fractures but non-iron materials -- phosphates, etc -- excluded as the crystals grew, pushing impurities into the interstitial melt, and then stopped growing as they encroached upon one another. The cracks contain colorful stuff. I guess they got wet from cooling immersions during sanding, if not already wet from millennia outdoors. Yellow, red, blue, green, orange "flowers" grow out of the cracks, I guess as water evaporates in the dry Tucson air. Face down overnight on a sponge of fine white foam rubber, the polished face leaves a colorful imprint that after a few days turns to uniform rust orange. I hope these are not alien life forms!Too much imagination? And are my two samples unique? Norton's book suggests not. But get your own meteorite and see what you find.
Continued February 2002:
Bob came back for the 2002 Gem and Mineral Show, this time actively seeking meteorites, and gave me a few Campo de Cielo pieces, nicely cleaned up and varnished, in the 1-2 pound range. Grinding a corner of each to label it "Campo" lest future archaeologists of Tucson go nutty with speculations, I see that the inner structure is like the one I had last year from Ebay.
And he brought something from Odessa, TX, already cut and polished. This meteorite is coarse octahedrite (7% Ni, 1/2% Co, 1/4% P), and it arrived 50,000 years ago, in both respects like the one responsible for Barringer Crater in northern Arizona. It shows "Widmanstatten figures", oriented patches of kamacite and taenite like the several-mm grains packed into my Campo, but seemingly without dirty interstices where Campo's growing grains had collided. Again, here are views variously oriented to the Sun so you can see the many isolated patches are uniformly and differently oriented.
And he brought home a sample of the 1% of meteorite falls that are "stony", with surface fused by great heat during encounter with the Earth's atmosphere at something like 5 miles/sec. Sliced open it reveals scattered flecks of metal. Iron with several % Nickel? Maybe this is the sort of stuff that, were it to melt, and were it many miles in diameter, would segregate an iron core that could cool to make such crystals are we saw.
The web provides plenty of good information (two links to the same site in Alaska) on the diversity of meteorites.
I posted the beginnings of this little Adventure to my web site a year ago. It was noticed by a citizen of Mongolia who happened upon a 40 kilogram lump of iron last summer that had punched a hole in a cave roof and lay inside, I guess in wet conditions, for ... well, no one knows how long. He sought advice about marketing the lump and offered to send me a little sample to characterize. If so, there will be another Adventure in Discovery.
Now go create your own and report back some results to SAS!
And a postscript about 25 March and 31 March, when observers in the Americas might have witnessed a Rainbow Moon:At Tucson on the 25th visibility was perfect but, just as on 25 January, the Moon rose over the mountains an hour too late, already just inside the primary rainbow and skewered by the secondary rainbow's arc of color. Further to the east, timing must have been better, but I have heard no report of (implicitly primary) Rainbow Moon.
And on Easter morning 31 March
the Moon set at 08:23 AM while exiting the rainbow circle a few days after full moon. Its phase angle was about 40 deg and increasing, but I wanted 41-42deg. Beautiful weather, nice snapshots at 07:50, 5-6 deg above the western horizon in this photo, in the color band of the secondary rainbow (180-151=39 deg phase angle), but again a couple hours too early (as far west as Tucson) to see it exiting the primary rainbow.
Next chance 20 July.
Copyright 2002 by A.T.Winfree. All rights reserved. Used by permission.
