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19.9 grams. 30 by 27 by 23 mm.   CM2

TKW 12.6 kg. Observed fall 20 September 1950, in Calloway County, Kentucky, US.




   

Steve writes:

Around 1:35 AM on September 20, 1950, a fireball "as bright as day" was seen traversing southeast over Illinois towards Kentucky, and after several sonic booms were heard, it exploded at an altitude of 30 miles with a force that reportedly jarred windows over a thousand-square-mile area and as far away as Paris in neighboring Tennessee.

Shortly after the fall, Dr. Charles P. Olivier, the President of the American Meteor Society, used eyewitness reports to calculate the meteorite’s trajectory and a likely impact site approximately 9 miles east of Murray, Kentucky. Then, on October 22, 1950, a party from Vanderbilt University visited the area to search for fragments. Intermittent rain hampered their search, and although witnesses claimed hearing many masses fall in nearby woods, only a few small ones separated by as much as 3 miles were found. Local farmers gave the search party additional pieces, bringing the total weight of the initial find to about 7 kg.

At 3.4 kg, the largest recovered stone was initially heard whistling through the air before striking the ground; the 6 inch diameter specimen embedded itself in a well-used and compacted path to a depth about equal to its diameter, just 26 feet from the home of Mr. Ernest Barnett. Many of the other pieces fragmented upon impact, with one puncturing the roof of a house owned by a Mr. Wilkinson and falling onto the floor; because the floor was not damaged, researchers concluded the fragment had, during its fall through Earth’s atmosphere, slowed to approximately the terminal velocity of a freely falling body.

Subsequent searches would result in an additional 5.6 kg being found, bringing Murray’s TKW to 12.6 kg. "Diligent collector" Hugh Howard had acquired 7.3 kg of Murray’s fragments, but much to Howard’s dismay, the largest 3.4 kg piece was not among them, as it remained locked inside a display case inside Vanderbilt University’s former observatory. So in 1959, he sold his collection to the Smithsonian, then shortly afterwards enrolled at Vanderbilt as an undergraduate. After visiting the observatory several times to devise a plan, Howard stole the university’s 3.4 kg meteorite, replacing it with a painted clay replica. But while university scientists discovered the theft a few days later when they noticed the meteorite looked blacker than usual, they kept the fake on display for fear the thief would destroy the original if the theft was exposed. Howard’s fingerprints were found on the display case, and because the meteorite was valued at $2000 (about $21,000 in today’s dollars), authorities subsequently arrested him in Arlington, Virginia on grand larceny charges, for which he served time in prison, where he was later murdered.

(Interestingly, the Murray meteorite is not a part of Vanderbilt’s collection, as the university had no paperwork to prove ownership; today, it’s housed in Arizona State University’s collection after it was purchased by ASU from an unidentified individual assumed to have been the legal owner after the meteorite’s subsequent recovery.)

Murray, a CM2 carbonaceous chondrite, is a breccia that has experienced varying degrees of aqueous alteration on its parent asteroid. Its matrix is primarily graphitic carbon with inclusions of refractory residue and small chondrules composed mostly of olivine. Besides tiny diamonds and corundum, the meteorite also contains pre-solar grains and noble gas components with anomalous isotopic compositions that predate the birth of our solar system – even older than Murray’s calcium aluminum inclusions (CAIs are some of the first solid matter that coalesced in the solar nebula prior to planetary formation). Because of this, some of Murray’s silicon carbide grains have even been used to help with dating of our Milky Way galaxy, as isotopic anomalies of the meteorite’s silicon, nitrogen, and carbon suggest some of its minerals may be circumstellar grains from carbon-rich stars.

Carbonaceous chondrites like Murray share similar oxygen isotope ratios and have nearly identical Mg/Si proportions as the Sun’s. Further, gases in these types of meteorites are divided into two categories: "solar" (light rare gases such as helium and neon); and "primordial" (heavier gases like argon and xenon). Gases in the Murray meteorite are primarily solar and are similar in composition to the solar wind.

CM2 meteorites are also characterized by their nickel-bearing sulfides: mass spectrometer analyses of Murray’s rhenium and osmium elements indicate the meteorite may have been formed from different earlier precursor materials than iron meteorites. And like the more famous Murchison meteorite of the same type, Murray also contains both a wide range of organic compounds and numerous amino acids – 17 total, 11 of which are unique from the 20 found in proteins on Earth. (That said, over 2700 articles to date reference Murray, making it a well-studied meteorite too.)

Our 19.9 gram, 30 by 27 by 23 mm piece (Photo 1) has fusion crust over not quite half its surface that takes three forms: traditional black on one side (Photo 2); brownish on another side (likely tainted by brown terrestrial material, as seen in Photo 3); and areas with a frothy, almost fibrous-looking texture (Photo 4). On broken surfaces, chondrules typical of CM2 meteorites are quite evident (a variety of which are shown in Photo 5), as well as some less typical chondrules (two examples are shown in Photo 6). Additionally, a few CAIs can be seen (Photo 7 shows one on a broken edge from two different angles).

Over the 18 months, I’ve been having issues with degrading eyesight that have limited my ability to work with my microscopes, and in particular, do crossed- or parallel-eyes 3D photo viewing. Additionally, I know some of Paul’s readers struggle with these kinds of images, and even red-cyan anaglyphs are problematic, as not everyone has the appropriate glasses required to view them. In my November 29th, 2020 MPOD, I included some crude attempts at 2D projections of 3D features using animated GIFs, but more recently I’ve been experimenting with improvements on this approach. By way of comparisons of these various techniques, Photo 8 provides a traditional 2D view of an interesting chondrule in this meteorite; Photos 9 and 10 offer crossed- and parallel-eyes image pairs (see viewing notes at the end of this post); and Video 1 is an attempt at a 2D projection (from a different angle) to give an idea of what the area looks like in 3D, but without the need for special glasses or strenuous eye gymnastics. (My apologies – I wasn’t personally able to verify Photos 8 or 9, but the method I had previously developed to generate these kinds of images worked reliably for the images in my prior MPODs.)

An additional comparative example is offered in Photo 11’s traditional 2D image of a gouge-like feature across the surface of the meteorite (which continues across the meteorite to the area seen in Photo 4), the crossed- and parallel-eyes views of this area shown in Photos 12 and 13, and Video 2’s corresponding 2D video projection.

[I’d be interested in any comments folks may have about these comparisons – in particular whether, for future posts, the 2D video projections are adequate replacements for the crossed- and parallel-eyes 3D images. From what I can tell, the projections aren’t quite as sharp, but I’m working on ways that might help to address that limitation.]

Finally, I don’t find most CM2 meteorites all that interesting in thin section but for those that do, here’s a gigapixel scan of Murray

Viewing Notes:

For the crossed- or parallel-eyes images, it may help to start at lower zooms, then when you’ve achieved focus, slowly increase the zoom as appropriate.

For the Gigapan image, switch to full screen mode by clicking on the diagonal arrows to the upper right of the image; then use your mouse to pan around and its wheel to zoom in or out.