We did come up with our own version of a fanciful idea, though, which sounds like something out of a Star Trek movie.
Imagine that the Khatyrka meteor was the result of a collision between an ordinary carbonaceous chondritic meteor and an alien spaceship. One could then imagine that the never-seen-before combination of aluminum-copper metal in the Khatyrka meteor might be a remnant of that spaceship. It has always been a fun explanation for us to fantasize and joke about, especially because it would mean that our quasicrystal was ultimately proof of life on other planets.
Of course, all this is in jest. The point of the laughable alien spaceship theory is that, as crazy as it sounds, it is harder to disprove that theory than any of the more plausible possibilities we considered, all of which we have successfully managed to test and disprove.
But if the alien theory was a joke, what was the real secret of how and when our natural quasicrystal formed?
TWENTY-TWO
* * *
NATURE’S SECRET
In less than a year after returning home from Kamchatka, our team had already obtained an overwhelming amount of new evidence. We had proven beyond a reasonable doubt that nature made quasicrystals long before humans fabricated them in the laboratory, and that the examples we recovered in Kamchatka were not of this world. They were visitors from outer space.
We could have stopped there, declared victory, and moved on to other research. But neither my nor Luca’s DNA permitted that. Our curiosity was more stoked than ever and we were totally committed to finding out where our meteorite originated, when it formed, and how it was created. There was no simple way to answer all those questions. The only way forward was to try everything. Simultaneously.
Leave no stone unturned. That had been my mantra ever since the first natural quasicrystal was discovered in a long-forgotten museum sample. In the wake of our expedition that all-out approach was more apropos than ever.
Dig out every detail from the natural samples we had brought back from Kamchatka. Design experiments to reproduce the extreme conditions in outer space so we could test our theories using man-made alloys. Identify new ways to find the original source of the Khatyrka meteorite. Collect and investigate meteorites similar to Khatyrka for natural quasicrystals or for other supposedly “forbidden” metallic aluminum alloys. And finally, figure out how to do all of those things at the same time because no one could ever know in advance how long any of them might take or which, if any, of these ideas would turn out to be the most fruitful.
As a result, our research efforts since 2012 have been unusually diverse, with novel and occasionally risky experiments. New teams of scientists, each with an array of highly specialized knowledge, have been enlisted to help pursue our quest. We have had our share of painful failures along the way. But what remains most impressive to me is the extraordinary progress and remarkable insights we have realized in such a short amount of time.
ALUMINUM WORMS AND MINERAL LADDERS
We began with Grain #125. Of all the grains we recovered from the Listvenitovyi, it had the longest and clearest example of a contact between an oxygen-bearing silicate and khatyrkite, the crystal aluminum-copper alloy that is the most abundant metal in our samples. Studying the textures near the contact seemed like a promising approach to take in order to try to understand the powerful forces that created the unusual combination of minerals.
One of our earliest team members, Lincoln Hollister, was the ideal person to lead the investigation. Lincoln and I began working together in January of 2009, just a few days after our initial discovery of a natural quasicrystal. He was renowned for his ability to piece together the history of rocks based on their structure and composition, which was exactly the kind of analysis we needed. Lincoln had officially retired from Princeton the same month we began our expedition to Kamchatka but insisted he had no intention of withdrawing from the project. He loved the challenge of being at the forefront of trailblazing research.
Our first new team member would be Chaney Lin, a graduate student who had come to Princeton in the fall of 2011 to study theoretical physics with me. Once he was exposed to the mysteries and puzzles surrounding natural quasicrystals, he was hooked. Just like the rest of us.
Chaney started off with a summer project aimed at finding new meteorite samples in the dozens of bags of material we brought back from Kamchatka. Luca had already completed two full passes through the hundreds of thousands of grains, so it seemed a good time to enlist a fresh set of eyes. Chaney’s long-term goal was to become a theoretical physicist, which involves more math than microscopy. So before he could examine any grains to see if they had the right chemical composition, he needed to learn how to use an electron microscope, which is a delicate business in its own right.
With tutelage from Nan Yao, director of the Princeton Imaging Center, Chaney soon became one of the best electron microscopists on campus. He had both the patience and skill to extract precise and meaningful information from our tiny materials. By the end of his summer assignment, Chaney and another graduate student had completed a third pass through all the material. They discovered two additional meteorite samples, which was a great cause for celebration.
Chaney then decided to keep working on our investigation, in addition to his work on theoretical physics. As an undergraduate, he had spent the last four years on the East Coast at New York University. But having grown up in Los Angeles, Chaney retained an ample amount of the laid-back demeanor one often finds in a Californian. Among his many positive traits, he could accept criticism without becoming defensive or emotional. He would always listen to my feedback with a receptive smile before responding with thoughtful and creative comments. I decided he would be an ideal protégé for Lincoln, who had earned a reputation on campus as a wonderful, but demanding, mentor.
When I introduced Chaney and Lincoln, seen below, the two of them hit it off instantly and voilà! They dove into analyzing every minute component of Grain #125, beginning with contacts between the silicate and the khatyrkite.
Chaney soon made his first major scientific breakthrough. Using an electron microprobe to study Grain #125, he determined that the wormy threads in the khatyrkite metal were nearly pure aluminum, which was something never seen definitively in any mineral before. Finding that impossible substance, along with the impossible metallic aluminum alloys, greatly enhanced the mystery of the Khatyrka meteorite. Chaney presented the evidence of pure aluminum to Lincoln and me in his usual, understated manner. But it was evident from his ear-to-ear smile that he was bursting with pride at the discovery.
Lincoln expertly interpreted Chaney’s image, shown below. He pointed out that the regular texture of the dark, wormy aluminum threads between the channels of whitish khatyrkite—a mixture of one part copper and two parts aluminum—was a sure sign that the metal grain had somehow completely melted and then rapidly cooled.
If the original liquid mix was one part copper and slightly more than two parts pure aluminum, Lincoln said, it would naturally separate as it cooled and solidify into thick strips of khatyrkite with thin wormy threads of excess aluminum, which was exactly what we were seeing in Grain #125.
Studying the silicate material, the darker substance on the other side of the metal-silicate contact, with the electron microscope was more problematic. When Chaney and Lincoln first viewed it in the scanning electron microscope and checked its chemistry with the electron microprobe, they found an unusual composition and texture that they could not readily identify. With Lincoln looking on, Chaney worked for many weeks and used a barrage of creative techniques to solve the mystery. Nothing worked.
The two of them finally decided the problem was that the composition varied wildly across the microscopic space of just a few interatomic distances. The microprobe could only report the average composition over a much greater area, which had the effect of blurring fine-scale variations. We needed to find another experimental approach that could resolve differences in composition occurring ove
r very short distances.
After consultation with Luca Bindi and Nan Yao, we devised a plan using a special piece of equipment known as an FIB—a focused ion beam. It would be a risky surgical operation that we had not used on any of the other grains so far. The FIB would cut and remove an ultrathin slice from the puzzling area of the sample. Then the slice would be studied with a transmission electron microscope, which, unlike the microprobe, was powerful enough to measure compositional differences over very short distances.
It would take a full six months to perform the FIB surgery and take the required measurements. We needed Nan Yao’s expertise to prepare the sample. First, he carefully reviewed it with Chaney, Lincoln, and me. Then, he painstakingly deposited an extremely narrow strip of platinum on top of the tiny sample in a predetermined place that appeared to have the greatest compositional variation. The location is seen in the previous image along the dotted line. The total width of the platinum strip Nan applied was less than one-hundredth of the thickness of a human hair.
The sample was then sent to FIB expert Jamil Clarke at Hitachi High Technologies in South Carolina. He would focus an intense ion beam on the sample and blast away material surrounding Nan’s tiny platinum strip. The platinum was thick enough to repel the ions, so the slice of material that lay directly beneath the metal was sure to remain intact.
The ion beam created a depression all around the platinum strip. Within the depression was a gossamer-thin wall of meteorite material, which was left standing in the microscopic crater like a fragile butterfly wing. With exquisite care, Jamil detached the delicate piece from the rest of the sample and shipped everything back to us.
The nearly transparent slice was barely visible when we opened the package. One sneeze, I thought, and the sample would be lost. Once we were able to examine it under the transmission electron microscope, we understood why the microprobe had never been able to get a clear read on its composition and texture. Instead of being a uniform layer composed of a single mineral, it looked like a complex, microfine mess. And that revelation opened the door to another series of important discoveries.
The slice was originally composed of silicate material that would normally be contained in the matrix material commonly seen outside the chondrule of a carbonaceous chondrite meteorite. But there was one significant difference. In this case, the image showed that the silicate material had melted and then rapidly cooled. It felt like the story was beginning to come together, because that was consistent with the wormy aluminum threads we had already found in the other portion of the grain, which also indicated that it had been melted and then rapidly cooled.
Because the cooling of the silicate had happened so quickly, the microfine mess that was revealed by the transmission electron microscope caught the violent, ancient process in suspended animation. The liquid had formed rivers and streams between remnants that had not melted, and each of the streams had rapidly solidified into a texture that looked like a ladder (as shown on the next page).
The white rungs on the ladder consisted of amorphous silicon dioxide, a glassy substance. Even more significantly, the dark rungs on the ladder consisted of a rare mineral called ahrensite. Just like stishovite, which had been found in one of our other samples, ahrensite can only form at ultra-high pressures. Chaney and Lincoln determined that the pressure must have been at least 50,000 times the normal atmospheric pressure on Earth. Temperatures would have had to have reached at least 2,000 degrees Fahrenheit in order to melt both the aluminum and copper.
As we continued to study the rest of the silicate in Grain #125 beyond the FIB slice, we recognized that it was composed of minerals arranged in shapes reminiscent of the loose matrix in the grain Glenn and I had reviewed together shortly after returning home from the expedition. The difference was that, this time, the matrix mineral grains were crushed together in a dense lump, which was exactly what one would expect if it had undergone a high-velocity impact with another asteroid in space. The impact would generate a shock wave that would squeeze and compress the loose matrix material into the shapes we were seeing under the microscope. And it would have melted the matrix in certain places where the temperature and pressure were especially high. With the discovery of ladders composed of ahrensite and silica and the observation of crushed matrix material, we now had explicit, quantitative proof that the Khatyrka meteorite had undergone one of the most powerful impacts ever detected in a CV3 carbonaceous chondrite.
Everything we had learned so far was further confirmation that the Khatyrka meteorite was exceptional. Luca and I were more energized than ever and ready to pursue the next open questions.
Were the natural quasicrystals already part of the Khatyrka meteorite when it began forming in the nascent solar nebula 4.5 billion years ago? Or were they created at a later time as a result of a collision?
Lincoln favored the second theory, which was that the natural quasicrystals formed after an intense impact. He found it more likely that aluminum and copper were chemically bonded to more typical chondritic meteorite minerals beforehand. Some of the minerals would have melted due to high pressures and temperatures caused by the impact, he theorized, freeing their atoms to form both the quasicrystal and the two “impossible” crystalline aluminum-copper alloys, khatyrkite and cupalite, which were found in the sample.
Glenn MacPherson’s favorite theory, on the other hand, was that the quasicrystals and aluminum-copper alloys had existed from the start. He believed it was more likely that the pure aluminum and copper had condensed directly from the solar nebular gas during the earliest stages of the solar system and had been part of Khatyrka all along.
It was not immediately obvious how we were going to distinguish between the two theories. Luca and I needed to come up with a different kind of experiment. But what? we wondered.
LOST IN SPACE
My philosophy of leaving no stone unturned occasionally caused problems.
While Chaney and Lincoln continued to search for more clues in Grain #125, Luca and I looked for a new way to study our samples. We desperately wanted to find new, nondestructive tests. Samples from the expedition were a very limited resource, and we wanted to preserve as much material as possible for multiple rounds of testing.
The process used to prepare our samples for the electron microscope was becoming a problem. We had to first embed the sample in a special holder filled with hot epoxy; then let it cool; and finally cut through the encased material to expose a smooth surface that could be studied.
The solidified epoxy helped keep the sample intact during slicing but introduced a problem unique to our material. The heat of the epoxy tended to fracture contacts between the metal and silicate. Our samples were especially vulnerable because of the difference in thermal expansion rates between aluminum-copper alloys and silicates. We were trying to study contacts between those materials and needed them left as undisturbed as possible.
A promising alternative was X-ray tomography, essentially a CAT scan for minerals. The test can identify minerals within a sample and produce an extraordinarily useful three-dimensional reconstruction. It had already become a well-established technology for human medical diagnostics but was still a relatively new technique for studying minerals. It could not approach the fine resolution we had already achieved with the FIB experiment and was also not as precise as the electron microprobes we were already using. But it offered one major advantage: It would not require the destructive hot epoxy and slicing procedures.
Luca and I had both read about the emerging technology and decided to arrange a trial experiment. Luca was able to obtain access to a low-resolution machine. So he tested part of a sample that had not been epoxied. The results looked promising, so I arranged to perform more precise scans at the University of Texas High-Resolution X-ray Computed Tomography Facility, which had some of the best machines in the world. All I needed to do was provide the lab with clean samples that had not been epoxied.
The only grains that remained unaltered
at this point were two of the samples Luca was working with in Florence. So those were the grains that would be sent to Texas. Luca carefully packed up the two samples, Grain #124 and Grain #126, using the same methods he had been using to send me samples for the previous five years. He hand-carried the padded box to the Air Express office in Florence, as usual, and mailed it off to me in Princeton.
And then, nothing. That was the last we ever saw of it. Air Express lost all trace of our priceless, tiny grains.
I was aghast. Absolutely, positively horrified. Our expedition team had beaten the odds and traveled thousands of miles to the easternmost edge of Russia, traversed the tundra, crossed the turbulent Khatyrka River, evaded enormous Kamchatka brown bears, battled relentless mosquitoes, dug up tons of nearly frozen clay in freezing water with our bare hands, fought our way back to civilization through storms, conveyed our sifted material out of Russia, painstakingly sorted through millions of grains only to have some incompetent, faceless person misplace two of our most valuable finds?
For the next few months, I kept a semi-frantic watch on my mailbox as Luca hounded Air Express.
Did the package make it out of Italy? Was it stuck in customs, baggage claim, or buried in the back of a delivery truck? What about the computerized tracking system?
Increasingly desperate, Luca tried to enlist the shipping company’s help by explaining how exceedingly rare the grains were, how difficult they had been for us to obtain, and how important they were to scientific research and our understanding of the fundamental nature of matter.
Luca became progressively agitated as time wore on. But the Italian Air Express office just shrugged off the incident. They never managed to figure out what happened to our package. Worst of all, the staff never seemed to care.
The Second Kind of Impossible Page 28